Dr. Michael Loranty Associate Professor of Geography August 6, 2018 Dr. Kirsten Thonicke Associate Editor, Biogeosciences

Re: bg-2018-201

Dear Dr. Thonicke,

My coauthors and I were pleased to learn that you are in agreement with our responses to comments provided by reviewers on our manuscript “Reviews and Syntheses: Changing ecosystem influences on soil thermal regimes in northern high-latitude permafrost regions”. As requested, please find below our point-by-point response to reviewer comments followed by a track-changes version of the manuscript. Thank you for further consideration of our manuscript.

Best Regards, Michael Loranty & Coauthors

Response to reviewer comments: We thank both reviewers for the insightful and constructive comments, and are happy that they appreciate the value of our review in helping to identify important knowledge gaps regarding relationships between ecosystems and permafrost thermal regimes. The manuscript is greatly improved as a consequence of substantive revisions made in response to these comments. Specifically we have: 1) provided a more thorough and systematic treatment of the ground thermal regime and incorporated this more fully into the overall framework of the review, 2) more fully synthesized the findings of existing studies and identified concrete research questions that need to be addressed, and 3) addressed all of the minor issues. Below we provide our responses to specific reviewer comments. Reviewer comments are shown in blue Times New Roman font, and our responses follow directly in black Cambria font. We indicate where we have revised the manuscript in response to each comment, and provide two versions of the revised manuscript, one with track-changes highlighted, and a second final version with all changes accepted. In our response to specific comments we indicate page and section numbers where revisions are found in the tracked-changes version of the revised manuscript so that the editor and reviewers can easily see them.

Anonymous Referee #1 The authors summarize a wide a range of findings on interactions between vegetation, hydrology and soil temperatures in permafrost. A review paper on these complex processes could fill an important gap in the literature. Unfortunately, I am not sure whether the manuscript in its present form achieves this aim. Rather than synthesizing a large spectrum of studies, the manuscript feels disjointed at times. For instance, the impact of hydrological changes is treated separately for winter and summer, thus neglecting important interactions and potential feedbacks. The manuscript also falls short of fulfilling the promise contained in the title, namely the elucidation of the soil thermal dynamics. While the ground heat flux at the soil surface is discussed, many other important aspects of the soil’s thermal regime, such as mean permafrost temperatures, temperature profiles, seasonal amplitudes, ground ice formation, etc., are given very short shrift. I hope that the following comments will be useful to the authors. We are glad the reviewer sees the utility of our paper in filling an important gap in the literature, and appreciate these helpful comments. The manuscript is improved as a result of a more comprehensive inclusion of the ground thermal regime and greater synthesis. Below we respond to specific comments and indicate where we have made changes.

1) Thermal dynamics As stated above, I found the discussion of the soil thermal dynamics incomplete. While the ground heat flux is clearly an important factor, it does not tell the whole story. Also, it is coupled to the subsurface temperature profile, so that is difficult to consider in isolation. These issues are confounded by the fact that the relevant time scales at which the ground heat flux varies are barely discussed. For instance, it is apparently implicitly assumed that the values are averaged over at least a diurnal cycle. Furthermore, the interactions between winter and summer processes are largely left out. In our focus on G as a unifying process and context for considering ecosystem effect s on permafrost it is clear that we failed to comprehensively consider the full thermal regime. Consequently we have modified the beginning of section 2 (p5-7). A new paragraph at the beginning of section 2 explicitly describes the important components of the annual soil thermal regime and how they are quantified. We have also added language to emphasize the importance of above- and below-ground controls on G (p6). Here we should note that in the original manuscript we chose not to emphasize many of these belowground aspects because factors such as the permafrost temperature and the vertical temperature profile are affected by non-ecosystem factors such as long-term climate, geologic and geomorphic history, permafrost genesis, etc., and so are beyond the scope of this review. We also explicitly note our focus on seasonal to annual variability in the soil thermal regime (p 7). Throughout the manuscript processes are now discussed in the context of how changes in G relate back to seasonal and annual aspects of the thermal regime. Interactions between summer and winter are addressed in response to the following context.

2) Summer and winter-time processes I felt there was a lack of balance and integration across the annual cycle, and the manuscript thus falls short of its objective to synthesize disparate information. In addition to the problems with the description of the ground heat flux, I had similar reservations about the discussion of the thermal conductivity. I missed a discussion of how the water/ice content modifies the soil thermal conductivity at below-zero temperatures (not explicitly mentioned), and what the impacts on the soil thermal dynamics are. Also, the impact of snow cover on summer-time conditions (soil moisture, deeper soil temperatures, etc.) is not really discussed. Thank you for pointing this out. Section 2 is extensively revised with expanded discussion of wintertime processes and how summer and winter processes are integrated. Sections 2.1 and 2.2 have been combined into a single section focused on canopy processes. It retains the same organization as the previous version, but includes a paragraph at the end discussing integrated effects of canopy processes across the annual cycle, and also identifies clear hypotheses and directions for future research. In sections 2.2 and 2.3 (formerly 2.3 and 2.4) we have also added discussion of seasonal interactions where appropriate. This includes discussion of how water/ice affect thermal conductivity at sub- zero temperatures. In addition we have substantially revised portion of section 2.4 (formerly 2.5) to focus more explicitly on process interactions that impact the soil thermal regime across annual timescales. With this more direct synthesis of results we are able to offer more specific directions for future research.

3) Heterogeneity and variability I believe the co-variability of soil and vegetation properties could be highlighted more clearly, as it has a strong influence on future changes and also on the presently observed patterns of spatial variability. For instance, bryophytes in adjacent wet and dry microtopographical positions often differ greatly in their physical properties. Such interactions can modify observed patterns of e.g. the relation between soil moisture and thaw depths. These issues in interpreting observational (as opposed to experimental data) are not acknowledged very clearly. We have highlighted these points more clearly in our revisions to section 2, and highlighted issues associated with interpreting observational vs. experimental data in our revisions to section 2.5.

4) Synthesis I would welcome a greater attempt at synthesizing previous findings, for instance by coming up with testable hypotheses. At present, there are many statements that process X may be important/not important or positive/negative, depending on multiple other factors. By highlighting open questions, or hypothesizing about the most important interactions, the manuscript would be more exciting to read. For instance, the discussion of conductive vs advective heat fluxes would be more informative if the conditions under which large advective contributions are hypothesized to occur (or where they tend to be observed; e.g. in fens in discontinuous permafrost), were mentioned. Thank you for highlighting this. We have revised the manuscript to more clearly identify open research questions and develop hypotheses regarding the directionality and importance of process interactions. These are included at the end of each appropriate paragraphs and sections, and summarized in the conclusions section.

Minor issues 1) Energy balance 1 The coupled nature of the surface energy balance and the subsurface dynamics is not portrayed very well. For instance, the following sentence suggests that above- ground processes (rather than above and below-ground processes) determine the surface temperature: ’Once energy has been absorbed at the ground surface and TSG is elevated, soil KT will dictate how much of this energy is transferred downward into the soil’. As described above, we have included more thorough discussion of belowground processes as a component of the expanded focus on the soil thermal regime.

2) Energy balance 2 I feel that several important influences of vegetation canopies on the energy balance are neglected (e.g., roughness, longwave radiation from vegetation canopies). We have modified Figure 1 and expanded our discussion of canopy influences on energy partitioning (p9-10) to include these important processes.

3) line 581 ponding is an important aspect in this context Agreed, we have amended the sentence to reflect this.

Anonymous Referee #2 Permafrost grounds will undergo pronounced changes in a warmer climate. In the current manuscript the authors focus on how high latitude terrestrial ecosystems influence surface energy fluxes of permafrost soils, and therefore the current soil thermal state and fate of future permafrost degradation. They discuss many aspects of ecosystem/vegetation interactions with the soil thermal regime – interactions which are key to predict future changes in permafrost conditions, but which are not represented (or only represented in a very simplified manner) in current Earth System Models. The authors consider individual processes not in isolation but especially discuss a broad picture of interaction among key processes. Given that current understanding of vegetation- permafrost interactions is incomplete, and that the topic touches on an important aspect for model improvement, I consider the paper of broader interest to the readership of Biogeosciences. Thank you, what you describe is exactly the aim of our review and we are pleased this came across in the manuscript.

Major aspects 1. The multitude of aspects discussed in the manuscript makes it rather difficult for the reader to extract which key processes are likely to govern permafrost-vegetation interactions (under present day conditions and under future climate change). The authors put a lot of effort in discussing a broad spectrum of vegetation-permafrost ground interactions which all influence permafrost soil thermal regimes. Many examples of these interactions reveal the possibility of either a net positive or a net negative feedback, depending on factors such as local topography, climate, soil conditions, etc. A “synthesis” of current knowledge about ecosystem changes and related impacts on permafrost soil conditions would have added value if the discussed aspects of vegetation-permafrost interaction in this manuscript would be summarized such that the reader can judge the broad-scale importance/representativeness of individual processes. In this regard an additional table or figure would be very helpful, which summarizes the discussed aspects in the text and which could list/illustrate a) the key physical process chains discussed in this manuscript, indicating whether the interactions are likely to result in a net positive of negative feedback (on ground temperatures, or on carbon cycling), or stating that the sign is unclear given current knowledge b) the factors which drive the sign of the feedback (e.g. topography, climate) To the degree possible, it would also be interesting to illustrate in this table/figure whether feedbacks will rather amplify or dampen under expected Arctic climate change, and (in line with the discussion of fire impacts on page 21) whether changes are reversible or irreversible (on human timescales). We agree that the manuscript covers so many processes that it is hard to keep track of them all. In order to accomplish this we chose to enhance Figure 4 rather than adding an additional figure or table. Many of the feedback processes were already included in Figure 4, and their linkages to other process were not illustrated elsewhere. So this seemed like a logical place to do this. As we worked through the manuscript we did indeed attempt to create a diagram illustrating all of the process linkages, their impacts on permafrost thermal regimes, key drivers, and associated climate feedbacks. However it quickly became apparent there were simply too many connections to explicitly illustrate them all, for example using block and arrows as we did in Figure 2. Thus we adopted a modified table approach in our revision of Figure 4.

2. A key uncertainty of future high latitude ecosystem changes will come from changes in the hydrologic regime, determined by changes in precipitation, evaporation, and drainage. Projections of these changes are highly uncertain. This aspect should be discussed in the manuscript as future high latitude vegetation responses will follow rather different trajectories for wetter or drier conditions (compared to today). In this context: Fig. 4 assumes a reduction in future (?) moss cover, and an increase in vegetation canopy cover. What are the assumptions behind made here? Thank you for highlighting this point. We have included discussion of hydrologic uncertainty where appropriate throughout sections 3-5, and in Figure 2. As described above, Figure 4 has been revised to provide more details regarding key changes and feedbacks.

3. One objective of the paper is stated as: “ to identify key challenges and research questions that need to be addressed to better constrain how continued climate- mediated ecosystem changes will affect soil thermal dynamics in the permafrost zone.” I might have overseen a discussion of this aspect in the manuscript, but at least in the conclusion section a reference is only made by stating that integrated analyses of processes are needed. A discussion of more concrete aspects would be helpful. Reviewer 1 also raised this concern and we have revised the manuscript to provide more explicit and informative synthesis of the information presented. Within each section we have summarized key process interactions that are poorly understand and where possible hypothesize regarding the likely impact on permafrost thermal regimes. We have also worked to synthesize key processes across spatial and temporal scales more explicitly, and revised the conclusions to provide a clearer description of the key research challenges and questions.

Minor aspects L61: double occurrence of sentence The duplicated sentence has been removed.

L 79/80: can you give a reference here? Yes, we have amended the sentence and added a reference.

L 126: what is meant by “internal energy transfers”? We modified this sentence to indicate that internal energy transfers refer to energy fluxes within the soil associated with water phase changes and temperature gradients within the soil.

L 269: Kt depends also on the thermal state (ratio of liquid to frozen water) The sentence has been amended to reflect this.

L688: “available evidence. . .” can you give a reference here? This sentence was meant to synthesize information presented in the preceding sentences and has been revised accordingly.

L 1507: (H) instead of (S) This typo is corrected.

Figure 2: what is meant by “Climate” as change agent – increases in temperature?, what about climate change induced changes in precipitation? The figure has been amended to indicate climate warming as the driver. In addition we have added a indicating to acknowledge climate induced changes in precipitation, and the associated uncertainty as discussed in our response to your comment above.

Figure 3, L1534: can you give numbers here? Yes – we have included approximate active layer depths for each site; ~40cm for the high- density and ~90cm for the low-density.

Figure 4: OLT is not explained This has been addressed through the figure revisions described above. 1 Reviews and Syntheses: Changing ecosystem influences on soil thermal regimes in northern 2 high-latitude permafrost regions 3 4 Michael M. Loranty1, Benjamin W. Abbott2, Daan Blok3, Thomas A. Douglas4, Howard E. 5 Epstein5, Bruce C. Forbes6, Benjamin M. Jones7, Alexander L. Kholodov8, Heather Kropp1, Avni 6 Malhotra9, Steven D. Mamet10, Isla H. Myers-Smith11, Susan M. Natali12, Jonathan A 7 O’Donnell13, Gareth K. Phoenix14, Adrian V. Rocha15, Oliver Sonnentag16, Ken D. Tape17, 8 Donald A. Walker18 9 10 1Department of Geography, Colgate University, Hamilton, NY 13346 USA 11 2Department of Plant and Wildlife Sciences, Brigham Young University, Provo, UT 84602 USA 12 3Department of Physical Geography and Ecosystem Science, Lund University, S-223 62 Lund, 13 Sweden 14 4U.S. Army Cold Regions Research and Engineering Laboratory Fort Wainwright, Alaska 99703 15 USA 16 5Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904 USA 17 6Arctic Centre, University of Lapland, FI-96101, Rovaniemi, Finland 18 7 U.S. Geological Survey Alaska Science Center, Anchorage, AK 99508 USA 19 8Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775 USA 20 9Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National 21 Laboratory, Oak Ridge, TN 37831-6301, USA 22 10Department of Soil Science, University of Saskatchewan, Saskatoon, SK S7N 5A8 Canada 23 11School of GeoSciences, University of Edinburgh, Edinburgh, UK 24 12Woods Hole Research Center, Falmouth, MA 02540 USA 25 13Arctic Network, National Park Service, Anchorage, AK 99501 USA 26 14Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield, 27 S10 2TN, United Kingdom 28 15Department of Biological Sciences and the Environmental Change Initiative, University of 29 Notre Dame, Notre Dame 46556 USA 30 16Département de géographie, Université de Montréal, Montréal, QC H2V 3W8, Canada 31 17Institute of Northern Engineering, Water & Environmental Research Center, University of 32 Alaska, Fairbanks, AK 99775 USA 33 18Institute of Arctic Biology, University of Alaska Fairbanks, AK 99775 USA 34 35 36 Correspondence to: Michael M. Loranty, email - [email protected], phone – 315-228-6057 37 38 39 40 41 42 43 44 45 46

1 47 Abstract

48 Permafrost sSoils in arctic and boreal ecosystems store twice as much the amount of

49 currentcarbon as the atmosphere, a portion of whichic carbon that may be mobilized and

50 released to the atmosphere as greenhouse gases whenas high-latitude soils soils thaw under a

51 warming climatewarm. This permafrost carbon climate feedback is among the most globally

52 important terrestrial biosphere feedbacks to climate warming, yet its magnitude remains highly

53 uncertain. Some of theThis uncertainty in the timing and magnitude of the permafrost climate

54 feedback lies stems from complex interactions between ecosystem properties and soil thermal

55 dynamicsin predicting the rates and spatial extent of permafrost thaw and subsequent carbon

56 cycle processes. Terrestrial ecosystems fundamentally regulate the response of permafrost to

57 climate change by influencinges on surface energy partitioning exert strong control on

58 permafrost soil thermal dynamicsand the thermal properties of soil itself and are critical for

59 understanding permafrost soil responses to climate change and disturbance. Here we review

60 how arctic and boreal ecosystem processes influence thermal dynamics in permafrost soils and

61 characterize how these linkages may evolvekey ecosystem changes that regulate permafrostin

62 responses to climate change. While many of the ecosystem characteristics and processes

63 affecting soil thermal dynamics have been examined in isolationindividually (e.g. vegetation, soil

64 moisture, and soil structure), interactions between among these processes are less well

65 understood. In particular connections between vegetation, soil moisture, and soil thermal

66 properties affecting permafrost conditions could benefit from additional research. In particular,

67 connections between vegetation, soil moisture, and soil thermal properties affecting permafrost

68 could benefit from additional research.

2 69 Changes in ecosystem distribution type and vegetation characteristics will alter spatial

70 patterns of interactions between climate and permafrost. In addition to shrub expansion, other

71 vegetation responses to changes in climate and rapidly changing disturbance regimes will all

72 affect ecosystem surface energy partitioning in ways that are important for permafrost. Lastly,

73 changes in vegetation and ecosystem distribution will lead to regional and global biophysical and

74 biogeochemical climate feedbacks that may compound or offset local impacts on permafrost

75 soils. Consequently, accurate prediction of the permafrost carbon climate feedback will require

76 detailed understanding of changes in terrestrial ecosystem distribution and function, which

77 depend on and the net effects of multiple feedback processes operating across scales in space and

78 time.

79 1 Introduction

80 Permafrost, or is perennially frozen ground, that underlies approximately 24% of

81 northern hemisphere land masses, primarily in arctic and boreal regions (Brown et al., 1998).

82 Soils in permafrost ecosystems have a seasonally thawed active layer that develops each

83 summer. Soil oOrganic carbon and nutrients stored in the active layer are seasonally subjected to

84 mineralization, uptake by plants and microbes, and lateral hydrological transport, as components

85 of contemporary biogeochemical cycles. Carbon and nutrients locked in perennially frozen

86 ground are considerably less active, sometimes often remaining isolated from global

87 biogeochemical cycles for millions of yearsmillennia (Froese et al., 2008). However, changes in

88 temperature, associated with recent climatic change are warming soils in many high-latitude

89 regions (Romanovsky et al., 2010), introducing permafrost carbon and nutrients to modern

90 biogeochemical cycles (Schuur et al., 2015). Microbial activity may release Ssome carbon and

91 nutrients may be released to the atmosphere by microbial activity in the form of carbon dioxide,

3 92 methane, and nitrous oxide, greenhouse gases that contribute to further warming (e.g. Koven et

93 al., 2011; Abbott & Jones, 2015; Voigt et al., 2017). While the magnitude of this permafrost-

94 climate feedback remains uncertain, it is considered one of the largest terrestrial feedbacks to

95 climate change, potentially enhancing human-induced emissions by 22-40% by the end of the

96 century (Schuur et al., 2013; 2015; Comyn-Platt et al., 2018)(Schuur et al., 2013; 2015).

97 A major source of uncertainty in estimating the timing and magnitude of the permafrost

98 climate feedback is the complexity of the soil thermal response of permafrost ecosystems to

99 atmospheric warming. Permafrost soil temperature and its response to climatic change are highly

100 variable across space and time (Jorgenson et al., 2010), owing to multiple biophysical

101 interactions that modulate soil thermal regimes across arctic and boreal regions (Romanovsky et

102 al., 2010). Moving northward,In general, permafrost temperature and active layer thickness

103 generally decrease,s andwhile permafrost thickness and spatial extent increase along a northward

104 climatic gradient. In more northern locations, the areal distribution of permafrost may be

105 continuous (> 90% areal extent), whereas at lower latitudes discontinuous, sporadic, and isolated

106 permafrost (> 50-90%, 10-50%, and < 10% areal extent, respectively) (Brown et al., 1998) have

107 large areas that are not perennially frozen. This general latitudinal gradient is interrupted by

108 considerable local variability in active layer and permafrost thickness and temperature due to

109 differences in local climate, vegetation, soil properties, hydrology, topography, and snow

110 characteristics. These factors can exert increase or decrease the responsiveness of permafrost soil

111 temperatures to climate positive and negative effects on permafrost thermal state, mediating a

112 high degree of spatial and temporal variability in the relationship between air and permafrost soil

113 temperatures (Shur & Jorgenson, 2007; Jorgenson et al., 2010). Understanding how ecosystem

114 characteristics influence local and regional permafrost temperature is critical to interpreting

4 115 variability in rates of recent permafrost temperature increases (Romanovsky et al., 2010), and to

116 predicting the magnitude and timing of the permafrost climate feedback. However, links between

117 permafrost and climate could fundamentally change as arctic and boreal vegetation (e.g. Pearson

118 et al., 2013) and disturbance regimes (e.g. Kasischke & Turetsky, 2006) shift in responserespond

119 to climate change.

120 HereIn this paper, we review how ecosystem structural and functional properties

121 influence permafrost soil thermal dynamics in arctic and boreal regions. We focus on how

122 ecosystem responses to a changing climate alter the thermal balance of permafrost soils (energy

123 moving into and out of permafrost soil) and how these thermal dynamics translate into seasonal

124 and interannual temperature shifts. Our objectives are to 1) identify and review the key

125 mechanisms by which terrestrial ecosystem structure and function influence permafrost soil

126 thermal dynamics; 2) characterize changes in these ecosystem properties associated with altered

127 climate and disturbance regimes; 3) identify and characterize potential feedbacks and

128 uncertainties arising from multiple opposing processes operating across spatial and temporal

129 scales; and 4) identify key challenges and research questions that need to be addressed to better

130 that could improve understanding of constrain how continued climate-mediated ecosystem

131 changes will affect soil thermal dynamics in the permafrost zone.

132

133 2 Ecosystem influences controls on permafrost soil thermal dynamics

134 Permafrost soil thermal regimes can be characterized by four seasonal phases that occur over an

135 annual time scaleannually. In spring, soils thaw onset occurs as day length increases energy

136 inputs and air temperatures, and snow melts. Thaw onset occurs fairly rapidly, typically over a

137 period of several days to weeks. During the summer, thaw period soils accumulate energy

5 138 resulting in warming and deepening of the active layer and warming of both frozen and unfrozen

139 material. In autumn, soil freeze-back occurs as day length and air temperatures decrease. The

140 length of the freeze-back period varies widely, from days to several months, and is heavily

141 dependent on soil moisture content. Finally, the winter freezing period is typically characterized

142 by energy losses to the atmosphere and declining soil temperatures until day length increases

143 available energy in the spring and the annual cycle begins again. The permafrost soil thermal

144 regime is complex because it varies with depth, and the four phases are connected. Key metrics

145 used to characterize the soil thermal regime include the length of the freeze-back and summer

146 thaw periods, mean annual temperature, the annual amplitude of mean temperature, and the ratio

147 of air to soil freezing/thawing degree days (i.e., n-factors), among others. (e.g. Romanovsky &

148 Osterkamp, 1995; Cable et al., 2016).

149 Soil thermal dynamics in the permafrost zone are governed by ground-atmosphere energy

150 exchange and internal energy transfers associated with phase changes of water and temperature

151 gradients within the soil. The simplified thermal balance at the ground surface is the difference

152 between net radiation (RN) absorbed by a vegetation-free, snow-free, and ice-free land surface,

153 and energy loss via turbulent sensible (H), latent (LE), and ground (G) heat fluxes. RN is the

154 difference between incoming and outgoing longwave (LW) and shortwave (SW) radiation where

155 net LW is a function of atmospheric and surface temperatures, and net SW is a function of

156 incoming solar radiation and surface albedo. In terrestrial ecosystems G is therefore modulated

157 by vegetation function and structure, snow cover, topography, and hydrology (Smith, 1975; Betts

158 & Ball, 1997; Eaton et al., 2001; Zhang, 2005; Stiegler et al., 2016a; Helbig et al., 2016b2016a).

159 Vegetation exerts strong controls on albedo, surface conductance, and surface temperature (Betts

160 & Ball, 1997; Betts et al., 1999; Helbig et al., 2016b2016a), and consequently partitioning of the

6 161 surface energy balance into its component fluxes (Eugster et al., 2000). These energy balance

162 controls vary diurnally, seasonally, and spatially across arctic and boreal ecosystems (e.g.

163 Beringer et al., 2005), and are sensitive to natural and anthropogenic disturbances (Helbig et al.,

164 2016a2016b).

165 Unlike lower-latitude ecosystems where G constitutes relatively a relatively small

166 fraction of the surface energy balance, G in permafrost regions is comparable in magnitude to

167 Though usually small compared to gross soil-atmospheric heat fluxes (H and LE) due to

168 relatively large temperature gradients between the ground surface and permafrost table (Eugster

169 et al., 2000; Langer et al., 2011a; 2011b)., G is critically important, because it is the transfer of

170 heat between the ground surface and the active layer and permafrost. G occurs primarily by

171 thermal conduction, and is a function of the temperature gradient between the ground surface and

172 the permafrost table (Kane et al., 2001; but see Fan et al., 2011), and the thermal conductivity

173 (KT) of the soil. Thus, variability in thermal dynamics of active layer and permafrost soils are

174 most generally controlled by factors influencing: 1) the temperature gradient between the ground

175 surface and permafrost at a given depth, and 2) the KT of active layer and permafrost soil

176 substrates (Figure 1). Ground surface temperature (TSG)The amount of energy available for G is

177 governed by energy dynamics of the atmosphere and overlying plant canopies, and ground cover

178 influences on albedo, H, and LE (Figure 1). Ground surface temperature TSG is different from the

179 land surface temperature (TSL), a measure typically used to assess ecosystem-climate interactions

180 (e.g. Urban et al., 2013), because TSL includes tall-statured overlying vegetation canopies,

181 whereas TSG includes only ground-cover vegetation (e.g., mosses and lichens), bare soil, or plant

182 litter that functionally represents the ground surface. Once energy has beenis absorbed at the

183 ground surface and TSG is elevated, soil KT and the surface-permafrost temperature gradient will

7 184 dictate how much of this energy is transferred downward into the soil. Here we focus on TSG and

185 KT because they are more dynamic than permafrost temperature and will mediate permafrost

186 responses to climate and associated carbon cycle consequences, particularly in the coming

187 decades to centuries. It is also important to note that G varies on diurnal, seasonal, and annual

188 timescales. We focus on factors that affect G on seasonal and annual timescales because they are

189 indicative of permafrost warming and thawing, and are thus most relevant for understanding

190 changes to the thermal regime that will impact greenhouse gas fluxes from the soil in the coming

191 decades and centuries. In the following subsections we review the ecological factors that affect

192 individual phases of the soil thermal regime and then consider interactions across the annual

193 cycle.

194 195 196 197 Figure 1. Key ecosystem controls on surface energy partitioning in relation to permafrost soil

198 thermal dynamics (energy fluxes are indicated by orange arrows). Net radiation (RN) is balanced

199 by sensible (SH), latent (LE), and ground (G) heat fluxes (energy fluxes are indicated by orange

200 arrows). Ground surface temperature (TSG) and soil thermal conductivity (KT) exert strong

201 controls on G and are strongly influenced by a variety of ecosystem controls (indicated in dark

8 202 gray boxes; red and blue text denote soil cooling and warming effects, respectively). Controls on

203 air (TA) and permafrost (TPf) temperatures are driven largely by climate, and we assume that

204 ecosystem impacts on these variables are negligible at short timescales (e.g., seasonal to

205 annualyear) and small spatial scales (e.g., m2 to km2) relative to factors highlighted in dark

206 boxes.

207

208 2.1 Vegetation canopies during the growing seasoncanopy effects on G

209 Vegetation canopies attenuate incoming solar radiation ( Juszak et al.,“Arctic shrub

210 effects on NDVI, summer albedo and soil shading,” 2014; Juszak et al., 2016), thereby reducing

211 radiation at the ground surface and subsequently TSG. Canopy removal and addition experiments

212 illustrate that shrub canopies insulate tundra soils in summer, maintaining soil temperatures

213 upwards of 2 °C cooler than adjacent tall shrub-free areas (Bewley et al., 2007; Blok et al., 2010;

214 Myers-Smith & Hik, 2013; Nauta et al., 2014). Canopy shading has also been linked to

215 decreaseds soil temperatures in both evergreen (Jean & Payette, 2014a; 2014b; Roy-Léveillée et

216 al., 2014; Fisher et al., 2016) and deciduous (Iwahana et al., 2005; Fedorov et al., 2016)

217 needleleaf boreal forests. Canopy removal experiments have resulted in substantial soil warming,

218 permafrost thaw and subsidence in ice-rich tundra (Blok et al., 2010; Myers-Smith & Hik, 2013;

219 Nauta et al., 2014) and deciduous needleleaf forests (Iwahana et al., 2005; Fedorov et al., 2016).

220 In the latter case, ecosystem recovery and winter processes lead to permafrost stabilization in the

221 decades after clearing (Fedorov et al., 2016). However manipulation experiments may increase

222 soil moisture and thus KT (described below) via reductions in transpiration that may not occur

223 when vegetation change occurs naturally. Increases vegetation stature will tend to decrease TSG

224 and resulting in local soil cooling during the summer months when plant canopies are present.

9 225 Whereas increases in tree and shrub cover reduce solar radiation at the ground surface,

226 the increased canopy stature and complexity generally reduces canopy albedo leading to an

227 overall increase of the canopy RN (Beringer et al., 2005; Chapin et al., 2005; Sturm et al., 2005;

228 Loranty et al., 2011). However, albedo may increase when shrubs replace bare ground or wet

229 tundra (Blok et al., 2011b; Gamon et al., 2012) or depending on changes in community

230 composition or structure (Williamson et al., 2016). During the growing season these albedo

231 differences are relatively small (Juszak et al., 2016). Increased surface roughness, with shrub or

232 tree expansion also enhances heat transfer to the atmosphere, however, and associated changes in

233 RN and H have not yet been linked to soil thermal dynamics at the ecosystem scale (Beringer et

234 al., 2005; Helbig et al., 2016b; Göckede et al., 2017). Vegetation canopies may enhance LW

235 radiation inputs at the ground surface by re-radiating absorbed SW radiation, however most

236 research has focused on LW enhancement effects on snowmelt (Webster et al., 2016), and so the

237 growing season effects of LW enhancement on G in permafrost ecosystems remain largely

238 unstudied. Moreover, observations Observations of lower TSL for boreal forest canopies relative

239 to adjacent non-forested lands due to higher LE flux (Li et al., 2015; Helbig et al., 2016b)

240 highlight the importance of canopy controls on transpiration when considering how vegetation

241 change affects land surface energy partitioning and atmospheric temperatures. Within vegetation

242 types growing season with higher LE reduce the amount of energy available for H and G (Boike

243 et al., 2008), however this is also related to variability in moisture inputs and can alter soil

244 moisture dynamics, both of which also affect G, as discussed in following sections. In summary,

245 during the growing season there is no clear evidence for altered ecosystem scale G associated

246 with local evaporative cooling (Li et al., 2015) or increased sensible heating as a function of

10 247 canopy albedo (Beringer et al., 2005), likely because these effects are overwhelmed by canopy

248 light attenuation.

249

250 2.2 Vegetation canopies during the non-growing season

251 Snow covers much of the arctic and boreal regions for long periods each year and is a

252 critical driver of ground temperature (Goodrich, 1982; Stieglitz, 2003). Deep and/or low-

253 density snow has low KT and therefore reduces heat flux from the ground to the atmosphere

254 during the non-growing season when air temperatures are typically colder than soil temperatures.

255 Snow depth is initially controlled by the timing and intensity of snowfall, but wind can

256 redistribute snow according to local topography, vegetation structure, landscape position and

257 wind direction, leading to high heterogeneity in snow cover and depth (Walker et al., 2001;

258 Kershaw & McCulloch, 2007). Snow physical and insulative properties can also vary on the

259 scale of broad ecoregions as a result of differences in air temperature, wind, precipitation, and

260 vegetation cover (Sturm et al., 1995). For example, high thermal conductivity and density of

261 snow in tundra relative to boreal ecosystems has been linked to differences in soil temperatures

262 (Gouttevin et al., 2012; Mamet & Kershaw, 2013). Snow cover in the shoulder seasons (freeze-

263 up back and thaw periods) can cool soils as a result of albedo effects, but generally ground

264 insulation from snow cover during the extended winter period dominates the snow effects on G.

265 For example, across the Alaskan arctic, ground surface temperatures are estimated to be 4 °C to 9

266 °C warmer as a result of higher snow cover (Zhang, 2005).

267 In tundra, shrub canopies trap blowing snow, leading to localized deepening of snow

268 cover and higher winter soil temperatures (Sturm et al., 2001; Liston et al., 2002; Sturm et al.,

269 2005; Marsh et al., 2010; Myers-Smith & Hik, 2013; Domine et al., 2015). However, shrub

11 270 canopies can bend in winter under the snowpack leading to potentially different amounts of snow

271 trapping in years with heavy wet snow versus dry snow in early winter (Marsh et al., 2010;

272 Ménard et al., 2014). But eEven buried vegetation can lead to turbulent airflow that transports

273 snow into complex patterns (Filhol & Sturm, 2015), which resulting createsin spatially variable

274 ground temperatures within a given year. In some cases vegetation-snow interactions can also

275 have a negative effect on winter ground temperature, leading to soil cooling. In northeast Siberia,

276 large graminoid tussocks exposed above the snowpack in early winter create gaps in the

277 insulating snow layer, which leads to lower ground temperatures, earlier active layer freezing

278 and cooling of surface permafrost (Kholodov et al., 2012).

279 In the boreal forest, the presence of trees strongly reduces the wind regime and snow

280 redistribution typical of tundra (Baldocchi et al., 2000). While there is less wind-distribution in

281 boreal forests than in the more open tundra, tree composition and density impact affect snow

282 distribution and depth through interception of snow by the canopy branches and subsequent

283 evaporation and sublimation. This results in lower snow inputs in dense forests and areas of

284 shallow snow underneath individual trees (Rasmus et al., 2011). This winter effect of tree

285 density on snow cover may, in part, explain the negative relationship found between larch stand

286 density and ground thaw (Webb et al., 2017) and is consistent with the effects of winter warming

287 experiments on summertime active layer dynamics (e.g. Natali et al., 2011). However at treeline

288 or areas with patchy tree cover, forests can trap blowing snow, leading to decreased heat

289 losselevated soil temperatures from soil in winter (Roy-Léveillée et al., 2014)

290 Tall-statured vegetation canopies that protrude above the snowpack decrease land surface

291 albedo. While the accompanying increases in RN will lead to sensible heating of the atmosphere

292 at regional to local scales (Chapin et al., 2005), they do not have a direct influence first order

12 293 effect on TSG or KT. In the spring thaw period when snow covers the landscape and solar

294 radiation is high, this increase in RN is largest (Liston et al., 2002; Pomeroy et al., 2006; Marsh et

295 al., 2010) and may accelerate snow melt (Sturm et al., 2005; Loranty et al., 2011). This could

296 lead to a longer snow-free season and greater G during the growing seasonssummer thaw period,

297 however, this snow-reducing effect can be offset by the snow-trapping effects of vegetation

298 (Sturm et al., 2005). Changes in the length of the snow-free season because of altered canopy

299 albedo could lead to changes in G; however, such an effect has not been observed. While canopy

300 albedo does not directly influence G at the ecosystem scale, regional climate feedbacks

301 associated with albedo changes (described below) may influence permafrost thermal dynamics

302 (Lawrence & Swenson, 2011; Bonfils et al., 2012).

303 Across the annual cycle, the net effect of vegetation canopies on soil thermal regimes

304 remains unclear. Relatively few studies have simultaneously examined the role of summer

305 energy partitioning and winter snow trapping on G or soil temperatures. It is also important to

306 consider the relative contributions of seasonal variation in ecosystem influences on permafrost

307 thermal dynamics, and the potential for temporal autocorrelation at annual timescales. Myers-

308 Smith and Hik (2013) found that winter warming associated with snow-trapping by shrub

309 canopies elevated soil temperatures by 4-5 °C whereas canopy shading led to 2 °C cooling in

310 summer. Similarly, relative to non-forested palsas, forested palsas in eastern Canada exhibited

311 winter soil warming associated with snow trapping but slower rates of permafrost thaw due to

312 summer cooling associated with thicker organic layers and canopy shading (Jean & Payette,

313 2014a; 2014b). Additionally, these studies observed delayed freeze-up and later spring thaw

314 associated with late fall precipitation that resulted in complex relationships between annual air

315 and soil temperatures and active layer depths (Jean & Payette, 2014b). Canopy snow trapping

13 316 influences on winter soil temperature or G is likely affected by shrub or forest patch size,

317 however this has not been explicitly examined. Conversely, the influence of canopy shading and

318 LW enhancement on summer soil temperature should increase with vegetation stature and

319 density, but vary little with patch size. At the ecosystem scale canopy influences on albedo have

320 not been shown to impact the ground thermal regime. Thus it is likely that the magnitude of

321 vegetation canopy influences on the annual permafrost soil thermal regime will be controlled

322 jointly by vegetation stature, density, and patch size influences on snow-redistribution. The

323 studies mentioned above also highlight the importance covariation in overstory and understory

324 vegetation and canopy influences on soil moisture, which will be addressed in the following

325 sections. The magnitude of these effects likely varies spatially with patch size and climatic

326 controls, making it difficult to distinguish the relative importance of summer versus winter

327 processes, as well as potential links across successive growing seasons.

328

329 2.3 2 Groundcover impacts on ground surface temperature

330 Ground cover in permafrost ecosystems may include bare soil, plant litter, lichens, and/or

331 mosses. Unlike vascular plant canopies, moss and lichen are in close thermal contact with the

332 underlying soil layers so heat can be transferred from the vegetation into the soil (and vice versa)

333 via conduction (Yi et al., 2009; e.g. O'Donnell et al., 2009a). During the growing season,

334 dDifferences in albedo and LE are the primary causes of variability in TSG among ground cover

335 types. During winter ground cover is masked by snow, and KT is the dominant factor affecting G

336 (described below). Under moist snow-free conditions, non-vascular evaporation rates are

337 generally high, leading to surface cooling (Heijmans et al., 2004a; 2004b). Under dry conditions

338 taxonomic level differences in physiological responses to drought (Heijmans et al., 2004b), can

14 339 lead to large differences in TSG (Stoy et al., 2012). Increased LE from bare soil after

340 experimental- (Blok et al., 2011a) and disturbance-induced (Rocha & Shaver, 2011) moss

341 removal illustrates the importance of non-vascular plant physiology, and highlights the relatively

342 high potential for evaporative cooling from bare soil surfaces. Low hydraulic conductivity in

343 mosses relative to organic and mineral soils may result in suppression of LE once moisture held

344 in surface vegetation is depleted, whereas higher hydraulic conductivity in underlying soil layers

345 may allow for evaporation of deeper soil moisture and increased LE observed with moss removal

346 (Rocha & Shaver, 2011; Blok et al., 2011a). Albedo differences between common moss and

347 lichen species may also contribute to large differences in TSG; in ways that either amplify or

348 ameliorate decrease the effects of physiological differences in evaporative cooling (Stoy et al.,

349 2012; Higgins & Garon-Labrecque, 2018; Loranty et al., 2018)(Stoy et al., 2012; Loranty et al.,

350 2018). Variability in ground cover can correspond to large differences in TSG that depend on the

351 joint effects of albedo and LE, and are strongly dependent on available moisture. However the

352 extent to which an increase in TSG leads to an increase in G depends upon KT of the groundcover

353 and soil as well their soil moisture/ice content layers.

354

355 2.4 3 Impacts of ground cover and soil properties on thermal conductivity

356 Soil KT , which often includes the moss layer where present, affects the rate of heat

357 transfer through the soil profile across a temperature gradient between the ground surface and the

358 soil at a given depth. KT varies throughout the soil profile with soil moisture and composition.

359 Under dry conditions, mosses have among thevery lowest KT , followed by organic and then

360 mineral soils (Hinzman et al., 1991; O'Donnell et al., 2009a). Moss and organic soil layers have

361 very low KT owing to high porosity, and KT typically increases with soil bulk density (Hinzman

15 362 et al., 1991; O'Donnell et al., 2009a). Mineral soils typically have higher KT than organic soils

363 (Kane et al., 1989; Hinzman et al., 1991; Romanovsky & Osterkamp, 2000), and fine textured

364 clay mineral soils have lower KT than silt or sand (Johansen, 1977). In general, ecosystems with

365 thick moss and organic soil (e.g., peat) layers with low bulk density tend to have low G and

366 shallow active layers, all else held equal (Woo et al., 2007; Fisher et al., 2016).

367 Soil and moss mMoisture content influences their thermal dynamics of soil and moss in a

368 variety of important ways. Linear increases in KT with moisture content (O'Donnell et al., 2009a;

369 Soudzilovskaia et al., 2013) have strong impacts on G, soil temperatures, and active layer

370 dynamics. Under saturated conditions, KT values of mineral soils remain higher than in organic

371 soils and mosses (Hinzman et al., 1991; Romanovsky & Osterkamp, 2000; O'Donnell et al.,

372 2009a), so the general pattern of increasing KT with depth/bulk density is maintained. Local- and

373 ecosystem-scale observations of warmer soil temperatures and deeper thaw depths in areas of

374 perennially elevated soil moisture (Hinkel et al., 2001; Hinkel & Nelson, 2003; e.g. Shiklomanov

375 et al., 2010; Curasi et al., 2016) indicate increases in KT outweigh the concurrent increase in

376 specific heat capacity associated with increasing moisture content. Similarly, interannual

377 variability in soil moisture and active layer thickness are positively related across a range of

378 spatial scales (Iijima et al., 2010; Park et al., 2013). Across soil types, KT increases in winter

379 when soils freeze (Romanovsky & Osterkamp, 1997), and also with soil ice content meaning that

380 increased soil moisture will increase summer and winter KT (Langer et al., 2011b).

381 Liquid water and water vapor can also warm soils through non-conductive heat transfer

382 (Hinkel & Outcalt, 1994; i.e. water movement; Kane et al., 2001). Here, the timing and source of

383 water is important. For example, infiltration of snowmelt in spring does not deliver substantial

384 heat to the soil because the water temperature is very close to freezing (Hinkel et al., 2001) and

16 385 the near-surface soil horizons are mostly frozen. Alternatively, condensation of water vapor in

386 frozen soils can lead to fairly rapid temperature increases during spring melt (Hinkel & Outcalt,

387 1994). Heat delivery from groundwater flow has been implicated as a cause for permafrost

388 degradation in areas of discontinuous permafrost in interior Alaska (Jorgenson et al., 2010). The

389 hydraulic properties of soil horizons are especially important in this regard. Unsaturated peat and

390 organic-soil horizons with large interconnected pore spaces generally promote non-conductive

391 transport of heat in soils unless the substrate is dry enough that it absorbs water.

392 The relative importance of non- conductive heat transfer on permafrost thermal

393 dynamics is difficult to determine. Observations of elevated soil temperature, active layer

394 thickness, and thermal erosion in areas with poorly drained or inundated soils (Woo, 1990; e.g.

395 Jorgenson et al., 2010; Curasi et al., 2016) suggest the effects of soil moisture on KT may have

396 stronger influences than convective processes on soil thermal dynamics. However, several recent

397 studies indicate that heat advected in groundwater may promote permafrost thaw (de Grandpré et

398 al., 2012; Sjöberg et al., 2016). This process is likely most important in fens, water tracks, and

399 areas of discontinuous permafrost, and less important in areas of continuous permafrost with thin

400 organic layers because mineral soils generally have low hydraulic conductivity. Soil moisture

401 distribution within the soil profile is important as well; dry surface organic layers with low KT

402 may buffer against warmer air temperatures even though deeper soils may have high KT

403 associated with moisture and soil composition (Rocha & Shaver, 2011; Göckede et al., 2017).

404 Observations of co-varying heterogeneity in soil structure, temperature, and moisture also

405 illustrate the importance of spatio-temporal variability in soil moisture and KT for understanding

406 permafrost soil thermal dynamics (Boike et al., 1998).

17 407 In wet soils the large latent heat content of soil moisture can delay freezing of the active

408 layer (i.e., extend the freeze-up back duration; Romanovsky & Osterkamp 2000). The period

409 during which soil active layer temperatures remain constant near 0 °C as latent heat is released

410 form soil moisture is commonly referred to as the ‘zero-curtain’ (Outcalt et al., 1990). Longer

411 zero-curtain periods promote warmer winter active layer and permafrost temperatures (Outcalt et

412 al., 1990; Morse et al., 2015). Soil thaw during spring tends to occur more rapidly than freeze-up

413 back during autumn, despite the high latent heat required to thaw ground ice, likely due to

414 increases in KT associated with snowmelt infiltration and/or latent heat released by condensation

415 of water vapor (Hinkel & Outcalt, 1994). Excess ground ice deeper in the active layer or

416 permafrost requires larger amounts of latent heat energy to melt, and so typically buffer

417 permafrost soils against thaw (Halsey et al., 1995). However, when this type of ground ice does

418 melt, it can lead to an array of physical and ecological changes via thermokarst development

419 (Mamet et al., 2017), which further alter the soil thermal regime and can promote further

420 warming (Osterkamp et al., 2009; Kokelj & Jorgenson, 2013).

421 Across the seasonal cycle soil and ground cover thermal properties interact to affect the

422 thermal regime in complex ways that vary across ecosystem types. For example, a comparison

423 of wet and dry microsites within tundra ecosystems found warmer surface soils in dry microsites

424 due to lower heat capacity, however deeper soil layers in the dry microsite remained cooler

425 because of lower thermal conductivity of dry surface soils (Göckede et al., 2017). In wet

426 microsites greater soil moisture lengthened the fall freeze-back period meaning that soils were

427 warmer than dry microsites, however once soils froze, temperatures in the wet microsites

428 dropped rapidly and became cooler than dry microsites because of higher KT (Göckede et al.,

18 429 2017). This example illustrates how covariation in vegetation and soil properties within a single

430 ecosystems affect the soil thermal regimes in complex ways across the annual cycle.

431

432 2.5 4 Interacting ecosystem influences on ground heat fluxthe soil thermal regime

433 The mechanisms described in the previous sections are relatively well understood

434 individually and at seasonal timescales., Bbut when considered in concert, the relative

435 importancenet effect of specific processes on annual ground temperatures and thermal regimes is

436 often unclear. This is particularly true when ecological processes co-vary, or have opposing

437 effects on permafrost soil thermal dynamics. For example, is the effect of canopy shading

438 mitigated by LW enhancement, or amplified by reductions in soil KT resulting from plant

439 utilization of soil moisture? For example, cUsing successional gradients to answer such

440 questions like this is complicated by concurrent accumulation of organic soil, and canopy leaf

441 area, and soil moisture make it difficult to quantify the relative importance of each when

442 considering differences in active layer properties across successional gradients (Jorgenson et al.,

443 2010)[1]., Likewise,whilewhereas manipulativeon experiments nearly always involve side effects

444 and artefacts, for example, are often incapable of altering a single variable without affecting

445 another. Ccanopy manipulations most likely affect soil moisture, in unrealistic ways meaning

446 thatchanging soil thermal properties are manipulated along withand surface energy inputs

447 simultaneously (Fedorov et al., 2016). . None-the-lessOn the other hand, carefully designed

448 manipulations orand gradient studies do still provide the best optionavenue for studying single

449 and interactive processes, and for parametrizing models. Consequently, the magnitude of

450 permafrost soil temperature responses to ecological change is uncertain.

19 451 Though While there are a number of studies that have examined the role of variation in

452 vegetation canopy cover, soil moisture, and ground/soil thermal properties on the permafrost

453 thermal regime, few have fully isolated the relative contribution of each process to variation in

454 active layer thickness or soil temperatures (Jiang et al., 2015). For example, in addition to

455 increasing radiation at the ground surface, canopy removal experiments (Blok et al., 2010; e.g.

456 Fedorov et al., 2016) may also elevate soil moisture via reductions in plant water use. In a A

457 recent study by Fisher et al. (2016) examining examined the impact of multiple processes factors

458 on active layer thickness in Canadian boreal forest and found overstory leaf area to be most

459 important, followed by moss thickness and understory leaf area. Further, this study revealed that

460 moisture in deeper soil layers modified the impacts of vegetation whereas surface soil moisture

461 did not (Fisher et al., 2016). However this study did not explicitly consider how active vegetation

462 canopy effects on snow-cover, or soil moisture influences on freeze-back and winter soil

463 temperature might contribute to variability in active layer depth. Ecosystem influences on

464 moisture distribution throughout the soil profile, particularly in relation to evapotranspiration, are

465 not well characterized and will likely become increasingly important with continued climate

466 warming (Swann et al., 2010).

467 Further complexity is added when processes are considered across the annual cycle. The

468 extent to which vegetation canopy effects on snow-distribution impact growing season soil

469 moisture, either via direct moisture inputs or affects on growing season length, has not been

470 thoroughly investigated. A study examining interannual variability in snow cover found that in

471 that growing season energy partitioning was similar in a wet-fen after winters with above- and

472 below-average snowfall (Stiegler et al., 2016b). However, in a nearby dry heath below average

473 snowfall resulted in earlier snowmelt and reduced soil moisture during the lengthened growing

20 474 season, which in turn suppressed LE and G (Stiegler et al., 2016b). Future research should focus

475 on disentangling complex series of interactions between vegetation, soil properties, snow

476 redistribution, and soil moisture across annual cycles of the soil thermal regime. Covariation in

477 vegetation and soil characteristics and their influences on soil thermal regimes within ecosystems

478 (Boike et al., 2008) and regions (Cable et al., 2016) may help to interpret empirical relationships

479 between ecological and thermal variables at a range of scales.

480 It is also important to consider the relative contributions of seasonal variation in

481 ecosystem influences on permafrost thermal dynamics, and the potential for temporal

482 autocorrelation at annual timescales. Myers-Smith and Hik (2013) found that winter warming

483 associated with snow-trapping by shrub canopies elevated soil temperatures by 4-5 °C whereas

484 canopy shading led to 2 °C cooling in summer. Similarly, relative to non-forested palsas,

485 forested palsas in eastern Canada exhibited winter soil warming associated with snow trapping

486 but slower rates of permafrost thaw due to summer cooling associated with thicker organic layers

487 and canopy shading (Jean & Payette, 2014a; 2014b). Additionally, these studies observed

488 delayed freeze-up and later spring thaw associated with late fall precipitation that resulted in

489 complex relationships between annual air and soil temperatures and active layer depths (Jean &

490 Payette, 2014b). The magnitude of these effects likely varies spatially with patch size and

491 climatic controls, making it difficult to distinguish the relative importance of summer versus

492 winter processes, as well as potential links across successive growing seasons.

493 Disentangling the relative impacts of multiple ecosystem characteristics on G will

494 become increasingly important as ecological responses to continued climate warming may lead

495 to shifts in ecosystem distribution (Pearson et al., 2013; Abbott et al., 2016), potentially resulting

496 in novel ecosystems with no current eco-climatic analogs (Macias-Fauria et al., 2012). Because

21 497 ecosystems influence permafrost soil thermal dynamics in a variety of ways, such shifts in

498 ecosystem distribution are likely towill fundamentally alter rates of permafrost thaw with

499 projected future warming. This will occur directly via altered ecosystem surface energy

500 dynamics that affect G and indirectly through changes to the surface energy balance that feed

501 back to climate (e.g., Figure 1). The following sections describe ongoing and anticipated

502 ecosystem responses to climate and associated changes to soil thermal regimes G via impacts on

503 GTSG or KS, and then the associated regional to global scale atmospheric feedbacks.

504

505 3 Implications of Eenvironmentalcosystem change with implications for permafrost

506 thermal dynamics

507 Vegetation productivity and community composition are changing in response to longer

508 and warmer growing seasons associated with amplified climate warming across the Arctic.

509 Relationships between air temperature and soil thermal dynamics regimes vary with ecosystem

510 properties and will therefore evolve as ecosystems respond to climate change. Ecosystem

511 structural and functional characteristics that influence soil thermal dynamics may be altered

512 directly by ecosystem responses to climate change, or indirectly by climatic alteration of

513 disturbance processes that in turn modify ecosystems (e.g. O'Donnell et al., 2011a). In this

514 section, we outline key ecosystem changes arising from direct and indirect climate responses

515 (summarized in Figure 2), and describe how these changes are likely to affect permafrost soil

516 thermal dynamics regimes via impacts on processes described above.

22 517 518

519 Figure 2. Summary of key drivers of ecosystem change, and the associated ecosystem responses

520 observed (solid lines) or hypothesized (dashed lines) in permafrost ecosystems. Arrows (èè)

521 indicate transition from the current (left) to a new (right) ecosystem type, and the symbol delta

522 (Δ) indicates a change in the associated ecosystem property. Ecosystem types are defined as

523 follows: DBF = Deciduous Broadleaf Forest; DNF = Deciduous Needleleaf Forest; ENF =

524 Evergreen Needleleaf Forest; GRM = Graminoid Dominated Ecosystem; SHR = Shrub

525 Dominated Ecosystem; WET = Wetland Ecosystem; All = Any Initial Ecosystem type.

526 Ecosystem properties are: LAI = Leaf Area Index, and SOC = Soil Organic Carbon.

527

528 3.1 Vegetation change in response to climate

529 In tundra ecosystems, increases in vegetation productivity inferred from satellite

530 observations (Jia et al., 2003; Beck & Goetz, 2011) have been linked to shrub expansion and

23 531 accelerated annual growth at locations throughout the Arctic (Tape et al., 2006; Forbes et al.,

532 2010; Macias-Fauria et al., 2012; Frost & Epstein, 2014). However, warming experiments

533 indicate that productivity increases may occur without shifts in the dominant vegetation type

534 (Walker et al., 2006; Elmendorf et al., 2012b), and dendroecological observations illustrate that

535 shrub responses to temperature are moderated by moisture and nutrient availability and are

536 highly heterogeneous in space and time (Zamin & Grogan, 2012; Myers-Smith et al., 2015;

537 Ackerman et al., 2017). Despite the high degree of heterogeneity in tundra vegetation responses

538 to warming (Elmendorf et al., 2012a), there are several consistent changes that include increased

539 vegetation height, increased litter production, decreased moss cover (Elmendorf et al., 2012b),

540 and increased graminoid cover in lowland permafrost features (Malmer et al., 2005; Johansson et

541 al., 2006; Malhotra & Roulet, 2015). However, reductions in greenness in some regions (referred

542 to as ‘browning’) driven by, for example, reduced summer warmth index (Bhatt et al., 2013) or

543 acute ‘browning events’ from disturbances such as winter frost droughts (Bjerke et al., 2014;

544 Phoenix & Bjerke, 2016) add complexity to predicting vegetation change and hence subsequent

545 impacts on permafrost.

546 Enhanced tundra vegetation productivity may reduce summer soil temperatures via

547 ground shading and increase winter soil temperatures via effects on snow depth and density. The

548 effect of declining moss cover will depend on the balance between reduced insulation (i.e. KT)

549 and latent cooling associated with increased soil evaporation. Vegetation change may also alter

550 organic soil accumulation rates via altered litter quality and quantity (Cornelissen et al., 2007).

551 This overall effect on soil KT will depend on the net effects of changing litter inputs, lability, and

552 decomposition rates with warming (Hobbie, 1996; Hobbie & Gough, 2004; Cornelissen et al.,

553 2007; Christiansen et al., 2018; Lynch et al., 2018).

24 554 Belowground vegetation dynamics are more difficult to study, but recent

555 observations indicate that the below ground growing season length (period of unfrozen

556 temperatures allowing for plant growth) can be greater than that aboveground (Blume-Werry et

557 al., 2015; Radville et al., 2016). These differences likely vary with depth due to effects related to

558 the progression of soil freezing and thawing (Rydén & Kostov, 1980). Thus, rooting depth and

559 lateral root distributions will influence the below-ground phenology differentially for deep-

560 rooted (e.g., sedge) versus shallow-rooted (e.g., shrub) species (Bardgett et al., 2014; Iversen et

561 al., 2015), which may alter soil moisture via plant water uptake under future warming related

562 vegetation change increased active layer depth. The changing above- and below-ground growth

563 phenology of tundra plants (Blume-Werry et al., 2015; Iversen et al., 2015; Radville et al., 2016)

564 could also favor the proliferation of certain functional groups or species creating potential

565 feedbacks to vegetation change. In addition to belowground phenology, total root production

566 could also increase in response to warming (e.g. Xue et al., 2015). However, increased nutrient

567 availability from warming could decrease root production relative to aboveground production

568 (Keuper et al., 2012; Poorter et al., 2012). The net effect of climate change induced belowground

569 changes on soil thermodynamics is unclear.Improved understanding of interactions between root

570 dynamics and soil moisture may help to understand thermal changes in permafrost soils during

571 the summer thaw and fall freeze-back periods.

572 Determining the net effect of tundra vegetation productivity changes on soil thermal

573 regimes requires improved understanding of the magnitude and spatial extent of changes in

574 vegetation stature and rooting dynamics. Enhanced tundra vegetation productivity may reduce

575 summer soil temperatures via ground shading and increase winter soil temperatures via effects

576 on snow depth and density. The effect of declining moss cover will depend on the balance

25 577 between reduced insulation (i.e., KT) and latent cooling associated with increased soil

578 evaporation. Vegetation change may also alter organic soil accumulation rates via altered litter

579 quality and quantity (Cornelissen et al., 2007). This overall effect on soil KT will depend on the

580 net effects of changing litter inputs, lability, and decomposition rates with warming (Hobbie,

581 1996; Hobbie & Gough, 2004; Cornelissen et al., 2007; Christiansen et al., 2018; Lynch et al.,

582 2018). Overall the effects of vegetation change on snow redistribution and soil moisture will

583 likely have the strongest influence on soil thermal regimes.

584

585

586 Boreal forest responses to climate in recent decades were generally more heterogeneous

587 than those observed in tundra ecosystems due to a variety of interacting factors including species

588 differences in physiology, disturbance regimes, and successional dynamics. Initial satellite

589 observations of boreal forest productivity increases (Myneni et al., 1997) have slowed or even

590 reversed in recent decades (Beck & Goetz, 2011; Guay et al., 2014). Tree ring analyses confirm

591 productivity declines associated with temperature induced drought stress in interior Alaska

592 boreal forests (Barber et al., 2000; Walker & Johnstone, 2014; Juday et al., 2015; Walker et al.,

593 2015), and have been used to corroborate satellite observations (Beck et al., 2011). Similarly,

594 drought-induced mortality has been observed at the southern margins of Canadian boreal forests

595 (Peng et al., 2011) where correspondence between satellite and tree ring records have also been

596 observed (Berner et al., 2011). In Siberia, positive forest responses to air temperatures observed

597 in tree rings and satellite observations near latitudinal tree lines give way to declines in tree

598 growth further south (Lloyd et al., 2010; Berner et al., 2013). These results are in line with

599 ecosystem-scale observations of suppressed transpiration under high vapor pressure deficits and

26 600 low soil moisture conditions (Lopez C et al., 2007; Kropp et al., 2017). More generally, forests

601 growing on continuous permafrost exhibit more widespread productivity increases (Loranty et

602 al., 2016), suggesting that permafrost may buffer against drought stress. However, waterlogged

603 soil resulting from permafrost thaw can also lead to unstable soils and forest mortality (Baltzer et

604 al., 2014; Iijima et al., 2014; Helbig et al., 2016a).

605 The extent to which ongoing boreal forest productivity changes influence permafrost soil

606 thermal dynamics is not entirely clear. If forest canopy cover changes with productivity (e.g.,

607 canopy infilling or increased leaf area), then changes in ground shading and LW dynamics could

608 alter ground thermal regimes. Increases in forest cover have been observed in northern Siberia

609 (Frost & Epstein, 2014); however, it is unclear whether the cause is climate warming or

610 ecosystem recovery after fire. Conversely, productivity declines are more pronounced in high-

611 density forests (Bunn & Goetz, 2006) and, consequently, browning trends associated with

612 mortality in southern boreal forests (Peng et al., 2011) may increase radiation at the ground

613 surface. Additionally, if browning is indicative of drought stress, vegetation may enhance the

614 insulation of organic soils by further depleting of soil moisture via plant water uptake (Fisher et

615 al., 2016). Forest mortality and declines in canopy cover in southern boreal forests as a

616 consequence of permafrost thaw (Helbig et al., 2016a) may feedback positively to permafrost

617 thaw. A clearer understanding of boreal forest structural and ecohydrological changes associated

618 with widespread productivity changes is necessary.Functional changes (e.g., stomatal

619 suppression of transpiration in response to drought) occur more quickly than structural changes,

620 so boreal forest effects on soil moisture will likely be an important driver of changes in soil

621 thermal regimes. In addition there has been relatively little work on how the effects of forest

27 622 distribution on snow cover alters G in winter, and this will also become increasingly important as

623 forests change.

624 3.2 Wildfire disturbance

625 Wildfire is the dominant disturbance in the boreal forest and is increasingly present in

626 arctic tundra. Wildfire influences surface energy dynamics via impacts on vegetation and surface

627 soil properties, likely accelerating permafrost thaw (Burn, 1998; Viereck et al., 2008; O'Donnell

628 et al., 2011a; Jafarov et al., 2013; Brown et al., 2015; Jones et al., 2015).Vegetation combustion

629 and mortality increases radiation at the ground surface. The combustion and charring of moss

630 and organic soil lowers albedo and increases KtKT, leading to warmer soils with deeper active

631 layers in the decades following a fire. (Yoshikawa et al., 2003; Liljedahl et al., 2007; Rocha &

632 Shaver, 2011; French et al., 2016). In boreal forests, loss of canopy cover increases albedo

633 during the snow-covered period (Jin et al., 2002; Lyons et al., 2008; Jin et al., 2012), which may

634 result in local atmospheric cooling (Lee et al., 2011). However, such atmospheric cooling has not

635 been linked to soil climate, and canopy loss may also result in a deeper snowpack, which inhibits

636 ground cooling during winter (Kershaw, 2001). In general, wildfire effects on permafrost soil

637 climate are primarily the result of altered growing season surface energy dynamics.

638 The magnitude of wildfire effects on soil temperature is closely linked to burn severity,

639 as indicated by the degree of organic soil combustion and the post-fire organic horizon thickness

640 (Kasischke & Johnstone, 2005). Post-fire recovery of the organic-soil horizon can allow recovery

641 of soil temperature and active layer thickness to pre-fire conditions (Rocha et al., 2012).

642 However, relatively warm discontinuous zone permafrost is often ecosystem-protected by

643 vegetation and organic horizons (Shur & Jorgenson, 2007), thus loss or reduction of organic soil

644 may result in the irreversible thaw or loss of permafrost (Romanovsky et al., 2010; Jiang et al.,

28 645 2015). Site-based model simulations suggest that fire-driven change in organic-horizon thickness

646 is the most important factor driving post-fire soil temperature and permafrost dynamics (Jiang et

647 al., 2015).

648 Wildfire impacts on permafrost also vary spatially with ecosystems and topography. For

649 instance south-facing forest stands tend to burn more severely than north-facing stands (Kane et

650 al., 2007). Further, poorly drained toe-slopes burn less severely than more moderately drained

651 upslope landscapes. These topographic effects on burn severity can strongly influence the

652 response of soil temperature and permafrost to fire (O'Donnell et al., 2009b). The loss of

653 transpiration due to the combustion of trees may result in wetter soils in recently burned stands

654 compared to unburned stands (O'Donnell et al., 2011a). However, other studies have

655 documented drier soils in burned relative to unburned stands (Jorgenson et al., 2013),

656 particularly at sites underlain by coarse-grained, hydrologically conductive soils. Post-fire

657 thawing of permafrost can increase the hydraulic conductivity of mineral soils due to ice loss,

658 leading to enhanced infiltration of soil water and soil drainage. Post-fire changes in soil moisture

659 and drainage can function as either a positive or negative feedback to permafrost thaw

660 (O'Donnell et al., 2011b). Recent evidence also indicates that mineral soil texture is an important

661 control on post-fire permafrost dynamics (Nossov et al., 2013).

662 While the magnitude of fire effects on G and active layer depth is typically governed by

663 burn severity, the persistence of these changes depends on ecosystem recovery (Jorgenson et al.,

664 2013). Albedo returns to pre-fire levels within several years after fire (Jin et al., 2012) due to

665 fairly rapid recovery of vegetation (Mack et al., 2008). Recovery of moss and re-accumulation of

666 the organic-soil horizon further facilitate recovery of soil temperatures and permafrost, and may

667 occur within several decades (e.g. Loranty et al., 2014b). Finally, recovery of vegetation

29 668 canopies over decades to centuries gradually reduces incident radiation at the ground surface to

669 pre-fire levels. The effects of fire on TSG and permafrost are well understood, and it may be

670 reasonable to expect similar effects in the future that are amplified as fire exposes permafrost

671 soils to increasingly warmer atmospheric temperatures. However, changes in the severity and

672 extent of wildfires can result in new ecosystem dynamics with implications for permafrost that

673 do not confer linearly from current eco-climatic conditions.

674 Recent warming at high latitudes has increased the spatial extent, frequency, and severity

675 of wildfires in North America (Turetsky et al., 2011; Rocha et al., 2012) to levels that are

676 unprecedented in recent millennia (Hu et al., 2010; Kelly et al., 2013). Fire regimes in boreal

677 forests in Eurasia remain poorly characterized (Kukavskaya et al., 2012), though several studies

678 indicate that fire extent and frequency are likely increasing with climate warming (Kharuk et al.,

679 2008; 2013; Ponomarev et al., 2016). Circumpolar wildfire in the boreal forest and arctic tundra

680 are projected to substantially increase by the end of the century due to direct climate forcing and

681 ecosystem responses Circumpolar wildfire in the Boreal Forest and Arctic tundra are projected to

682 substantially increase by the end of the century due to direct climate forcing and ecosystem

683 responses(Abbott et al., 2016). Recovery of soil thermal regimes and permafrost after fire is

684 strongly influenced by ecosystem recovery, and recent studies have established links between

685 burn severity and post-fire succession (Johnstone et al., 2010; Alexander et al., 2018).

686 Consequently, in North America burn severity is likely the dominant factor controlling the

687 effects of wildfire on permafrost soil thermal dynamicsregimes both through direct influences on

688 soil thermal regimes and indirectly through influences on post fire succession.

689 In boreal North America, low-severity fires in upland black spruce forest typically foster

690 self-replacing post-fire vegetation trajectories while high-burn severity fosters a transition to

30 691 deciduous dominated forests. (Johnstone et al., 2010). In addition to changes in canopy effects

692 on ground shading, this transition also leads to reductions in post-fire accumulation of the soil

693 organic layer (Alexander & Mack, 2015). Observations of mean annual soil temperatures that are

694 1-2 °C colder in soils underlying black spruce forests compared to deciduous forests (Jorgenson

695 et al., 2010; Fisher et al., 2016) indicate that burn severity influences on post-fire succession will

696 lead to alternate soil temperature and permafrost recovery pathways as well.

31 697

698

a( b( Unburned( Burned(

Fire(Break(

699 700 701 Figure 3. Impacts of fire on ecosystem structure in Siberian larch forests. A firebreak near the

702 town of Cherskii (a) shows the contrast between burned and unburned areas ~30 10 years post-

703 fire, where apparent larch and shrub recruitment failure has resulted a transition to graminoid

704 dominance (b; detail of burned area). Nearby in a ~70 year old burn scar high-density (c) and

705 low-density (d) forests illustrate the impacts of fire severity on canopy cover, and correspond to

706 large differences in soil thermal regimes and active layers depths of ~40 cm in the high-density

707 stand and ~90 cm in the low-density stand (M. Loranty, unpublished data). Photos M. Loranty.

708

709

32 710 In Siberian larch forests, post-fire recovery is impacted by fire severity and seed dispersal

711 (Figure 3). High burn severity fires promote high rates of seedling recruitment and subsequent

712 forest stand density (Sofronov & Volokitina, 2010; Alexander et al., 2018) when dispersal is not

713 limited. But since larch are not serotinous and seed rain varies from year to year, high burn

714 severity does not guarantee succession to high-density forests. Recovery tends to be slow and

715 highly variable (Berner et al., 2012; Alexander et al., 2012b). Wide ranges of post-fire moss

716 accumulation and forest regrowth have been observed, though consequences for permafrost are

717 unclear (Furayev et al., 2001). Observed declines in permafrost thaw depth with increasing

718 canopy cover (Webb et al., 2017) support the notion of a link between fire severity and

719 permafrost soil thermal dynamics. However, the combined effects of fire and climatic warming

720 and drying could lead to widespread conversion of larch forests to steppe (Tchebakova et al.,

721 2009), whereas declines in fire could result in increased cover of evergreen needleleaf species

722 (Schulze et al., 2012). Thus the impacts of fire on permafrost in Siberia remain uncertainwill

723 depend on the combined effects of climate and fire severity.

724 In tundra ecosystems fire is becoming increasingly common (Rocha et al., 2012). Fire-

725 induced transitions from graminoid- to shrub-dominated ecosystems have been observed in

726 several instances (Landhäusser & Wein, 1993; Racine et al., 2004; Jones et al., 2013), while in

727 others recovery of graminoid-dominated ecosystems has occurred, especially when fire leads to

728 ponding (Vavrek et al., 1999; Barrett et al., 2012; Loranty et al., 2014b). If unusually large

729 tundra fires with high burn severity (e.g. Jones et al., 2009) occur more regularly fire induced

730 transitions from graminoid to shrub tundra may become more common (Jones et al., 2013; Lantz

731 et al., 2013). A shift to shrub dominance could buffer permafrost soils from continued climate

732 warming during summer (e.g Blok et al., 2010; Myers-Smith & Hik, 2013) or promote warmer

33 733 soils in winter (Lantz et al., 2013; Myers-Smith & Hik, 2013) at the ecosystem-scale depending

734 on how topography and the spatial distribution of shrubs impact snow redistribution (Essery &

735 Pomeroy, 2004; Ménard et al., 2014)., In addition, there is evidence that thermal erosion as a

736 consequence of fire may facilitate shrub transitions, especially in areas of ice-rich permafrost

737 (Bret-Harte et al., 2013; Jones et al., 2013), and the associated changes in local hydrology and

738 topography will also impact soil temperature dynamicsthermal regimes.

739 Across arctic and boreal ecosystems increased fire extent and severity will increase

740 summer G leading to warmer soils with deeper active layers that take longer to freeze-back in

741 fall and thus reduce the time for heat loss in winter across larger portions of the permafrost

742 region. Post-fire ecosystem recovery will determine the trajectory of soil thermal regimes in

743 coming decades to centuries. In tundra and Siberian larch forests shifts toward increased canopy

744 cover may help thermal regimes recover more quickly and buffer against continued warming.

745 However the link between fire severity and increased canopy cover is not certain. In North

746 American boreal forests increased deciduous cover after high severity soils may prevent full

747 recovery of the soil thermal regime after severe fires (i.e., warmer soils) and loss of permafrost in

748 areas where discontinuous permafrost is ecosystem protected (Jorgenson et al., 2010).

749 3.3 Permafrost thaw, thermokarst disturbance, and hydrologic change

750 Permafrost thaw can occur in two primary modes, as determined by depending on pre-

751 thaw ground ice content. In terrain underlain by low ground ice content (typically < 20% by

752 volume), the soil profile can thaw from the top down without disturbing the surface in what is

753 termed thaw-stable permafrost degradation (Jorgenson et al., 2001). Alternatively, in ice-rich

754 terrain, when ground ice volume exceeds unfrozen soil pore space (usually > 60%), permafrost

755 thaw causes surface subsidence or collapse, termed thermokarst (Kokelj & Jorgenson, 2013).

34 756 Thermokarst is the predominant disturbance in arctic tundra and is an important disturbance in

757 boreal forests underlain by permafrost (Lara et al., 2016). Recent evidence indicates increasing

758 prevalence of thermokarst features during the last half-century (Jorgenson et al., 2006; 2013;

759 Liljedahl et al., 2016; Mamet et al., 2017), though circum-arctic prevalence and change of

760 thermokarst extent are poorly constrained (Yoshikawa & Hinzman, 2003; Lantz & Kokelj, 2008;

761 Olefeldt et al., 2016). Thermokarst features form over the course of weeks to decades, can

762 involve centimeters to meters of ground surface displacement, and typically lead to dramatic

763 changes in ecosystem vegetation and soil properties (e.g. Osterkamp et al., 2000; Douglas et al.,

764 2016; Wagner et al., 2018)(e.g. Osterkamp et al., 2000; Douglas et al., 2016). Ecological

765 responses to thermokarst formation can act as either positive or negative feedbacks to continued

766 thaw, depending on how thermokarst formation affects vegetation and hydrology, including

767 snow cover (Kokelj & Jorgenson, 2013). Thermokarst could affect 20–50% of the permafrost

768 zone by the end of the century, according to projections of permafrost degradation and the

769 distribution of ground ice (Zhang et al., 2000; Slater & Lawrence, 2013; Abbott & Jones, 2015).

770 Upland thermokarst in the discontinuous permafrost zone already impacts 12% of the overall

771 landscape in some areas and up to 35% of some vegetation classes (Belshe et al., 2013).

772 Following initial thaw, hydrologic conditions play an important role in the subsequent

773 evolution of thermokarst features because the high thermal conductivity of water can increase

774 heat flux to the active layer and permafrost (Nauta et al., 2015). Lowland and upland thermokarst

775 may have contrasting effects on surface hydrology, with lowland thermokarst initially increasing

776 wetness (e.g. O'Donnell et al., 2012), but eventually leading to greater drainage if permafrost is

777 completely degraded (Anthony et al., 2014). Upland thermokarst can either increase or decrease

778 surface wetness, depending on soil conditions and local topography (Abbott et al., 2015; Abbott

35 779 & Jones, 2015; Mu et al., 2017). Redistribution of water to thermokarst pits and gullies can lead

780 to drying in adjacent areas that have not subsided (Osterkamp et al., 2009). In winter, increases

781 in snow accumulation in thermokarst depressions insulates soils (Stieglitz, 2003).

782 Ecological responses to thermokarst formation can act as either positive or negative

783 feedbacks to continued thaw, depending on how thermokarst formation affects vegetation and

784 hydrology, including snow cover (Kokelj & Jorgenson, 2013). Thermokarst impacts vegetation

785 and soils in a variety of ways. Active layer detachments in uplands remove vegetation and

786 organic soil, increasing energy inputs to deeper soil layers. In upland tundra, shifts from

787 graminoid- to shrub-dominated vegetation communities have been observed with thaw, though

788 communities varied locally with microtopography created by thermokarst features themselves

789 (Schuur et al., 2007). In boreal forests, thermokarst and permafrost thaw can cause transitions to

790 wetlands or aquatic ecosystems (Jorgenson & Osterkamp, 2005); whereas, vegetation community

791 shifts are more subtle in uplands (Jorgenson et al., 2013). Permafrost thaw may also lead to a

792 more nutrient- rich environment (Keuper et al., 2012; Harms et al., 2014), but this depends on

793 local soil properties. The succession of aquatic or terrestrial vegetation can curb thaw through

794 negative feedbacks associated with canopy cover and organic soil accumulation and aggrade

795 permafrost (Briggs et al., 2014). Hydrologic changes associated with thermokarst likely have a

796 stronger influence on the soil thermal regime that associated ecosystem changes, in part because

797 the former occur more rapidly than the latter. Under thaw stable conditions there is the

798 possibility that enhanced vegetation productivity could lead to summer soil cooling, however the

799 effects on soil composition and moisture, and snow distribution will also affect the thermal

800 regime and are as yet unclear.

801

36 802 3.4 Zoogenic disturbance

803 A large portion of the circumpolar Arctic is grazed by reindeer and caribou (both

804 Rangifer tarandus L.), and their grazing and trampling causes important long-term vegetation

805 shifts, namely inhibition of shrub proliferation (Olofsson et al., 2004b; Forbes & Kumpula,

806 2009; Olofsson et al., 2009; Plante et al., 2014; Väisänen et al., 2014). Besides direct

807 consumption of lichen and green biomass, large semi-domestic reindeer herds of northwest

808 Eurasia also exert a variety of impacts on biotic and abiotic components of Arctic and sub-Arctic

809 tundra ecosystems that have implications for permafrost thermal dynamicsregimes. For example,

810 as reindeer reduce vertical structure of vascular and nonvascular vegetation, they tend to

811 decrease albedo (Beest et al., 2016) and reduce thermal conductivity at the ground level

812 (Olofsson, 2006; Fauria et al., 2008), which can lead to warmer soils (Olofsson et al., 2001; van

813 der Wal et al., 2001; Olofsson et al., 2004b). Recent research has revealed that the consequences

814 of climate warming on tundra carbon balance are determined by reindeer grazing history (Zimov

815 et al., 2012; Väisänen et al., 2014). Grazing by small mammals also influences arctic plant

816 communities (Olofsson et al., 2004a). The extent to which ongoing vegetation change across the

817 Arctic is a result historic grazing patterns is unclear. However, it is plausible that social and/or

818 ecoclimatic drivers that change the distribution or behavior of grazing mammals have impacted

819 permafrost ecosystems in ways that affect the soil thermal regime. Historic and future grazing

820 and trampling impacts on vegetation communities and soils will continue to be important for

821 understanding permafrost soil temperature responses to climate.More targeted research is

822 necessary to elucidate links between grazing, ecosystem vegetation and soil characteristics, and

823 soil thermal regimes.

824

37 825 3.5 Anthropogenic disturbance

826 The most extensive direct anthropogenic disturbances within the permafrost zone occur

827 in three regions that have experienced widespread hydrocarbon exploration and extraction

828 activities: the North Slope of Alaska, the Mackenzie River Delta in Canada, and northwest

829 Russia, including the Nenets and Yamal-Nenets Autonomous Okrugs. The types of terrestrial

830 degradation commonly associated with the petroleum industry have historically included rutting

831 from tracked vehicles; seismic survey trails; pipelines, drilling pads and roads and the excavation

832 of the gravel and sand quarries necessary for their construction (Walker et al., 1987; Huntington

833 et al., 2013). A single pass of a vehicle over thawed ground can create ruts with increased KT due

834 to increased bulk density and soil moisture, while altered local hydrology can drain downslope

835 wetlands and, in both cases, lead to vegetation changes that persist for decades (Forbes, 1993;

836 1998). As a result of these combined factors, the increase from scale of impact to scale of

837 response can be several orders of magnitude (Forbes et al., 2001). It has also been demonstrated

838 that even relatively small-scale, low intensity disturbances in winter, like seismic surveys over

839 snow-covered terrain, reduce microtopography, and increase ground temperatures and active

840 layer thaw depths (Crampton, 1977) (Kershaw, 1983).

841 More recently, gravel roads and pads have become common, however this elevated

842 infrastructure causes other unanticipated impacts to the permafrost from accumulated dust, snow

843 drifts, and roadside flooding (Walker & Everett, 1987; 1991; Auerbach et al., 1997; Raynolds et

844 al., 2014). Over time, the warmer environments adjacent to roads have led to strips of earlier

845 phenology and shrub vegetation and even trees along both sides of most roads and buried

846 pipeline berms in the Low Arctic (Gill et al., 2014). Aeolian sand and dust associated with gravel

847 roads or quarries can affect tundra vegetation and soils up to 1 km from the point source (Forbes,

38 848 1995; Myers-Smith et al., 2006). At present, there is a concern that climate warming and

849 infrastructure are combining to enhance melting of the top surface of ice-wedges, leading to

850 more extensive ice-wedge thermokarst (Raynolds et al., 2014; Liljedahl et al., 2016) and

851 cryogenic landslides (Leibman et al., 2014) in areas of intensive development. The proportion

852 of permafrost ecosystems affected by anthropogenic disturbance is not well quantified, but it will

853 continue to increase in coming decades.

854

855 4 Local versus regional ecosystem feedbacks on permafrost thermal dynamics

856 Interactions between ecosystem scale microclimate feedbacks and regional or global

857 climate feedbacks stemming from ecological change are complex and represent a key source of

858 uncertainty related to understanding permafrost soil responses to continued climate warming. If

859 changing ecosystem characteristics influencing permafrost thermal dynamics described above

860 are widespread, the accompanying changes in land surface water and energy exchange will feed

861 back to influence regional climate, and changes in greenhouse gas dynamics will feed back on

862 global climate (Chapin et al., 2000b). Therefore, ecosystem changes that alter local permafrost

863 soil thermal dynamics may also lead to regional and global climate feedbacks that compound or

864 offset ecosystem-scale effects (Figure 4Table 1).

865

39 Table 1. Key ecosystem changes, associated drivers, and feedback effects on local soil climate and regional to global climate.

Ecosystem Property Drivers of Change1 Local Feedbacks2 Regional-Global Feedbacks3

Canopy cover/density Climate warming (+) TSg- Ground Shading Albedo

increases more likely, unless Hydrologic change (?) TSg - LW Enhancement Increased EvapotranspiraKon widespread weJng occurs or Fire severity (+/-) KT - Soil moisture uKlizaKon Carbon SequestraKon under certain condiKons aLer fire. Thermokarst (-) KT - Snow trapping Permafrost thaw (+) Grazing (-/?) Anthropogenic (+/?)

Soil moisture Climate warming (+/-) KT Increased EvapotranspiraKon

uncertain; dependent on Hydrologic change (+) TSg- EvaporaKon Carbon SequestraKon vegetaKon, soil, climate, Fire severity (+/-) Greenhouse gas emissions topography, ground ice, and Thermokarst (+/-) whether permafrost is conKnuous Permafrost thaw (-) Anthropogenic (+/?)

Moss cover/organic layer Climate warming (?) KT EvapotranspiraKon thickness Hydrologic change (?) TSg- EvaporaKon Carbon SequestraKon

uncertain; dependent on Fire severity (-) overstory vegetaKon, topography, Thermokarst (+/-) and soil moisture Permafrost thaw (+/-)

Grazing (-) Anthropogenic (+/?)

1 Parentheses indicate whether driver is likely to cause an increase (+) or decrease (-) in ecosystem properKes, or if the direcKon of the relaKonship is unclear. 2 Effects of changing ecosystems property on local soil temperatures. Red and blue indicate posiKve and negaKve effects on soil temperature, respecKvely. 3 Regional and Global climate feedbacks associated with changing ecosystem properKes. Red and blue colors indicate posiKve and negaKve feedbacks respecKvely, gray indicates uncertain feedback effects. 866 867 868 Figure 4. Key ecosystem changes and their associated feedback effects on local soil climate, 869 regional atmospheric climate, and global climate. The + beneath canopy cover indicates an 870 assumed increase across the permafrost region, while the – beneath organic thickness and moss 871 cover indicates an assumed decrease. The change in soil moisture will depend on both changes in 872 ecosystem-scale hydrologic cycling, as well as changes in regional hydrology driven by climate, 873 and is assumed to be unknown. Blue text indicates negative feedbacks (cooling effect), red text 874 indicates positive feedbacks (warming effects), and gray text indicates feedbacks where the 875 direction is not known. 876

877 4.1 Regional biogeochemical climate feedbacks

878 The net biogeochemical climate effects of ecosystem change across the permafrost

879 regions will be a balance of changes in CO2 uptake that accompany shifts in vegetation, and

880 changes in CO2 and CH4 release associated with shifts in autotrophic and heterotrophic

881 respiration, and fire and thermokarst disturbance. These feedback effects will be global in extent

40 882 and will not contribute directly to regional variability in permafrost thaw because greenhouse

883 gasses are well mixed in the atmosphere. Changes in the net CO2 balance remain uncertain, but a

884 recent expert survey suggests that over the next century increases in vegetation productivity may

885 not be large enough to offset increases in carbon release to the atmosphere (Abbott et al., 2016).

886 In tundra ecosystems, this conclusion is in line with projections of future biomass distribution

887 (Pearson et al., 2013) and atmospheric inversions showing that increased autumn CO2 efflux

888 offsets increases in uptake during the growing season (Welp et al., 2016; Commane et al., 2017).

889 In boreal forests, carbon cycle changes are more complex; long-term trends in the annual

890 amplitude of atmospheric CO2 concentrations (Graven et al., 2013; Forkel et al., 2016) suggest

891 increases in biological activity while satellite observations and tree ring analyses suggest

892 widespread declines in productivity (Beck et al., 2011). Further, model analyses indicate a

893 weakening terrestrial carbon sink associated with declining uptake, increases in respiration, and

894 disturbance (Hayes et al., 2011), which is crucially important in boreal forests (Bond-Lamberty

895 et al., 2013).

896 The net CO2 effect of wildfire has typically been considered to be close to zero for

897 evergreen needleleaf forests in interior Alaska over historic fire return intervals (Randerson et

898 al., 2006). However, the combined effects of climate warming and fire tend to reduce ecosystem

899 carbon storage by thawing permafrost (Harden et al., 2000; O'Donnell et al., 2011b; Douglas et

900 al., 2014). Model simulations that include permafrost dynamics indicate ecosystem carbon losses

901 may become larger in the future with continued warming and intensification of the fire regime,

902 particularly for dry upland sites (Genet et al., 2013; Jafarov et al., 2013). These studies do not

903 account for potential changes in post-fire vegetation communities (Alexander et al., 2012a)

904 however, the net effects of vegetation shifts on ecosystem carbon storage appear to be minimal

41 905 (Alexander & Mack, 2015). In tundra ecosystems larger and more severe fires lead to large soil

906 C losses (Mack et al., 2011) that may be sustained over time due to permafrost thaw (Jones et al.,

907 2013; 2015). Taken together, this evidence suggests that Across the permafrost region, available

908 evidence suggests that fire will likely lead to net carbon losses in the coming decades to

909 centuries across the permafrost region, thus acting as a positive feedback to climate warming

910 with associated effects on permafrost soils (Abbott et al., 2016). The biophysical climate

911 feedbacks associated with fire are more immediate and will be stronger than the carbon cycle

912 feedbacks (Randerson et al., 2006).

913 The effects of thermokarst on greenhouse gas dynamics depend largely on associated

914 hydrological changes. With increased drainage and surface drying, increased oxidation rates

915 reduce carbon accumulation (Robinson & Moore, 2000) and enhance CO2 release (Frolking et

916 al., 2006), and reduce CH4 production (Abbott & Jones, 2015). When ground thaw is associated

917 with increased soil saturation, CH4 production and emissions are increased (Johansson et al.,

918 2006; Olefeldt et al., 2012; Abbott & Jones, 2015; Malhotra & Roulet, 2015; Natali et al., 2015),

919 which can shift tundra from a net CH4 sink (Jorgensen et al., 2015) into a CH4 source (Nauta et

920 al., 2015). Thermokarst may also increase lateral transport of soil organic matter, which can

921 decrease CO2 release (Abbott & Jones, 2015) and alter carbon processing downslope.

922 Thermokarst lakes emit CH4, particularly along actively thawing lake margins (Walter et al.,

923 2007; 2008), and CO2 (Kling et al., 1991; Algesten et al., 2004). However at millennial

924 timescales, thermokarst lakes can sequester carbon as lake sediments and peat accumulate (Jones

925 et al., 2012; Anthony et al., 2014). Currently thermokarst landscapes comprise upwards of 20%

926 of the permafrost region (Olefeldt et al., 2016), however their current and future impacts on the

927 global carbon balance remain poorly constrained.

42 928

929 4.2 Regional biophysical climate feedbacks

930 The biophysical effects of ecosystem change arising from shifts in surface energy

931 partitioning have climate feedback effects at scales ranging from local to regional and global.

932 Whereas biogeochemical climate feedbacks will influence global temperature in conjunction

933 with many other carbon cycle processes, biophysical feedbacks operating at local and regional

934 scales are likely to influence the spatial and temporal patterns of permafrost thaw with continued

935 warming. As described in the previous sections, changes in vegetation composition and structure

936 alter soil thermal dynamics via changes in G during the snow-free season (Chapin et al., 2000a;

937 Beringer et al., 2005). However, changes in G associated with vegetation change will also be

938 accompanied by changes in H and LE that may feedback to G, depending upon the scale of

939 impact.

940 Decadal ecosystem responses to climate inferred from ‘greening’ or ‘browning’ trends

941 are the most spatially pervasive change affecting vegetation in the permafrost zone (Loranty et

942 al., 2016). Increases in leaf area and/or vegetation stature will generally reduce albedo, and these

943 effects are particularly pronounced during the spring and fall if enhanced productivity leads to

944 increased snow-masking by vegetation (Sturm et al., 2005; Loranty et al., 2014a). Reductions in

945 albedo will lead to sensible heating of the atmosphere (Chapin et al., 2005) that may counteract

946 the effects of canopy shading on G, if albedo reduction occurs at sufficiently large spatial scales

947 (Lawrence & Swenson, 2011; Bonfils et al., 2012). The magnitude and spatial extent of

948 vegetation height increases are crucial to determine the net feedback strength, but these

949 quantities remain largely unknown.

43 950 A second important but relatively unexplored feedback relates to evaporative cooling of

951 the land surface associated with increases in LE (but see Swann et al., 2010; Helbig et al.,

952 2016a)(but see Swann et al., 2010; Helbig et al., 2016b). Productivity increases are likely

953 accompanied by increases in evapotranspiration (Zhang et al., 2009), which have been shown to

954 mitigate temperature increases at global scales by increased cloud cover, which may reduce

955 incoming short-wave radiation reaching the Earth’s surface (Zeng et al., 2017). During the

956 growing season, this cooling could effectively reduce the degree of atmospheric sensible heating

957 associated with increased albedo, and would be particularly important if there is no change in

958 snow masking by vegetation (e.g., greening in tundra without shrub expansion, or in closed

959 canopy boreal forest). However, the extent to which latent cooling with enhanced productivity

960 may offset sensible heating associated with albedo decreases is uncertain for several reasons.

961 First, model experiments simulating shrub expansion, for example, utilize canopy

962 parameterizations for deciduous boreal tree species, because arctic shrub canopy physiology has

963 not been thoroughly characterized (e.g. Bonfils et al., 2012). Second, existing observations

964 indicate an increasing degree of stomatal control on evapotranspiration with vegetation stature

965 (Eugster et al., 2000; Kasurinen et al., 2014), indicating that LE will not necessarily continue to

966 increase with climate warming, which is supported by the emergence of browning trends.

967 Additionally, climatic changes in arctic hydrology are highly uncertain and likely to vary

968 spatially (Francis et al., 2009), meaning that LE may be limited by hydrology in some places but

969 not others. Lastly, disturbance processes will also alter surface energy dynamics through short-

970 term direct impacts on ecosystem structure and long-term impacts on post-disturbance

971 succession (as described above).

972

44 973 5 Conclusions

974 The effects of climatic change on permafrost thermal dynamics across the arctic and

975 boreal biomes will be strongly affected bydepends directly on terrestrial ecosystem influences

976 properties, which mediateon surface energy partitioning and transmission through the soil

977 profilethermal characteristics. Relationships between permafrost and climate vary spatially with

978 ecosystems properties and processes, and these patterns in the relationship between permafrost

979 and climate will change over time as ecosystems respond to climatevary through time on event to

980 millennial timescales. These changes changing nature of permafrost thermal regimes will be

981 driven by surface energy feedbacks operating on local-, regional-, and global-scales. Complex

982 interactions among many of these feedbacks create uncertainty surrounding the timing and

983 magnitude of the permafrost carbon feedback.

984 Interactions among ecosystem processes are not well understood and represent a key

985 source of uncertainty in the relationship between permafrost soils and climate.Continued

986 ecosystem- scale research focused on several key process interactions will improve our

987 understanding of ecological influences on soil thermal regimes. In particular, soil moisture alters

988 soil thermal conductivity, however the The influence of vegetation plant water use on spatial

989 and temporal variability in soil moisture is unclear.. F Future work should seek to elucidate

990 interactions between vegetation and soil moisture. Similarly, concurrent cThe extent to which

991 changes in decomposition rates and the litter substrate quantity and quality of available substrate

992 may have strongalter influences on the insulating effects of ground cover and the soil organic

993 layer is also unclear and could benefit from continued research. More research on relationships

994 between the spatial distribution of vegetation canopies and the insulative properties of snow is

995 also needed, especially in boreal forests. Lastly, more studies should involve year-round data

45 996 collection focused on understanding time-lags and the cumulative effects of seasonal processes.

997 In particular the net thermal effects of canopy shading versus snow-trapping, seasonally lagged

998 effects of snow cover, and seasonally lagged effects of soil moisture could all be better

999 understood through focused observational studies., and changes in the distribution and

1000 productivity of mosses may have similar effects. Improved understanding of the ecosystem

1001 processes influencing soil moisture and thermal properties are necessary to understand the fate of

1002 permafrost.

1003 Improved process level understanding of ecosystem influences on soil thermal regimes

1004 will not be useful for predicting the fate of permafrost carbon unless the processes that control

1005 the timing, extent, and trajectories of ecosystem change are known. Holistic understanding of

1006 changes in vegetation and ecosystem distributions is another critically important topic for

1007 understanding the fate of permafrost. There has been a strong focus on graminoid-shrub

1008 transitions in tundra ecosystems, yet there are a number of other potential vegetation transitions,

1009 many mediated by disturbance, with equally important implications. Changes in boreal forest

1010 structure and function underlying productivity trends need to be elucidated. Continued work

1011 focused on understanding how changing fire regimes influence soils and post-fire succession is

1012 also important, especially in tundra and Siberian boreal forests. These changes are not spatially

1013 isolated, and compounding disturbances will likely become increasingly commonimportant to

1014 understand. In addition to vegetation changes, constraining the proportion of landscapes affected

1015 by drying versus waterlogging associated with initial permafrost thaw is central to predicting

1016 both soil organic matter stocks and vegetation responses to climate warming. Related, whether

1017 precipitation increases or decreases with climate warming remains highly uncertain, and this will

46 1018 exert strong influence on vegetation and ecosystem responses to climate as well as disturbance

1019 mediated ecosystem changes.

1020 Lastly, there is a high degree of uncertainty surrounding the net effects of opposing local

1021 and regional ecosystem feedbacks to permafrost soil temperatureschanges in ecosystem

1022 vegetation and soil characteristics that occur over sufficiently large spatial scales will affect soil

1023 thermal regimes via feedbacks to regional and global climate that with the potential to amplify or

1024 attenuate local ecosystem-scale feedbacks. For example, could wetland expansion associated

1025 with widespread permafrost thaw lead to regional cooling through increased albedo, or might

1026 warming as a result of increased methane emissions offset this? Or could increased

1027 evapotranspiration associated with enhanced vegetation productivity lead to surface cooling and

1028 cloud formation that cools soils in summer, or might the rise in atmospheric water vapor increase

1029 late summer precipitation and extend the fall freeze-back period? Complex feedback processes

1030 such as these will likely affect the trajectory of permafrost responses to climate. Model studies

1031 that have examined the net effects of feedbacks across scales typically focus on one type of

1032 vegetation change (e.g. shrub expansion), and so there is less information regarding interactions

1033 among feedbacks associated with multiple ongoing changes. Continued efforts to understand the

1034 fate of permafrost in response to climate will require integrated analyses of processes affecting

1035 permafrost soil thermal dynamicsregimes, changing circumpolar ecosystem distributions, and the

1036 net effects of resulting climate feedbacks operating across a range of spatial and temporal scales.

1037

1038

47 1039 Acknowledgments

1040 This project benefited from input from members of the Permafrost Carbon Network

1041 (www.permafrostcarbon.org). Supporting funding to the Permafrost Carbon Network was

1042 provided by the National Science Foundation Network Grant #955713 and the National Science

1043 Foundation Study of Environmental Arctic Change (SEARCH) Grant #1331083. MML was

1044 supported with funding from the U.S. National Science Foundation grant PLR-1417745. DB was

1045 supported by The Swedish Research Council (2015-00465) and Marie Skłodowska Curie

1046 Actions co-funding (INCA 600398). TAD acknowledges support from the U.S. Army Basic

1047 Research (6.1) Program. BCF was supported by the Academy of Finland (Decision #256991 and

1048 JPI Climate (Decision #291581). IMS received support from UK Natural Environment Research

1049 Council ShrubTundra Grant (NE/M016323/1). Any use of trade, product, or firm names is for

1050 descriptive purposes only and does not imply endorsement by the US Government. We thank

1051 two anonymous reviewers for comments that helped improve this manuscript.

1052

48 1053 References

1054 Abbott BW, Jones JB (2015) Permafrost collapse alters soil carbon stocks, respiration, CH 4, and 1055 N 2O in upland tundra. Global Change Biology, 21, 4570–4587. 1056 Abbott BW, Jones JB, Godsey SE, Larouche JR, Bowden WB (2015) Patterns and persistence of 1057 hydrologic carbon and nutrient export from collapsing upland permafrost. Biogeosciences, 1058 12, 3725–3740. 1059 Abbott BW, Jones JB, Schuur EAG et al. (2016) Biomass offsets little or none of permafrost 1060 carbon release from soils, streams, and wildfire: an expert assessment. Environmental 1061 Research Letters, 11, 1–13. 1062 Ackerman D, Griffin D, Hobbie SE, Finlay JC (2017) Arctic shrub growth trajectories differ 1063 across soil moisture levels. Global Change Biology, 69, 130–9. 1064 Alexander HD, Mack MC (2015) A Canopy Shift in Interior Alaskan Boreal Forests: 1065 Consequences for Above-and Belowground Carbon and Nitrogen Pools during Post-fire 1066 Succession. Ecosystems. 1067 Alexander HD, Mack MC, Goetz SJ, Beck PSA, Belshe EF (2012a) Implications of increased 1068 deciduous cover on stand structure and aboveground carbon pools of Alaskan boreal forests. 1069 Ecosphere, 3, art45. 1070 Alexander HD, Mack MC, Goetz SJ et al. (2012b) Carbon Accumulation Patterns During Post- 1071 Fire Succession in Cajander Larch (Larix cajanderi) Forests of Siberia. Ecosystems, 15, 1072 1065–1082. 1073 Alexander HD, Natali SM, Loranty MM et al. (2018) Impacts of increased soil burn severity on 1074 larch forest regeneration on permafrost soils of far northeastern Siberia. Forest Ecology And 1075 Management, 417, 144–153. 1076 Algesten G, Sobek S, Bergström AK, Ågren A, Tranvik LJ, Jansson M (2004) Role of lakes for 1077 organic carbon cycling in the boreal zone. Global Change Biology, 10, 141–147. 1078 Anthony KMW, Zimov SA, Grosse G et al. (2014) A shift of thermokarst lakes from carbon 1079 sources to sinks during the Holocene epoch. Nature Communications, 511, 452–456. 1080 Auerbach NA, Walker MD, Walker DA (1997) Effects of Roadside Disturbance on Substrate 1081 and Vegetation Properties in Arctic Tundra. Ecological Applications, 7, 218–235. 1082 Baldocchi D, Kelliher F, Black T, Jarvis P (2000) Climate and vegetation controls on boreal zone 1083 energy exchange. Global Change Biology, 6, 69–83. 1084 Baltzer JL, Veness T, Chasmer LE, Sniderhan AE, Quinton WL (2014) Forests on thawing 1085 permafrost: fragmentation, edge effects, and net forest loss. Global Change Biology, 20, 1086 824–834. 1087 Barber VA, Juday GP, Finney BP (2000) Reduced growth of Alaskan white spruce in the 1088 twentieth century from temperature-induced drought stress. Nature Communications, 405, 1089 668–673. 1090 Bardgett RD, Mommer L, De Vries FT (2014) Going underground: root traits as drivers of 1091 ecosystem processes. Trends in Ecology & Evolution, 29, 692–699. 1092 Barrett K, Rocha AV, van de Weg MJ, Shaver G (2012) Vegetation shifts observed in arctic 1093 tundra 17 years after fire. Remote Sensing Letters, 3, 729–736. 1094 Beck PSA, Goetz SJ (2011) Satellite observations of high northern latitude vegetation 1095 productivity changes between 1982 and 2008: ecological variability and regional differences. 1096 Environmental Research Letters, 6 045501. 1097 Beck PSA, Juday GP, Alix C et al. (2011) Changes in forest productivity across Alaska

49 1098 consistent with biome shift. Ecology Letters, 14, 373–379. 1099 Beest te M, Sitters J, Ménard CB, Olofsson J (2016) Reindeer grazing increases summer albedo 1100 by reducing shrub abundance in Arctic tundra. Environmental Research Letters, 11, 125013– 1101 14. 1102 Belshe EF, Schuur EAG, Grosse G (2013) Quantification of upland thermokarst features with 1103 high resolution remote sensing. Environmental Research Letters, 8, 035016. 1104 Beringer J, Chapin FS, Thompson CC, Mcguire AD (2005) Surface energy exchanges along a 1105 tundra-forest transition and feedbacks to climate. Agricultural and Forest Meteorology, 131, 1106 143–161. 1107 Berner LT, Beck PSA, Bunn AG, Goetz SJ (2013) Plant response to climate change along the 1108 forest-tundra ecotone in northeastern Siberia. Global Change Biology, 19, 3449–3462. 1109 Berner LT, Beck PSA, Bunn AG, Lloyd AH, Goetz SJ (2011) High-latitude tree growth and 1110 satellite vegetation indices: Correlations and trends in Russia and Canada (1982–2008). 1111 Journal of Geophysical Research, 116, G01015. 1112 Berner LT, Beck PSA, Loranty MM, Alexander HD, Mack MC, Goetz SJ (2012) Cajander larch 1113 (Larix cajanderi) biomass distribution, fire regime and post-fire recovery in northeastern 1114 Siberia. Biogeosciences, 9, 3943–3959. 1115 Betts AK, Ball J (1997) Albedo over the boreal forest. Journal of Geophysical Research, 102, 1116 28901–28909. 1117 Betts AK, Goulden M, Wofsy S (1999) Controls on evaporation in a boreal spruce forest. 1118 Journal of Climate. 1119 Bewley D, Pomeroy J, Essery R (2007) Solar Radiation Transfer Through a Subarctic Shrub 1120 Canopy. Arctic, Antarctic, and Alpine Research, 39, 365–374. 1121 Bhatt U, Walker D, Raynolds M et al. (2013) Recent Declines in Warming and Vegetation 1122 Greening Trends over Pan-Arctic Tundra. Remote Sensing, 5, 4229–4254. 1123 Bjerke JW, Karlsen SR, Høgda KA et al. (2014) Record-low primary productivity and high plant 1124 damage in the Nordic Arctic Region in 2012 caused by multiple weather events and pest 1125 outbreaks. Environmental Research Letters, 9, 084006. 1126 Blok D, Heijmans M, Schaepman-Strub G, Kononov A, Maximov T, Berendse F (2010) Shrub 1127 expansion may reduce summer permafrost thaw in Siberian tundra. Global Change Biology, 1128 16, 1296–1305. 1129 Blok D, Heijmans MMPD, Schaepman-Strub G, Ruijven J, Parmentier FJW, Maximov TC, 1130 Berendse F (2011a) The Cooling Capacity of Mosses: Controls on Water and Energy Fluxes 1131 in a Siberian Tundra Site. Ecosystems, 14, 1055–1065. 1132 Blok D, Schaepman-Strub G, Bartholomeus H, Heijmans MM, Maximov TC, Berendse F 1133 (2011b) The response of Arctic vegetation to the summer climate: relation between shrub 1134 cover, NDVI, surface albedo and temperature. Environmental Research Letters, 6, 035502. 1135 Blume-Werry G, Wilson SD, Kreyling J, Milbau A (2015) The hidden season: growing season is 1136 50% longer below than above ground along an arctic elevation gradient. New Phytologist, 1137 209, 978–986. 1138 Boike J, Roth K, Overduin PP (1998) Thermal and hydrologic dynamics of the active layer at a 1139 continuous permafrost site (Taymyr Peninsula, Siberia). Water Resources Research. 1140 Boike J, Wille C, Abnizova A (2008) Climatology and summer energy and water balance of 1141 polygonal tundra in the Lena River Delta, Siberia. Journal of Geophysical Research, 113, 1142 G03025–15. 1143 Bond-Lamberty B, Rocha AV, Calvin K, Holmes B, Wang C, Goulden ML (2013) Disturbance

50 1144 legacies and climate jointly drive tree growth and mortality in an intensively studied boreal 1145 forest. Global Change Biology, 20, 216–227. 1146 Bonfils CJW, Phillips TJ, Lawrence DM, Cameron-Smith P, Riley WJ, Subin ZM (2012) On the 1147 influence of shrub height and expansion on northern high latitude climate. Environmental 1148 Research Letters, 7, 015503. 1149 Bret-Harte MS, Mack MC, Shaver GR et al. (2013) The response of Arctic vegetation and soils 1150 following an unusually severe tundra fire. Philosophical Transactions of the Royal Society 1151 B: Biological Sciences, 368, 20120490–20120490. 1152 Briggs MA, Walvoord MA, McKenzie JM (2014) New permafrost is forming around shrinking 1153 Arctic lakes, but will it last? Geophysical Research Letters, 41, 1585–1592. 1154 Brown D, Jorgenson MT, Douglas TA et al. (2015) Interactive effects of wildfire and climate on 1155 permafrost degradation in Alaskan lowland forests. Journal of Geophysical Research 1156 Biogeosciences, 120, 1619–1637. 1157 Brown J, Ferrians OJ, Heginbottom JA, Melinikov ES (1998) Circum-arctic map of permafrost 1158 and ground ice conditions. 1159 Bunn AG, Goetz SJ (2006) Trends in satellite-observed circumpolar photosynthetic activity from 1160 1982 to 2003: The influence of seasonality, cover type, and vegetation density. Earth 1161 Interactions, 10, 12. 1162 Burn CR (1998) The response (1958-1997) of permafrost and near-surface ground temperatures 1163 to forest fire, Takhini River valley, southern Yukon Territory. Canadian Journal of Earth 1164 Sciences, 35, 184–199. 1165 Cable WL, Romanovsky VE, Jorgenson MT (2016) Scaling-up permafrost thermal 1166 measurements in western Alaska using an ecotype approach. The Cryosphere, 10, 2517– 1167 2532. 1168 Chapin F, Sturm M, Serreze M et al. (2005) Role of land-surface changes in Arctic summer 1169 warming. Science, 310, 657. 1170 Chapin FS III, Eugster W, McFadden J, Lynch A, Walker D (2000a) Summer differences among 1171 arctic ecosystems in regional climate forcing. Journal of Climate, 13, 2002–2010. 1172 Chapin FS III, McGuire A, Randerson J et al. (2000b) Arctic and boreal ecosystems of western 1173 North America as components of the climate system. Global Change Biology, 6, 211–223. 1174 Christiansen CT, Mack MC, DeMarco J, Grogan P (2018) Decomposition of Senesced Leaf 1175 Litter is Faster in Tall Compared to Low Birch Shrub Tundra. Ecosystems, 170, 809–16. 1176 Commane R, Lindaas J, Benmergui J et al. (2017) Carbon dioxide sources from Alaska driven by 1177 increasing early winter respiration from Arctic tundra. Proceedings Of The National 1178 Academy Of Sciences Of The United States Of America, 114, 5361–5366. 1179 Comyn-Platt E, Hayman G, huntingford C et al. (2018) Carbon budgets for 1.5 and 2 °C targets 1180 lowered by natural wetland and permafrost feedbacks. Nature Geoscience, 11, 568–573. 1181 Cornelissen JH, Van Bodegom PM, Aerts R et al. (2007) Global negative vegetation feedback to 1182 climate warming responses of leaf litter decomposition rates in cold biomes. Ecology Letters, 1183 10, 619–627. 1184 Crampton CB (1977) A study of the dynamics of hummocky microrelief in the Canadian north. 1185 Canadian Journal of Earth Sciences, 14, 639–649. 1186 Curasi SR, Loranty MM, Natali SM (2016) Water track distribution and effects on carbon 1187 dioxide flux in an eastern Siberian upland tundra landscape. Environmental Research 1188 Letters, 11, 1–12. 1189 de Grandpré I, Fortier D, Stephani E (2012) Degradation of permafrost beneath a road

51 1190 embankment enhanced by heat advected in groundwater. Canadian Journal of Earth 1191 Sciences, 49, 953–962. 1192 Domine F, Barrere M, Sarrazin D, Morin S, Arnaud L (2015) Automatic monitoring of the 1193 effective thermal conductivity of snow in a low-Arctic shrub tundra. The Cryosphere, 9, 1194 1265–1276. 1195 Douglas TA, Jones MC, Hiemstra CA, Arnold JR (2014) Sources and sinks of carbon in boreal 1196 ecosystems of interior Alaska: A review. Elementa: Science of the Anthropocene, 2, 1197 000032–39. 1198 Douglas TA, Jorgenson MT, Brown DRN et al. (2016) Degrading permafrost mapped with 1199 electrical resistivity tomography, airborne imagery and LiDAR, and seasonal thaw 1200 measurements. GEOPHYSICS, 81, WA71–WA85. 1201 Eaton AK, Rouse WR, Lafleur PM, Marsh P, Blanken PD (2001) Surface Energy Balance of the 1202 Western and Central Canadian Subarctic: Variations in the Energy Balance among Five 1203 Major Terrain Types. Journal of Climate, 14, 3692–3703. 1204 Elmendorf SC, Henry GHR, Hollister RD et al. (2012a) Global assessment of experimental 1205 climate warming on tundra vegetation: heterogeneity over space and time. Ecology Letters, 1206 15, 164–175. 1207 Elmendorf SC, Henry GHR, Hollister RD et al. (2012b) Plot-scale evidence of tundra vegetation 1208 change and links to recent summer warming. Nature Climate Change, 2, 1–5. 1209 Essery R, Pomeroy J (2004) Vegetation and topographic control of wind-blown snow 1210 distributions in distributed and aggregated simulations for an Arctic tundra basin. Journal of 1211 Hydrometeorology, 5, 735–744. 1212 Eugster W, Rouse W, Pielke R Sr et al. (2000) Land-atmosphere energy exchange in Arctic 1213 tundra and boreal forest: available data and feedbacks to climate. Global Change Biology, 6, 1214 84–115. 1215 Fan Z, Neff JC, Harden JW et al. (2011) Water and heat transport in boreal soils: Implications 1216 for soil response to climate change. Science of the Total Environment, The, 409, 1836–1842. 1217 Fauria MM, Helle T, Niva A (2008) Removal of the lichen mat by reindeer enhances tree growth 1218 in a northern Scots pine forest. Canadian journal of …. 1219 Fedorov AN, Iwahana G, Konstantinov PY et al. (2016) Variability of Permafrost and Landscape 1220 Conditions Following Clear Cutting of Larch Forest in Central Yakutia. Permafrost and 1221 Periglacial Processes, 1–8. 1222 Filhol S, Sturm M (2015) Snow bedforms: A review, new data, and a formation model. Journal 1223 of Geophysical Research: Earth Surface, 120, 1645–1669. 1224 Fisher JP, Estop Aragonés C, Thierry A et al. (2016) The influence of vegetation and soil 1225 characteristics on active-layer thickness of permafrost soils in boreal forest. Global Change 1226 Biology, 3127–3140. 1227 Forbes BC (1993) Aspects of natural recovery of soils, hydrology and vegetation at an 1228 abandoned high arctic settlement, Baffin Island, Canada. 1, 176–181. 1229 Forbes BC (1998) Cumulative impacts of vehicle traffic on high arctic tundra: soil temperature, 1230 plant biomass, species richness and mineral nutrition. Yellowknife, CA. 1231 Forbes BC (1995) Tundra disturbance studies, III: Short-term effects of Aeolian sand and dust, 1232 Yamal Region, Northwest Siberia. Environmental Conservation, 22, 335–344. 1233 Forbes BC, Kumpula T (2009) The Ecological Role and Geography of Reindeer ( Rangifer 1234 tarandus) in Northern Eurasia. Geography Compass, 3, 1356–1380. 1235 Forbes BC, Ebersole JJ, Strandberg B (2001) Anthropogenic disturbance and patch dynamics in

52 1236 circumpolar arctic ecosystems. Conservation Biology, 15, 954–969. 1237 Forbes BC, Fauria MM, Zetterberg P (2010) Russian Arctic warming and “greening” are closely 1238 tracked by tundra shrub willows. Global Change Biology, 16, 1542–1554. 1239 Forkel M, Carvalhais N, Roedenbeck C et al. (2016) Enhanced seasonal CO2 exchange caused 1240 by amplified plant productivity in northern ecosystems. Science, 351, 696–699. 1241 Francis JA, White DM, Cassano JJ et al. (2009) An arctic hydrologic system in transition: 1242 Feedbacks and impacts on terrestrial, marine, and human life. Journal of Geophysical 1243 Research, 114, G04019. 1244 French NH, Whitley MA, Jenkins LK (2016) Fire disturbance effects on land surface albedo in 1245 Alaskan tundra. Journal of Geophysical Research Biogeosciences, 121, 841–854. 1246 Froese DG, Westgate JA, Reyes AV, Enkin RJ, Preece SJ (2008) Ancient Permafrost and a 1247 Future, Warmer Arctic. Science, 321, 1648–1648. 1248 Frolking S, Roulet N, Fuglestvedt J (2006) How northern peatlands influence the Earth's 1249 radiative budget: Sustained methane emission versus sustained carbon sequestration. Journal 1250 of Geophysical Research, 111, G01008–10. 1251 Frost GV, Epstein HE (2014) Tall shrub and tree expansion in Siberian tundra ecotones since the 1252 1960s. Global Change Biology, 20, 1264–1277. 1253 Furayev V, Vaganov EA, Tchebakova NM, Valendik EN (2001) Effects of Fire and Climate on 1254 Successions and Structural Changes in The Siberian Boreal Forest. Eurasian Journal of 1255 Forest Research, 2, 1–15. 1256 Gamon JA, Kershaw GP, Williamson S, Hik DS (2012) Microtopographic patterns in an arctic 1257 baydjarakh field: do fine-grain patterns enforce landscape stability? Environmental Research 1258 Letters, 7, 015502. 1259 Genet H, Mcguire AD, Barrett K et al. (2013) Modeling the effects of fire severity and climate 1260 warming on active layer thickness and soil carbon storage of black spruce forests across the 1261 landscape in interior Alaska. Environmental Research Letters, 8, 045016. 1262 Gill HK, Lantz TC, O'Neill B, Kokelj SV (2014) Cumulative Impacts and Feedbacks of a Gravel 1263 Road on Shrub Tundra Ecosystems in the Peel Plateau, Northwest Territories, Canada. 1264 Arctic, Antarctic, and Alpine Research, 46, 947–961. 1265 Goodrich LE (1982) The influence of snow cover on the ground thermal regime. Canadian 1266 Geotechnical Journal, 19, 421–432. 1267 Gouttevin I, Ménégoz M, Domine F et al. (2012) How the insulating properties of snow affect 1268 soil carbon distribution in the continental pan-Arctic area. Journal of Geophysical Research, 1269 117, n/a–n/a. 1270 Göckede M, Kittler F, Kwon MJ et al. (2017) Shifted energy fluxes, increased Bowen ratios, and 1271 reduced thaw depths linked with drainage-induced changes in permafrost ecosystem 1272 structure. The Cryosphere, 11, 2975–2996. 1273 Graven HD, Keeling RF, Piper SC et al. (2013) Enhanced Seasonal Exchange of CO2 by 1274 Northern Ecosystems Since 1960. Science, 341, 1085–1089. 1275 Guay KC, Beck PSA, Berner LT, Goetz SJ, Baccini A, Buermann W (2014) Vegetation 1276 productivity patterns at high northern latitudes: a multi-sensor satellite data assessment. 1277 Global Change Biology, 20, 3147–3158. 1278 Halsey LA, Vitt DH, Zoltai SC (1995) Disequilibrium response of permafrost in boreal 1279 continental western Canada to climate change. Climatic Change, 30, 57–73. 1280 Harden JW, Trumbore SE, Stocks BJ, Hirsch A, Gower ST, O'neill KP, Kasischke ES (2000) 1281 The role of fire in the boreal carbon budget. Global Change Biology, 6, 174–184.

53 1282 Harms TK, Abbott BW, Jones JB (2014) Thermo-erosion gullies increase nitrogen available for 1283 hydrologic export. Biogeochemistry, 117, 299–311. 1284 Hayes DJ, Mcguire AD, Kicklighter DW, Gurney KR, Burnside TJ, Melillo JM (2011) Is the 1285 northern high-latitude land-based CO 2sink weakening? 25, n/a–n/a. 1286 Heijmans MM, Arp WJ, Chapin FS (2004a) Controls on moss evaporation in a boreal black 1287 spruce forest. 18. 1288 Heijmans MMPD, Arp WJ, Chapin FS III (2004b) Carbon dioxide and water vapour exchange 1289 from understory species in boreal forest. Agricultural and Forest Meteorology, 123, 135– 1290 147. 1291 Helbig M, Wischnewski K, Kljun N, Chasmer LE, Quinton WL, Detto M, Sonnentag O (2016a) 1292 Regional atmospheric cooling and wetting effect of permafrost thaw-induced boreal forest 1293 loss. Global Change Biology, 22, 4048–4066. 1294 Helbig M, Pappas C, Sonnentag O (2016b) Permafrost thaw and wildfire: Equally important 1295 drivers of boreal tree cover changes in the Taiga Plains, Canada. Geophysical Research 1296 Letters. 1297 Higgins KL, Garon-Labrecque M-È (2018) Fine-scale influences on thaw depth in a forested 1298 peat plateau landscape in the Northwest Territories, Canada: Vegetation trumps 1299 microtopography. Permafrost and Periglacial Processes, 29, 60–70. 1300 Hinkel KM, Nelson FE (2003) Spatial and temporal patterns of active layer thickness at 1301 Circumpolar Active Layer Monitoring (CALM) sites in northern Alaska, 1995–2000. 1302 Journal of Geophysical Research, 108. 1303 Hinkel KM, Outcalt SI (1994) Identification of heat-transfer processes during soil cooling, 1304 freezing, and thaw in central Alaska. Permafrost and Periglacial Processes, 5, 217–235. 1305 Hinkel KM, Paetzold F, Nelson FE, Bockheim JG (2001) Patterns of soil temperature and 1306 moisture in the active layer and upper permafrost at Barrow, Alaska: 1993–1999. Global and 1307 Planetary Change, 29, 293–309. 1308 Hinzman LD, Kane DL, Gieck RE, Everett KR (1991) Hydrologic and thermal properties of the 1309 active layer in the Alaskan Arctic. Cold Regions Science and Technology, 19, 95–110. 1310 Hobbie S (1996) Temperature and Plant Species Control Over Litter Decomposition in Alaskan 1311 Tundra. Ecological Monographs, 66, 503–522. 1312 Hobbie SE, Gough L (2004) Litter decomposition in moist acidic and non-acidic tundra with 1313 different glacial histories. Oecologia, 140, 113–124. 1314 Hu FS, Higuera PE, Walsh JE, Chapman WL, Duffy PA, Brubaker LB, Chipman ML (2010) 1315 Tundra burning in Alaska: Linkages to climatic change and sea ice retreat. Journal of 1316 Geophysical Research, 115, G04002. 1317 Huntington H, Arnbom T, Danielson F et al. (2013) Disturbance, feedbacks and conservation. In: 1318 Arctic Biodiversity Assessment Status and trends in Arctic biodiversity. Akureyri: 1319 Conservation of Arctic Flora and Fauna. 1320 Iijima Y, Fedorov AN, Park H, Suzuki K, Yabuki H, Maximov TC, Ohata T (2010) Abrupt 1321 increases in soil temperatures following increased precipitation in a permafrost region, 1322 central Lena River basin, Russia. Permafrost and Periglacial Processes, 21, 30–41. 1323 Iijima Y, Ohta T, Kotani A, Fedorov AN, Kodama Y, Maximov TC (2014) Sap flow changes in 1324 relation to permafrost degradation under increasing precipitation in an eastern Siberian larch 1325 forest. Ecohydrology, 7, 177–187. 1326 Iversen CM, Sloan VL, Sullivan PF et al. (2015) The unseen iceberg: plant roots in arctic tundra. 1327 New Phytologist, 205, 34–58.

54 1328 Iwahana G, Machimura T, Kobayashi Y (2005) Influence of forest clear‐cutting on the thermal 1329 and hydrological regime of the active layer near Yakutsk, eastern Siberia. Journal of 1330 Geophysical Research: Earth Surface. 1331 Jafarov EE, Romanovsky VE, Genet H, David McGuire A, Marchenko SS (2013) The effects of 1332 fire on the thermal stability of permafrost in lowland and upland black spruce forests of 1333 interior Alaska in a changing climate. Environmental Research Letters, 8, 035030. 1334 Jean M, Payette S (2014a) Dynamics of active layer in wooded palsas of northern Quebec. 1335 Geomorphology, 206, 87–96. 1336 Jean M, Payette S (2014b) Effect of Vegetation Cover on the Ground Thermal Regime of 1337 Wooded and Non-Wooded Palsas. Permafrost and Periglacial Processes, 25, 281–294. 1338 Jia G, Epstein H, Walker D (2003) Greening of arctic Alaska, 1981–2001. Geophysical Research 1339 Letters, 30, 2067. 1340 Jiang Y, Rocha AV, O'Donnell JA, Drysdale JA, Rastetter EB, Shaver GR, Zhuang Q (2015) 1341 Contrasting soil thermal responses to fire in Alaskan tundra and boreal forest. Journal of 1342 Geophysical Research: Earth Surface, 120, 363–378. 1343 Jin Y, Randerson JT, Goetz SJ, Beck PSA, Loranty MM, Goulden ML (2012) The influence of 1344 burn severity on postfire vegetation recovery and albedo change during early succession in 1345 North American boreal forests. Journal of Geophysical Research, 117. 1346 Jin Y, SCHAAF C, Gao F, Li X, STRAHLER A, Zeng X, Dickinson R (2002) How does snow 1347 impact the albedo of vegetated land surfaces as analyzed with MODIS data? Geophysical 1348 Research Letters, 29, 12–11. 1349 Johansen O (1977) Thermal conductivity of soils. 1350 Johansson T, Malmer N, Crill PM, Friborg T, ÅKERMAN JH, Mastepanov M, Christensen TR 1351 (2006) Decadal vegetation changes in a northern peatland, greenhouse gas fluxes and net 1352 radiative forcing. Global Change Biology, 12, 2352–2369. 1353 Johnstone JF, Hollingsworth TN, Chapin FS III, Mack MC (2010) Changes in fire regime break 1354 the legacy lock on successional trajectories in Alaskan boreal forest. Global Change Biology, 1355 16, 1281–1295. 1356 Jones BM, Breen AL, Gaglioti BV et al. (2013) Identification of unrecognized tundra fire events 1357 on the north slope of Alaska. Journal of Geophysical Research Biogeosciences, 118, 1334– 1358 1344. 1359 Jones BM, Grosse G, Arp CD, Miller E, Liu L, Hayes DJ, Larsen CF (2015) Recent Arctic 1360 tundra fire initiates widespread thermokarst development. Scientific Reports, 5, 15865. 1361 Jones BM, Kolden CA, Jandt R, Abatzoglou JT, Urban F, Arp CD (2009) Fire Behavior, 1362 Weather, and Burn Severity of the 2007 Anaktuvuk River Tundra Fire, North Slope, Alaska. 1363 Arctic, Antarctic, and Alpine Research, 41, 309–316. 1364 Jones MC, Grosse G, Jones BM, Walter Anthony K (2012) Peat accumulation in drained 1365 thermokarst lake basins in continuous, ice-rich permafrost, northern Seward Peninsula, 1366 Alaska. Journal of Geophysical Research, 117, n/a–n/a. 1367 Jorgensen CJ, Johansen KML, Westergaard-Nielsen A, Elberling B (2015) Net regional methane 1368 sink in High Arctic soils of northeast Greenland. Nature Geoscience, 8, 20–23. 1369 Jorgenson MT, Osterkamp TE (2005) Response of boreal ecosystems to varying modes of 1370 permafrost degradation. Canadian Journal of Forest Research, 35, 2100–2111. 1371 Jorgenson MT, HARDEN J, Kanevskiy M (2013) Reorganization of vegetation, hydrology and 1372 soil carbon after permafrost degradation across heterogeneous boreal landscapes. 1373 Environmental Research Letters.

55 1374 Jorgenson MT, Racine CH, Walters JC, Osterkamp TE (2001) Permafrost degradation and 1375 ecological changes associated with a warmingclimate in central Alaska. Climatic Change, 1376 48, 551–579. 1377 Jorgenson MT, Shur YL, Pullman ER (2006) Abrupt increase in permafrost degradation in 1378 Arctic Alaska. Geophysical Research Letters, 33, L02503. 1379 Jorgenson, Romanovsky V, Harden J et al. (2010) Resilience and vulnerability of permafrost to 1380 climate change. Candian Journal of Forest Research, 40, 1219–1236. 1381 Juday GP, Alix C, Grant TA III (2015) Spatial coherence and change of opposite white spruce 1382 temperature sensitivities on floodplains in Alaska confirms early-stage boreal biome shift. 1383 Forest Ecology And Management, 350, 46–61. 1384 Juszak, I., Erb, A. M., Maximov, T. C., & Schaepman-Strub, G. (2014). Arctic shrub effects on 1385 NDVI, summer albedo and soil shading. Remote Sensing of Environment, 153, 79–89. 1386 http://doi.org/10.1016/j.rse.2014.07.021 1387 Juszak I, Eugster W, Heijmans MMPD, Schaepman-Strub G (2016) Contrasting radiation and 1388 soil heat fluxes in Arctic shrub and wet sedge tundra. Biogeosciences, 13, 4049–4064. 1389 Kane DL, Hinkel KM, Goering DJ, Hinzman LD, Outcalt SI (2001) Non-conductive heat 1390 transfer associated with frozen soils. Global and Planetary Change, 29, 275–292. 1391 Kane DL, Hinzman LD, Benson CS, Everett KR (1989) Hydrology of Imnavait Creek, an arctic 1392 watershed. Ecography, 12, 262–269. 1393 Kane ES, Kasischke ES, Valentine DW, Turetsky MR, Mcguire AD (2007) Topographic 1394 influences on wildfire consumption of soil organic carbon in interior Alaska: Implications 1395 for black carbon accumulation. Journal of Geophysical Research, 112, n/a–n/a. 1396 Kasischke E, Johnstone J (2005) Variation in postfire organic layer thickness in a black spruce 1397 forest complex in interior Alaska and its effects on soil temperature and moisture. Canadian 1398 Journal of Forest Research, 35, 2164–2177. 1399 Kasischke E, Turetsky M (2006) Recent changes in the fire regime across the North American 1400 boreal region-spatial and temporal patterns of burning across Canada and Alaska. 1401 Geophysical Research Letters, 33, L09703. 1402 Kasurinen V, Alfredsen K, Kolari P et al. (2014) Latent heat exchange in the boreal and arctic 1403 biomes. Global Change Biology, 20, 3439–3456. 1404 Kelly R, Chipman ML, Higuera PE, Stefanova I, Brubaker LB, Hu FS (2013) Recent burning of 1405 boreal forests exceeds fire regime limits of the past 10,000 years. Proceedings of the 1406 National Academy of Sciences, 110, 13055–13060. 1407 Kershaw GP (1983) Some abiotic consequences of the CANOL Crude Oil Pipeline Project, 35 1408 years after abandonment. 595–600. 1409 Kershaw GP (2001) Snowpack Characteristics Following Wildfire on a Simulated Transport 1410 Corridor and Adjacent Subarctic Forest, Tulita, N.W.T., Canada. Arctic, Antarctic, and 1411 Alpine Research, 33, 131. 1412 Kershaw GP, McCulloch J (2007) Midwinter Snowpack Variation Across the Arctic Treeline, 1413 Churchill, Manitoba, Canada. Arctic, Antarctic, and Alpine Research, 39, 9–15. 1414 Keuper F, Bodegom PM, Dorrepaal E, Weedon JT, Hal J, Logtestijn RSP, Aerts R (2012) A 1415 frozen feast: thawing permafrost increases plant-available nitrogen in subarctic peatlands. 1416 Global Change Biology, 18, 1998–2007. 1417 Kharuk VI, Dvinskaya ML, Ranson KJ (2013) Fire return intervals within the northern boundary 1418 of the larch forest in Central Siberia. International Journal of Wildland Fire, 22, 207–6. 1419 Kharuk VI, Ranson KJ, Dvinskaya ML (2008) Wildfires dynamic in the larch dominance zone.

56 1420 Geophysical Research Letters, 35, L01402–6. 1421 Kholodov A, Gilichinsky D, Ostroumov V, Sorokovikov VA, Abramov AA, Davydov S, 1422 Romanovsky V (2012) Regional and local variability of modern natural changes in 1423 permafrost temperature in the Yakutian coastal lowlands, Northeastern Siberia. 1424 Kling GW, Kipphut GW, Miller MC (1991) Arctic lakes and streams as gas conduits to the 1425 atmosphere: implications for tundra carbon budgets. Science, 251, 298–301. 1426 Kokelj SV, Jorgenson MT (2013) Advances in Thermokarst Research. Permafrost and 1427 Periglacial Processes, 24, 108–119. 1428 Koven CD, Ringeval B, Friedlingstein P et al. (2011) Permafrost carbon-climate feedbacks 1429 accelerate global warming. Proceedings of the National Academy of Sciences, 108, 14769– 1430 14774. 1431 Kropp H, Loranty M, Alexander HD, Berner LT, Natali SM, Spawn SA (2017) Environmental 1432 constraints on transpiration and stomatal conductance in a Siberian Arctic boreal forest. 1433 Journal of Geophysical Research Biogeosciences, 209, 41–11. 1434 Kukavskaya EA, Soja AJ, Petkov AP, Ponomarev EI, Ivanova GA, Conard SG (2012) Fire 1435 emissions estimates in Siberia: evaluation of uncertainties in area burned, land cover, and 1436 fuel consumption. Canadian Journal of Forest Research, 43, 493–506. 1437 Landhäusser SM, Wein RW (1993) Postfire Vegetation Recovery and Tree Establishment at the 1438 Arctic Treeline: Climate-Change-Vegetation-Response Hypotheses. The Journal of Ecology, 1439 81, 665. 1440 Langer M, Westermann S, Muster S, Piel K, Boike J (2011a) The surface energy balance of a 1441 polygonal tundra site in northern Siberia – Part 1: Spring to fall. The Cryosphere, 5, 151– 1442 171. 1443 Langer M, Westermann S, Muster S, Piel K, Boike J (2011b) The surface energy balance of a 1444 polygonal tundra site in northern Siberia - Part 2: Winter. The Cryosphere, 5, 509–524. 1445 Lantz TC, Kokelj SV (2008) Increasing rates of retrogressive thaw slump activity in the 1446 Mackenzie Delta region, N.W.T., Canada. Geophysical Research Letters, 35, L06502. 1447 Lantz TC, Marsh P, Kokelj SV (2013) Recent shrub proliferation in the Mackenzie Delta uplands 1448 and microclimatic implications. Ecosystems, 16, 47–59. 1449 Lara MJ, Genet H, McGuire AD et al. (2016) Thermokarst rates intensify due to climate change 1450 and forest fragmentation in an Alaskan boreal forest lowland. Global Change Biology, 22, 1451 816–829. 1452 Lawrence DM, Swenson SC (2011) Permafrost response to increasing Arctic shrub abundance 1453 depends on the relative influence of shrubs on local soil cooling versus large-scale climate 1454 warming. Environmental Research Letters, 6, 045504. 1455 Lee X, Goulden ML, Hollinger DY, Barr A, Black TA (2011) Observed increase in local cooling 1456 effect of deforestation at higher latitudes. Nature Communications. 1457 Leibman M, Khomutov A, Kizyakov A (2014) Cryogenic landslides in the West-Siberian plain 1458 of Russia: classification, mechanisms, and landforms. In: Landslides in Cold Regions in the 1459 Context of Climate Change (eds Shan W, Guo Y, Mauri H, Strom A), pp. 143–162. Springer. 1460 Li Y, Zhao M, Motesharrei S, Mu Q, Kalnay E, Li S (2015) Local cooling and warming effects 1461 of forests based on satellite observations. Nature Communications, 6, 1–8. 1462 Liljedahl A, Hinzman L, Busey R, Yoshikawa K (2007) Physical short-term changes after a 1463 tussock tundra fire, Seward Peninsula, Alaska. Journal of Geophysical Research, 112, 165– 1464 13. 1465 Liljedahl AK, Boike J, Daanen RP et al. (2016) Pan-Arctic ice-wedge degradation in warming

57 1466 permafrost and its influence on tundra hydrology. Nature Geoscience, 9, 312–318. 1467 Liston GE, McFadden J, Sturm M, Pielke R (2002) Modelled changes in arctic tundra snow, 1468 energy and moisture fluxes due to increased shrubs. Global Change Biology. 1469 Lloyd AH, Bunn AG, BERNER L (2010) A latitudinal gradient in tree growth response to 1470 climate warming in the Siberian taiga. Global Change Biology, 17, 1935–1945. 1471 Lopez C ML, Saito H, Kobayashi Y, Shirota T, Iwahana G, Maximov TC, Fukuda M (2007) 1472 Interannual environmental-soil thawing rate variation and its control on transpiration from 1473 Larix cajanderi, Central Yakutia, Eastern Siberia. Journal of Hydrology, 338, 251–260. 1474 Loranty MM, Berner LT, Goetz SJ, Jin Y, Randerson JT (2014a) Vegetation controls on northern 1475 high latitude snow-albedo feedback: observations and CMIP5 model simulations. Global 1476 Change Biology, 20, 594–606. 1477 Loranty MM, Berner LT, Taber ED et al. (2018) Understory vegetation mediates permafrost 1478 active layer dynamics and carbon dioxide fluxes in open-canopy larch forests of northeastern 1479 Siberia (ed Rinnan R). PLoS ONE, 13, e0194014–17. 1480 Loranty MM, Goetz SJ, Beck PSA (2011) Tundra vegetation effects on pan-Arctic albedo. 1481 Environmental Research Letters, 6, 024014. 1482 Loranty MM, Liberman-Cribbin W, Berner LT, Natali SM, Goetz SJ, Alexander HD, Kholodov 1483 AL (2016) Spatial variation in vegetation productivity trends, fire disturbance, and soil 1484 carbon across arctic-boreal permafrost ecosystems. Environmental Research Letters, 11, 1– 1485 13. 1486 Loranty MM, Natali SM, Berner LT et al. (2014b) Siberian tundra ecosystem vegetation and 1487 carbon stocks four decades after wildfire. Journal of Geophysical Research Biogeosciences, 1488 119, 2144–2154. 1489 Lynch LM, Machmuller MB, Cotrufo MF, Paul EA, Wallenstein MD (2018) Tracking the fate of 1490 fresh carbon in the Arctic tundra: Will shrub expansion alter responses of soil organic matter 1491 to warming? Soil Biology and Biochemistry, 120, 134–144. 1492 Lyons EA, Jin Y, Randerson JT (2008) Changes in surface albedo after fire in boreal forest 1493 ecosystems of interior Alaska assessed using MODIS satellite observations. Journal of 1494 Geophysical Research, 113, 1–15. 1495 Macias-Fauria M, Forbes BC, Zetterberg P, Kumpula T (2012) Eurasian Arctic greening reveals 1496 teleconnections and the potential for structurally novel ecosystems. Nature Climate Change, 1497 2, 1–6. 1498 Mack MC, Bret-Harte MS, Hollingsworth TN, Jandt RR, Schuur EAG, Shaver GR, Verbyla DL 1499 (2011) Carbon loss from an unprecedented Arctic tundra wildfire. Nature Communications, 1500 475, 489–492. 1501 Mack MC, Treseder KK, Manies KL et al. (2008) Recovery of Aboveground Plant Biomass and 1502 Productivity After Fire in Mesic and Dry Black Spruce Forests of Interior Alaska. 1503 Ecosystems, 11, 209–225. 1504 Malhotra A, Roulet NT (2015) Environmental correlates of peatland carbon fluxes in a thawing 1505 landscape: do transitional thaw stages matter? Biogeosciences, 12, 3119–3130. 1506 Malmer N, Johansson T, Olsrud M, Christensen TR (2005) Vegetation, climatic changes and net 1507 carbon sequestration in a North-Scandinavian subarctic mire over 30 years. Global Change 1508 Biology, 11, 1895–1909. 1509 Mamet SD, Kershaw GP (2013) Multi-scale Analysis of Environmental Conditions and Conifer 1510 Seedling Distribution Across the Treeline Ecotone of Northern Manitoba, Canada. 1511 Ecosystems, 16, 295–309.

58 1512 Mamet SD, Chun KP, Kershaw GGL, Loranty MM, Peter Kershaw G (2017) Recent Increases in 1513 Permafrost Thaw Rates and Areal Loss of Palsas in the Western Northwest Territories, 1514 Canada. Permafrost and Periglacial Processes, 82, 45–15. 1515 Marsh P, Bartlett P, MackKay M, Pohl S, Lantz T (2010) Snowmelt energetics at a shrub tundra 1516 site in the western Canadian Arctic. Hydrological Processes, 24, 3603–3620. 1517 Ménard CB, Essery R, Pomeroy J (2014) Modelled sensitivity of the snow regime to topography, 1518 shrub fraction and shrub height. Hydrology and Earth System Sciences, 18, 2375–2392. 1519 Morse PD, Wolfe SA, Kokelj SV, Gaanderse AJR (2015) The Occurrence and Thermal 1520 Disequilibrium State of Permafrost in Forest Ecotopes of the Great Slave Region, Northwest 1521 Territories, Canada. Permafrost and Periglacial Processes, n/a–n/a. 1522 Mu CC, Abbott BW, Zhao Q et al. (2017) Permafrost collapse shifts alpine tundra to a carbon 1523 source but reduces N2O and CH4 release on the northern Qinghai-Tibetan Plateau. 1524 Geophysical Research Letters, 44, 8945–8952. 1525 Myers-Smith IH, Hik DS (2013) Shrub canopies influence soil temperatures but not nutrient 1526 dynamics: An experimental test of tundra snow-shrub interactions. Ecology and Evolution, 3, 1527 3683–3700. 1528 Myers-Smith IH, Arnesen BK, Thompson RM, Chapin FSI (2006) Cumulative impacts on 1529 Alaskan arctic tundra of a quarter century of road dust. Ecoscience, 13, 503–510. 1530 Myers-Smith IH, Elmendorf SC, Beck PSA et al. (2015) Climate sensitivity of shrub growth 1531 across the tundra biome. Nature Climate Change, 5, 887–891. 1532 Myneni R, Keeling C, Tucker C, Asrar G, Nemani R (1997) Increased plant growth in the 1533 northern high latitudes from 1981 to 1991. Nature Communications, 386, 698–701. 1534 Natali SM, Schuur EAG, Mauritz M et al. (2015) Permafrost thaw and soil moisture driving CO 1535 2 and CH 4 release from upland tundra. Journal of Geophysical Research Biogeosciences, 1536 120, 525–537. 1537 Natali SM, Schuur EAG, Trucco C, Hicks Pries CE, Crummer KG, Baron Lopez AF (2011) 1538 Effects of experimental warming of air, soil and permafrost on carbon balance in Alaskan 1539 tundra. Global Change Biology, 17, 1394–1407. 1540 Nauta AL, Heijmans MMPD, Blok D et al. (2014) Permafrost collapse after shrub removal shifts 1541 tundra ecosystem to a methane source. Nature Climate Change, 5, 67–70. 1542 Nauta AL, Heijmans MMPD, Blok D et al. (2015) Permafrost collapse after shrub removal shifts 1543 tundra ecosystem to a methane source. Nature Climate Change, 5, 67–70. 1544 Nossov DR, Torre Jorgenson M, Kielland K, Kanevskiy MZ (2013) Edaphic and microclimatic 1545 controls over permafrost response to fire in interior Alaska. Environmental Research Letters, 1546 8, 035013–13. 1547 O'Donnell JA, Harden JW, Mcguire AD, Romanovsky VE (2011a) Exploring the sensitivity of 1548 soil carbon dynamics to climate change, fire disturbance and permafrost thaw in a black 1549 spruce ecosystem. Biogeosciences, 8, 1367–1382. 1550 O'Donnell JA, Harden JW, Mcguire AD, Kanevskiy MZ, Jorgenson MT, Xu X (2011b) The 1551 effect of fire and permafrost interactions on soil carbon accumulation in an upland black 1552 spruce ecosystem of interior Alaska: implications for post‐thaw carbon loss. Global Change 1553 Biology, 17, 1461–1474. 1554 O'Donnell JA, Jorgenson MT, Harden JW, Mcguire AD, Kanevskiy MZ, Wickland KP (2012) 1555 The effects of permafrost thaw on soil hydrologic, thermal, and carbon dynamics in an 1556 Alaskan peatland. Ecosystems, 15, 213–229. 1557 O'Donnell JA, Romanovsky VE, Harden JW, Mcguire AD (2009a) The Effect of Moisture

59 1558 Content on the Thermal Conductivity of Moss and Organic Soil Horizons From Black 1559 Spruce Ecosystems in Interior Alaska. Soil Science, 174, 646–651. 1560 O'Donnell JA, Turetsky M, Harden J, Manies K, Pruett L, Shetler G, Neff J (2009b) Interactive 1561 Effects of Fire, Soil Climate, and Moss on CO 2 Fluxes in Black Spruce Ecosystems of 1562 Interior Alaska. Ecosystems, 12, 57–72. 1563 Olefeldt D, Goswami S, Grosse G, Hayes D (2016) Circumpolar distribution and carbon storage 1564 of thermokarst landscapes. Nature Communications, 7, 13043. 1565 Olefeldt D, Turetsky MR, Crill PM, Mcguire AD (2012) Environmental and physical controls on 1566 northern terrestrial methane emissions across permafrost zones. Global Change Biology, 19, 1567 589–603. 1568 Olofsson J (2006) Short- and long-term effects of changes in reindeer grazing pressure on tundra 1569 heath vegetation. Journal of Ecology, 94, 431–440. 1570 Olofsson J, Hulme PE, Oksanen L, Suominen O (2004a) Importance of large and small 1571 mammalian herbivores for the plant community structure in the forest tundra ecotone. Oikos, 1572 106, 324–334. 1573 Olofsson J, Kitti H, Rautiainen P, Stark S (2001) Effects of summer grazing by reindeer on 1574 composition of vegetation, productivity and nitrogen cycling. Ecography, 24, 13–24. 1575 Olofsson J, Oksanen L, Callaghan T, Hulme PE, Oksanen T, Suominen O (2009) Herbivores 1576 inhibit climate-driven shrub expansion on the tundra. Global Change Biology, 15, 2681– 1577 2693. 1578 Olofsson J, Stark S, Oksanen L (2004b) Reindeer influence on ecosystem processes in the 1579 tundra. Oikos, 105, 386–396. 1580 Osterkamp TE, Jorgenson MT, Schuur EAG, Shur YL, Kanevskiy MZ, Vogel JG, Tumskoy VE 1581 (2009) Physical and ecological changes associated with warming permafrost and thermokarst 1582 in Interior Alaska. Permafrost and Periglacial Processes, 20, 235–256. 1583 Osterkamp TE, Viereck L, Shur Y, Jorgenson MT, Racine C, Doyle A, Boone RD (2000) 1584 Observations of thermokarst and its impact on boreal forests in Alaska, USA. Arctic, 1585 Antarctic, and Alpine Research, 303–315. 1586 Outcalt SI, Nelson FE, Hinkel KM (1990) The zero‐curtain effect: Heat and mass transfer across 1587 an isothermal region in freezing soil. Water Resources Research, 26, 1509–1516. 1588 Park H, Walsh J, Fedorov AN, Sherstiukov AB, Iijima Y, Ohata T (2013) The influence of 1589 climate and hydrological variables on opposite anomaly in active-layer thickness between 1590 Eurasian and North American watersheds. The Cryosphere, 7, 631–645. 1591 Pearson RG, Phillips SJ, Loranty MM, Beck PSA, Damoulas T, Knight SJ, Goetz SJ (2013) 1592 Shifts in Arctic vegetation and associated feedbacks under climate change. Nature Climate 1593 Change, 3, 673–677. 1594 Peng C, Ma Z, Lei X et al. (2011) A drought-induced pervasive increase in tree mortality across 1595 Canada's boreal forests. Nature Climate Change, 1, 467–471. 1596 Phoenix GK, Bjerke JW (2016) Arctic browning: extreme events and trends reversing arctic 1597 greening. Global Change Biology, 22, 2960–2962. 1598 Plante S, Champagne E, Ropars P, Boudreau S, Lévesque E, Tremblay B, Tremblay J-P (2014) 1599 Shrub cover in northern Nunavik: can herbivores limit shrub expansion? Polar Biology, 37, 1600 611–619. 1601 Pomeroy JW, Bewley DS, Essery RLH et al. (2006) Shrub tundra snowmelt. Hydrological 1602 Processes, 20, 923–941. 1603 Ponomarev EI, Kharuk VI, Ranson KJ (2016) Wildfires dynamics in siberian larch forests.

60 1604 Forests, 7, 125. 1605 Poorter H, Niklas KJ, Reich PB, Oleksyn J, Poot P, Mommer L (2012) Biomass allocation to 1606 leaves, stems and roots: meta‐analyses of interspecific variation and environmental control. 1607 New Phytologist, 193, 30–50. 1608 Racine C, Jandt R, Meyers C, Dennis J (2004) Tundra fire and vegetation change along a 1609 hillslope on the Seward Peninsula, Alaska, USA. Arctic, Antarctic, and Alpine Research, 36, 1610 1–10. 1611 Radville L, McCormack ML, Post E, Eissenstat DM (2016) Root phenology in a changing 1612 climate. Journal Of Experimental Botany, erw062–12. 1613 Randerson JT, Liu H, Flanner MG et al. (2006) The Impact of Boreal Forest Fire on Climate 1614 Warming. Science, 314, 1130–1132. 1615 Rasmus S, Lundell R, Saarinen T (2011) Interactions between snow, canopy, and vegetation in a 1616 boreal coniferous forest. Plant Ecology & Diversity, 4, 55–65. 1617 Raynolds MK, Walker DA, Ambrosius KJ et al. (2014) Cumulative geoecological effects of 1618 62 years of infrastructure and climate change in ice-rich permafrost landscapes, Prudhoe Bay 1619 Oilfield, Alaska. Global Change Biology, 20, 1211–1224. 1620 Robinson SD, Moore TR (2000) The influence of permafrost and fire upon carbon accumulation 1621 in high boreal peatlands, Northwest Territories, Canada. Arctic, Antarctic, and Alpine 1622 Research, 32, 155. 1623 Rocha AV, Shaver GR (2011) Postfire energy exchange in arctic tundra: the importance and 1624 climatic implications of burn severity. Global Change Biology, 17, 2831–2841. 1625 Rocha AV, Loranty MM, Higuera PE et al. (2012) The footprint of Alaskan tundra fires during 1626 the past half-century: implications for surface properties and radiative forcing. 1627 Environmental Research Letters, 7, 044039. 1628 Romanovsky VE, Osterkamp TE (1995) Interannual variations of the thermal regime of the 1629 active layer and near‐surface permafrost in northern Alaska. Permafrost and Periglacial …. 1630 Romanovsky VE, Osterkamp TE (1997) Thawing of the active layer on the coastal plain of the 1631 Alaskan Arctic. Permafrost and Periglacial …. 1632 Romanovsky VE, Osterkamp TE (2000) Effects of unfrozen water on heat and mass transport 1633 processes in the active layer and permafrost. Permafrost and Periglacial Processes, 11, 219– 1634 239. 1635 Romanovsky VE, Smith SL, Christiansen HH (2010) Permafrost thermal state in the polar 1636 Northern Hemisphere during the international polar year 2007-2009: a synthesis. Permafrost 1637 and Periglacial Processes, 21, 106–116. 1638 Roy-Léveillée P, Burn CR, McDonald ID (2014) Vegetation-Permafrost Relations within the 1639 Forest-Tundra Ecotone near Old Crow, Northern Yukon, Canada. Permafrost and 1640 Periglacial Processes, 25, 127–135. 1641 Rydén BE, Kostov L (1980) Thawing and freezing in tundra soils. Ecological Bulletins. 1642 Schulze ED, Wirth C, Mollicone D et al. (2012) Factors promoting larch dominance in central 1643 Siberia: fire versus growth performance and implications for carbon dynamics at the 1644 boundary of evergreen and deciduous conifers. Biogeosciences, 9, 1405–1421. 1645 Schuur EAG, Abbott BW, Bowden WB et al. (2013) Expert assessment of vulnerability of 1646 permafrost carbon to climate change. Climatic Change, 119, 359–374. 1647 Schuur EAG, Crummer KG, Vogel JG, Mack MC (2007) Plant Species Composition and 1648 Productivity following Permafrost Thaw and Thermokarst in Alaskan Tundra. Ecosystems, 1649 10, 280–292.

61 1650 Schuur EAG, Mcguire AD, Schädel C et al. (2015) Climate change and the permafrost carbon 1651 feedback. Nature Communications, 520, 171–179. 1652 Shiklomanov NI, Streletskiy DA, Nelson FE et al. (2010) Decadal variations of active-layer 1653 thickness in moisture-controlled landscapes, Barrow, Alaska. Journal of Geophysical 1654 Research, 115, G00I04. 1655 Shur YL, Jorgenson MT (2007) Patterns of permafrost formation and degradation in relation to 1656 climate and ecosystems. Permafrost and Periglacial Processes, 18, 7–19. 1657 Sjöberg Y, Coon E, K Sannel AB et al. (2016) Thermal effects of groundwater flow through 1658 subarctic fens: A case study based on field observations and numerical modeling. Water 1659 Resources Research, 52, 1591–1606. 1660 Slater AG, Lawrence DM (2013) Diagnosing Present and Future Permafrost from Climate 1661 Models. Journal of Climate, 26, 5608–5623. 1662 Smith MW (1975) Microclimatic Influences on Ground Temperatures and Permafrost 1663 Distribution, Mackenzie Delta, Northwest Territories. Canadian Journal of Earth Sciences, 1664 12, 1421–1438. 1665 Sofronov M, Volokitina A (2010) Wildfire Ecology in Continuous Permafrost Zone. In: 1666 Permafrost Ecosystems Siberian Larch Forests (eds Osawa A, Zyryanova OA, Matsuura Y, 1667 Kajimoto T, Wein RW). Springer, New York. 1668 Soudzilovskaia NA, Van Bodegom PM, Cornelissen JHC (2013) Dominant bryophyte control 1669 over high-latitude soil temperature fluctuations predicted by heat transfer traits, field 1670 moisture regime and laws of thermal insulation (ed Schweitzer J). Functional Ecology, 27, 1671 1442–1454. 1672 Stiegler C, Johansson M, Christensen TR, Mastepanov M, Lindroth A (2016a) Tundra 1673 permafrost thaw causes significant shifts in energy partitioning. Tellus Series B-Chemical 1674 And Physical Meteorology, 68, 1–11. 1675 Stiegler C, Lund M, Christensen TR, Mastepanov M, Lindroth A (2016b) Two years with 1676 extreme and little snowfall: effects on energy partitioning and surface energy exchange in a 1677 high-Arctic tundra ecosystem. The Cryosphere, 10, 1395–1413. 1678 Stieglitz M (2003) The role of snow cover in the warming of arctic permafrost. Geophysical 1679 Research Letters, 30, 1–4. 1680 Stoy PC, Street LE, Johnson AV, Prieto-Blanco A, Ewing SA (2012) Temperature, Heat Flux, 1681 and Reflectance of Common Subarctic Mosses and Lichens under Field Conditions: Might 1682 Changes to Community Composition Impact Climate-Relevant Surface Fluxes? Arctic, 1683 Antarctic, and Alpine Research, 44, 500–508. 1684 Sturm M, Douglas T, Racine C, Liston GE (2005) Changing snow and shrub conditions affect 1685 albedo with global implications. Journal of Geophysical Research, 110, G01004. 1686 Sturm M, Holmgren J, Liston GE (1995) A seasonal snow cover classification system for local to 1687 global applications. Journal of Climate, 8, 1261–1283. 1688 Sturm M, McFadden J, Liston GE, Chapin FS III (2001) Snow–Shrub Interactions in Arctic 1689 Tundra: A Hypothesis with Climatic Implications. Journal of Climate. 1690 Swann AL, Fung IY, Levis S, Bonan GB, Doney SC (2010) Changes in Arctic vegetation 1691 amplify high-latitude warming through the greenhouse effect. Proceedings of the National 1692 Academy of Sciences, 107, 1295–1300. 1693 Tape K, Sturm M, Racine C (2006) The evidence for shrub expansion in Northern Alaska and 1694 the Pan-Arctic. Global Change Biology, 12, 686–702. 1695 Tchebakova N, Parfenova E, Soja A (2009) The effects of climate, permafrost and fire on

62 1696 vegetation change in Siberia in a changing climate. Environmental Research Letters, 4, 1697 045013. 1698 Turetsky MR, Kane ES, Harden JW, Ottmar RD, Manies KL, Hoy E, Kasischke ES (2011) 1699 Recent acceleration of biomass burning and carbon losses in Alaskan forests and peatlands. 1700 Nature Geoscience, 4, 27–31. 1701 Urban M, Forkel M, Schmullius C, Hese S, Hüttich C, Herold M (2013) Identification of land 1702 surface temperature and albedo trends in AVHRR Pathfinder data from 1982 to 2005 for 1703 northern Siberia. International Journal of Remote Sensing, 34, 4491–4507. 1704 van der Wal R, van Lieshout SMJ, Loonen MJJE (2001) Herbivore impact on moss depth, soil 1705 temperature and arctic plant growth. Polar Biology, 24, 29–32. 1706 Vavrek MC, Fetcher N, McGraw JB, Shaver GR, Chapin FS III, Bovard B (1999) Recovery of 1707 productivity and species diversity in tussock tundra following disturbance. Arctic, Antarctic, 1708 and Alpine Research, 254–258. 1709 Väisänen M, Ylänne H, Kaarlejärvi E, Sjögersten S, Olofsson J, Crout N, Stark S (2014) 1710 Consequences of warming on tundra carbon balance determined by reindeer grazing history. 1711 Nature Climate Change, 4, 384–388. 1712 Viereck LA, Werdin-Pfisterer NR, Adams PC, Yoshikawa K (2008) Effect of wildfire and 1713 fireline construction on the annual depth of thaw in a black spruce permafrost forest in 1714 interior Alaska: a 36-year record of recovery., pp. 1845–1850. 1715 Voigt C, Marushchak ME, Lamprecht RE et al. (2017) Increased nitrous oxide emissions from 1716 Arctic peatlands after permafrost thaw. Proceedings Of The National Academy Of Sciences 1717 Of The United States Of America, 114, 6238–6243. 1718 Wagner AM, Lindsey NJ, Dou S et al. (2018) Permafrost Degradation and Subsidence 1719 Observations during a Controlled Warming Experiment. Scientific Reports, 8, 106. 1720 Walker D, Everett K (1991) Loess ecosystems of northern Alaska: regional gradient and 1721 toposequence at Prudhoe Bay. Ecological Monographs, 437–464. 1722 Walker DA, Everett KR (1987) Road dust and its environmental impact on Alaskan taiga and 1723 tundra. Arctic and Alpine Research. 1724 Walker DA, Billings WD, De Molenaar JG (2001) Snow–vegetation interactions in tundra 1725 environments. Snow ecology: an interdisciplinary examination of snow-covered ecosystems, 1726 266–324. 1727 Walker DA, Webber PJ, Binnian EF, Everett KR, Lederer ND, Nordstrand EA, Walker MD 1728 (1987) Cumulative impacts of oil fields on northern Alaskan landscapes. Science, 238, 757– 1729 761. 1730 Walker M, Wahren C, Hollister R et al. (2006) Plant community responses to experimental 1731 warming across the tundra biome. Proceedings of the National Academy of Sciences, 103, 1732 1342–1346. 1733 Walker X, Johnstone JF (2014) Widespread negative correlations between black spruce growth 1734 and temperature across topographic moisture gradients in the boreal forest. 1–10. 1735 Walker XJ, Mack MC, Johnstone JF (2015) Stable carbon isotope analysis reveals widespread 1736 drought stress in boreal black spruce forests. Global Change Biology, 21, 3102–3113. 1737 Walter KM, Chanton JP, Chapin FS, Schuur EAG, Zimov SA (2008) Methane production and 1738 bubble emissions from arctic lakes: Isotopic implications for source pathways and ages. 1739 Journal of Geophysical Research, 113, G00A08. 1740 Walter KM, Smith LC, Stuart Chapin F (2007) Methane bubbling from northern lakes: present 1741 and future contributions to the global methane budget. Phiosophical Transactions of the

63 1742 Royal Society A, 365, 1657–1676. 1743 Webb EE, Heard K, Natali SM et al. (2017) Variability in Above and Belowground Carbon 1744 Stocks in a Siberian Larch Watershed. Biogeosciences Discussions, 1–39. 1745 Webster C, Rutter N, Zahner F, Jonas T (2016) Measurement of Incoming Radiation below 1746 Forest Canopies: A Comparison of Different Radiometer Configurations. Journal of 1747 Hydrometeorology, 17, 853–864. 1748 Welp LR, Patra PK, RÖDENBECK C, Nemani R, Bi J, Piper SC, Keeling RF (2016) Increasing 1749 summer net CO₂ uptake in high northern ecosystems inferred from atmospheric 1750 inversions and comparisons to remote-sensing NDVI. Atmospheric Chemistry and Physics, 1751 16, 9047–9066. 1752 Williamson SN, Barrio IC, Hik DS, Gamon JA (2016) Phenology and species determine 1753 growing-season albedo increase at the altitudinal limit of shrub growth in the sub-Arctic. 1754 Global Change Biology, 1–11. 1755 Woo M (1990) Consequences of climatic change for hydrology in permafrost zones. Journal of 1756 cold regions engineering, 4, 15–20. 1757 Woo M-K, Mollinga M, Smith SL (2007) Climate warming and active layer thaw in the boreal 1758 and tundra environments of the Mackenzie Valley. Canadian Journal of Earth Sciences, 44, 1759 733–743. 1760 Xue X, Peng F, You Q, Xu M, Dong S (2015) Belowground carbon responses to experimental 1761 warming regulated by soil moisture change in an alpine ecosystem of the Qinghai–Tibet 1762 Plateau. Ecology and Evolution, 5, 4063–4078. 1763 Yi S, Mcguire AD, Harden J et al. (2009) Interactions between soil thermal and hydrological 1764 dynamics in the response of Alaska ecosystems to fire disturbance. Journal of Geophysical 1765 Research, 114, 1–20. 1766 Yoshikawa K, Hinzman LD (2003) Shrinking thermokarst ponds and groundwater dynamics in 1767 discontinuous permafrost near council, Alaska. Permafrost and Periglacial Processes, 14, 1768 151–160. 1769 Yoshikawa K, Bolton WR, Romanovsky VE, Fukuda M, Hinzman LD (2003) Impacts of 1770 wildfire on the permafrost in the boreal forests of Interior Alaska. Journal of Geophysical 1771 Research, 108, 8148. 1772 Zamin TJ, Grogan P (2012) Birch shrub growth in the low Arctic: the relative importance of 1773 experimental warming, enhanced nutrient availability, snow depth and caribou exclusion. 1774 Environmental Research Letters, 7, 034027–10. 1775 Zeng Z, Piao S, Li LZX et al. (2017) Climate mitigation from vegetation biophysical feedbacks 1776 during the past three decades. Nature Climate Change, 351, 600–8. 1777 Zhang K, Kimball JS, Mu Q, Jones LA, Goetz SJ, Running SW (2009) Satellite based analysis of 1778 northern ET trends and associated changes in the regional water balance from 1983 to 2005. 1779 Journal of Hydrology, 379, 92–110. 1780 Zhang T (2005) Influence of the seasonal snow cover on the ground thermal regime: An 1781 overview. Reviews of Geophysics, 43, RG4002. 1782 Zhang T, Heginbottom JA, Barry RG, Brown J (2000) Further statistics on the distribution of 1783 permafrost and ground ice in the Northern Hemisphere. Polar Geography, 24, 126–131. 1784 Zimov SA, Zimov NS, Tikhonov AN, Chapin FS III (2012) Mammoth steppe: a high- 1785 productivity phenomenon. Quaternary Science Reviews, 57, 26–45. 1786 1787 Abbott BW, Jones JB (2015) Permafrost collapse alters soil carbon stocks, respiration, CH 4, and

64 1788 N 2O in upland tundra. Global Change Biology, 21, 4570–4587. 1789 Abbott BW, Jones JB, Godsey SE, Larouche JR, Bowden WB (2015) Patterns and persistence of 1790 hydrologic carbon and nutrient export from collapsing upland permafrost. Biogeosciences, 1791 12, 3725–3740. 1792 Abbott BW, Jones JB, Schuur EAG et al. (2016) Biomass offsets little or none of permafrost 1793 carbon release from soils, streams, and wildfire: an expert assessment. Environmental 1794 Research Letters, 11, 1–13. 1795 Ackerman D, Griffin D, Hobbie SE, Finlay JC (2017) Arctic shrub growth trajectories differ 1796 across soil moisture levels. Global Change Biology, 69, 130–9. 1797 Alexander HD, Mack MC (2015) A Canopy Shift in Interior Alaskan Boreal Forests: 1798 Consequences for Above-and Belowground Carbon and Nitrogen Pools during Post-fire 1799 Succession. Ecosystems. 1800 Alexander HD, Mack MC, Goetz SJ, Beck PSA, Belshe EF (2012a) Implications of increased 1801 deciduous cover on stand structure and aboveground carbon pools of Alaskan boreal forests. 1802 Ecosphere, 3, art45. 1803 Alexander HD, Mack MC, Goetz SJ et al. (2012b) Carbon Accumulation Patterns During Post- 1804 Fire Succession in Cajander Larch (Larix cajanderi) Forests of Siberia. Ecosystems, 15, 1805 1065–1082. 1806 Alexander HD, Natali SM, Loranty MM et al. (2018) Impacts of increased soil burn severity on 1807 larch forest regeneration on permafrost soils of far northeastern Siberia. Forest Ecology And 1808 Management, 417, 144–153. 1809 Algesten G, Sobek S, Bergström AK, Ågren A, Tranvik LJ, Jansson M (2004) Role of lakes for 1810 organic carbon cycling in the boreal zone. Global Change Biology, 10, 141–147. 1811 Anthony KMW, Zimov SA, Grosse G et al. (2014) A shift of thermokarst lakes from carbon 1812 sources to sinks during the Holocene epoch. Nature Communications, 511, 452–456. 1813 Auerbach NA, Walker MD, Walker DA (1997) Effects of Roadside Disturbance on Substrate 1814 and Vegetation Properties in Arctic Tundra. Ecological Applications, 7, 218–235. 1815 Baldocchi D, Kelliher F, Black T, Jarvis P (2000) Climate and vegetation controls on boreal zone 1816 energy exchange. Global Change Biology, 6, 69–83. 1817 Baltzer JL, Veness T, Chasmer LE, Sniderhan AE, Quinton WL (2014) Forests on thawing 1818 permafrost: fragmentation, edge effects, and net forest loss. Global Change Biology, 20, 1819 824–834. 1820 Barber VA, Juday GP, Finney BP (2000) Reduced growth of Alaskan white spruce in the 1821 twentieth century from temperature-induced drought stress. Nature Communications, 405, 1822 668–673. 1823 Bardgett RD, Mommer L, De Vries FT (2014) Going underground: root traits as drivers of 1824 ecosystem processes. Trends in Ecology & Evolution, 29, 692–699. 1825 Barrett K, Rocha AV, van de Weg MJ, Shaver G (2012) Vegetation shifts observed in arctic 1826 tundra 17 years after fire. Remote Sensing Letters, 3, 729–736. 1827 Beck PSA, Goetz SJ (2011) Satellite observations of high northern latitude vegetation 1828 productivity changes between 1982 and 2008: ecological variability and regional differences. 1829 Environmental Research Letters, 6 045501. 1830 Beck PSA, Juday GP, Alix C et al. (2011) Changes in forest productivity across Alaska 1831 consistent with biome shift. Ecology Letters, 14, 373–379. 1832 Beest te M, Sitters J, Ménard CB, Olofsson J (2016) Reindeer grazing increases summer albedo 1833 by reducing shrub abundance in Arctic tundra. Environmental Research Letters, 11, 125013–

65 1834 14. 1835 Belshe EF, Schuur EAG, Grosse G (2013) Quantification of upland thermokarst features with 1836 high resolution remote sensing. Environmental Research Letters, 8, 035016. 1837 Beringer J, Chapin FS, Thompson CC, Mcguire AD (2005) Surface energy exchanges along a 1838 tundra-forest transition and feedbacks to climate. Agricultural and Forest Meteorology, 131, 1839 143–161. 1840 Berner LT, Beck PSA, Bunn AG, Goetz SJ (2013) Plant response to climate change along the 1841 forest-tundra ecotone in northeastern Siberia. Global Change Biology, 19, 3449–3462. 1842 Berner LT, Beck PSA, Bunn AG, Lloyd AH, Goetz SJ (2011) High-latitude tree growth and 1843 satellite vegetation indices: Correlations and trends in Russia and Canada (1982–2008). 1844 Journal of Geophysical Research, 116, G01015. 1845 Berner LT, Beck PSA, Loranty MM, Alexander HD, Mack MC, Goetz SJ (2012) Cajander larch 1846 (Larix cajanderi) biomass distribution, fire regime and post-fire recovery in northeastern 1847 Siberia. Biogeosciences, 9, 3943–3959. 1848 Betts AK, Ball J (1997) Albedo over the boreal forest. Journal of Geophysical Research, 102, 1849 28901–28909. 1850 Betts AK, Goulden M, Wofsy S (1999) Controls on evaporation in a boreal spruce forest. 1851 Journal of Climate. 1852 Bewley D, Pomeroy J, Essery R (2007) Solar Radiation Transfer Through a Subarctic Shrub 1853 Canopy. Arctic, Antarctic, and Alpine Research, 39, 365–374. 1854 Bhatt U, Walker D, Raynolds M et al. (2013) Recent Declines in Warming and Vegetation 1855 Greening Trends over Pan-Arctic Tundra. Remote Sensing, 5, 4229–4254. 1856 Bjerke JW, Karlsen SR, Høgda KA et al. (2014) Record-low primary productivity and high plant 1857 damage in the Nordic Arctic Region in 2012 caused by multiple weather events and pest 1858 outbreaks. Environmental Research Letters, 9, 084006. 1859 Blok D, Heijmans M, Schaepman-Strub G, Kononov A, Maximov T, Berendse F (2010) Shrub 1860 expansion may reduce summer permafrost thaw in Siberian tundra. Global Change Biology, 1861 16, 1296–1305. 1862 Blok D, Heijmans MMPD, Schaepman-Strub G, Ruijven J, Parmentier FJW, Maximov TC, 1863 Berendse F (2011a) The Cooling Capacity of Mosses: Controls on Water and Energy Fluxes 1864 in a Siberian Tundra Site. Ecosystems, 14, 1055–1065. 1865 Blok D, Schaepman-Strub G, Bartholomeus H, Heijmans MM, Maximov TC, Berendse F 1866 (2011b) The response of Arctic vegetation to the summer climate: relation between shrub 1867 cover, NDVI, surface albedo and temperature. Environmental Research Letters, 6, 035502. 1868 Blume-Werry G, Wilson SD, Kreyling J, Milbau A (2015) The hidden season: growing season is 1869 50% longer below than above ground along an arctic elevation gradient. New Phytologist, 1870 209, 978–986. 1871 Boike J, Roth K, Overduin PP (1998) Thermal and hydrologic dynamics of the active layer at a 1872 continuous permafrost site (Taymyr Peninsula, Siberia). Water Resources Research. 1873 Boike J, Wille C, Abnizova A (2008) Climatology and summer energy and water balance of 1874 polygonal tundra in the Lena River Delta, Siberia. Journal of Geophysical Research, 113, 1875 G03025–15. 1876 Bond-Lamberty B, Rocha AV, Calvin K, Holmes B, Wang C, Goulden ML (2013) Disturbance 1877 legacies and climate jointly drive tree growth and mortality in an intensively studied boreal 1878 forest. Global Change Biology, 20, 216–227. 1879 Bonfils CJW, Phillips TJ, Lawrence DM, Cameron-Smith P, Riley WJ, Subin ZM (2012) On the

66 1880 influence of shrub height and expansion on northern high latitude climate. Environmental 1881 Research Letters, 7, 015503. 1882 Bret-Harte MS, Mack MC, Shaver GR et al. (2013) The response of Arctic vegetation and soils 1883 following an unusually severe tundra fire. Philosophical Transactions of the Royal Society 1884 B: Biological Sciences, 368, 20120490–20120490. 1885 Briggs MA, Walvoord MA, McKenzie JM (2014) New permafrost is forming around shrinking 1886 Arctic lakes, but will it last? Geophysical Research Letters, 41, 1585–1592. 1887 Brown D, Jorgenson MT, Douglas TA et al. (2015) Interactive effects of wildfire and climate on 1888 permafrost degradation in Alaskan lowland forests. Journal of Geophysical Research 1889 Biogeosciences, 120, 1619–1637. 1890 Brown J, Ferrians OJ, Heginbottom JA, Melinikov ES (1998) Circum-arctic map of permafrost 1891 and ground ice conditions. 1892 Bunn AG, Goetz SJ (2006) Trends in satellite-observed circumpolar photosynthetic activity from 1893 1982 to 2003: The influence of seasonality, cover type, and vegetation density. Earth 1894 Interactions, 10, 12. 1895 Burn CR (1998) The response (1958-1997) of permafrost and near-surface ground temperatures 1896 to forest fire, Takhini River valley, southern Yukon Territory. Canadian Journal of Earth 1897 Sciences, 35, 184–199. 1898 Cable WL, Romanovsky VE, Jorgenson MT (2016) Scaling-up permafrost thermal 1899 measurements in western Alaska using an ecotype approach. The Cryosphere, 10, 2517– 1900 2532. 1901 Chapin F, Sturm M, Serreze M et al. (2005) Role of land-surface changes in Arctic summer 1902 warming. Science, 310, 657. 1903 Chapin FS III, Eugster W, McFadden J, Lynch A, Walker D (2000a) Summer differences among 1904 arctic ecosystems in regional climate forcing. Journal of Climate, 13, 2002–2010. 1905 Chapin FS III, McGuire A, Randerson J et al. (2000b) Arctic and boreal ecosystems of western 1906 North America as components of the climate system. Global Change Biology, 6, 211–223. 1907 Christiansen CT, Mack MC, DeMarco J, Grogan P (2018) Decomposition of Senesced Leaf 1908 Litter is Faster in Tall Compared to Low Birch Shrub Tundra. Ecosystems, 170, 809–16. 1909 Commane R, Lindaas J, Benmergui J et al. (2017) Carbon dioxide sources from Alaska driven by 1910 increasing early winter respiration from Arctic tundra. Proceedings Of The National 1911 Academy Of Sciences Of The United States Of America, 114, 5361–5366. 1912 Cornelissen JH, Van Bodegom PM, Aerts R et al. (2007) Global negative vegetation feedback to 1913 climate warming responses of leaf litter decomposition rates in cold biomes. Ecology Letters, 1914 10, 619–627. 1915 Crampton CB (1977) A study of the dynamics of hummocky microrelief in the Canadian north. 1916 Canadian Journal of Earth Sciences, 14, 639–649. 1917 Curasi SR, Loranty MM, Natali SM (2016) Water track distribution and effects on carbon 1918 dioxide flux in an eastern Siberian upland tundra landscape. Environmental Research 1919 Letters, 11, 1–12. 1920 de Grandpré I, Fortier D, Stephani E (2012) Degradation of permafrost beneath a road 1921 embankment enhanced by heat advected in groundwater. Canadian Journal of Earth 1922 Sciences, 49, 953–962. 1923 Domine F, Barrere M, Sarrazin D, Morin S, Arnaud L (2015) Automatic monitoring of the 1924 effective thermal conductivity of snow in a low-Arctic shrub tundra. The Cryosphere, 9, 1925 1265–1276.

67 1926 Douglas TA, Jones MC, Hiemstra CA, Arnold JR (2014) Sources and sinks of carbon in boreal 1927 ecosystems of interior Alaska: A review. Elementa: Science of the Anthropocene, 2, 1928 000032–39. 1929 Douglas TA, Jorgenson MT, Brown DRN et al. (2016) Degrading permafrost mapped with 1930 electrical resistivity tomography, airborne imagery and LiDAR, and seasonal thaw 1931 measurements. GEOPHYSICS, 81, WA71–WA85. 1932 Eaton AK, Rouse WR, Lafleur PM, Marsh P, Blanken PD (2001) Surface Energy Balance of the 1933 Western and Central Canadian Subarctic: Variations in the Energy Balance among Five 1934 Major Terrain Types. Journal of Climate, 14, 3692–3703. 1935 Elmendorf SC, Henry GHR, Hollister RD et al. (2012a) Global assessment of experimental 1936 climate warming on tundra vegetation: heterogeneity over space and time. Ecology Letters, 1937 15, 164–175. 1938 Elmendorf SC, Henry GHR, Hollister RD et al. (2012b) Plot-scale evidence of tundra vegetation 1939 change and links to recent summer warming. Nature Climate Change, 2, 1–5. 1940 Essery R, Pomeroy J (2004) Vegetation and topographic control of wind-blown snow 1941 distributions in distributed and aggregated simulations for an Arctic tundra basin. Journal of 1942 Hydrometeorology, 5, 735–744. 1943 Eugster W, Rouse W, Pielke R Sr et al. (2000) Land-atmosphere energy exchange in Arctic 1944 tundra and boreal forest: available data and feedbacks to climate. Global Change Biology, 6, 1945 84–115. 1946 Fan Z, Neff JC, Harden JW et al. (2011) Water and heat transport in boreal soils: Implications 1947 for soil response to climate change. Science of the Total Environment, The, 409, 1836–1842. 1948 Fauria MM, Helle T, Niva A (2008) Removal of the lichen mat by reindeer enhances tree growth 1949 in a northern Scots pine forest. Canadian journal of …. 1950 Fedorov AN, Iwahana G, Konstantinov PY et al. (2016) Variability of Permafrost and Landscape 1951 Conditions Following Clear Cutting of Larch Forest in Central Yakutia. Permafrost and 1952 Periglacial Processes, 1–8. 1953 Filhol S, Sturm M (2015) Snow bedforms: A review, new data, and a formation model. Journal 1954 of Geophysical Research: Earth Surface, 120, 1645–1669. 1955 Fisher JP, Estop Aragonés C, Thierry A et al. (2016) The influence of vegetation and soil 1956 characteristics on active-layer thickness of permafrost soils in boreal forest. Global Change 1957 Biology, 3127–3140. 1958 Forbes BC (1993) Aspects of natural recovery of soils, hydrology and vegetation at an 1959 abandoned high arctic settlement, Baffin Island, Canada. 1, 176–181. 1960 Forbes BC (1998) Cumulative impacts of vehicle traffic on high arctic tundra: soil temperature, 1961 plant biomass, species richness and mineral nutrition. Yellowknife, CA. 1962 Forbes BC (1995) Tundra disturbance studies, III: Short-term effects of Aeolian sand and dust, 1963 Yamal Region, Northwest Siberia. Environmental Conservation, 22, 335–344. 1964 Forbes BC, Kumpula T (2009) The Ecological Role and Geography of Reindeer ( Rangifer 1965 tarandus) in Northern Eurasia. Geography Compass, 3, 1356–1380. 1966 Forbes BC, Ebersole JJ, Strandberg B (2001) Anthropogenic disturbance and patch dynamics in 1967 circumpolar arctic ecosystems. Conservation Biology, 15, 954–969. 1968 Forbes BC, Fauria MM, Zetterberg P (2010) Russian Arctic warming and “greening” are closely 1969 tracked by tundra shrub willows. Global Change Biology, 16, 1542–1554. 1970 Forkel M, Carvalhais N, Roedenbeck C et al. (2016) Enhanced seasonal CO2 exchange caused 1971 by amplified plant productivity in northern ecosystems. Science, 351, 696–699.

68 1972 Francis JA, White DM, Cassano JJ et al. (2009) An arctic hydrologic system in transition: 1973 Feedbacks and impacts on terrestrial, marine, and human life. Journal of Geophysical 1974 Research, 114, G04019. 1975 French NH, Whitley MA, Jenkins LK (2016) Fire disturbance effects on land surface albedo in 1976 Alaskan tundra. Journal of Geophysical Research Biogeosciences, 121, 841–854. 1977 Froese DG, Westgate JA, Reyes AV, Enkin RJ, Preece SJ (2008) Ancient Permafrost and a 1978 Future, Warmer Arctic. Science, 321, 1648–1648. 1979 Frolking S, Roulet N, Fuglestvedt J (2006) How northern peatlands influence the Earth's 1980 radiative budget: Sustained methane emission versus sustained carbon sequestration. Journal 1981 of Geophysical Research, 111, G01008–10. 1982 Frost GV, Epstein HE (2014) Tall shrub and tree expansion in Siberian tundra ecotones since the 1983 1960s. Global Change Biology, 20, 1264–1277. 1984 Furayev V, Vaganov EA, Tchebakova NM, Valendik EN (2001) Effects of Fire and Climate on 1985 Successions and Structural Changes in The Siberian Boreal Forest. Eurasian Journal of 1986 Forest Research, 2, 1–15. 1987 Gamon JA, Kershaw GP, Williamson S, Hik DS (2012) Microtopographic patterns in an arctic 1988 baydjarakh field: do fine-grain patterns enforce landscape stability? Environmental Research 1989 Letters, 7, 015502. 1990 Genet H, Mcguire AD, Barrett K et al. (2013) Modeling the effects of fire severity and climate 1991 warming on active layer thickness and soil carbon storage of black spruce forests across the 1992 landscape in interior Alaska. Environmental Research Letters, 8, 045016. 1993 Gill HK, Lantz TC, O'Neill B, Kokelj SV (2014) Cumulative Impacts and Feedbacks of a Gravel 1994 Road on Shrub Tundra Ecosystems in the Peel Plateau, Northwest Territories, Canada. 1995 Arctic, Antarctic, and Alpine Research, 46, 947–961. 1996 Goodrich LE (1982) The influence of snow cover on the ground thermal regime. Canadian 1997 Geotechnical Journal, 19, 421–432. 1998 Gouttevin I, Ménégoz M, Domine F et al. (2012) How the insulating properties of snow affect 1999 soil carbon distribution in the continental pan-Arctic area. Journal of Geophysical Research, 2000 117, n/a–n/a. 2001 Göckede M, Kittler F, Kwon MJ et al. (2017) Shifted energy fluxes, increased Bowen ratios, and 2002 reduced thaw depths linked with drainage-induced changes in permafrost ecosystem 2003 structure. The Cryosphere, 11, 2975–2996. 2004 Graven HD, Keeling RF, Piper SC et al. (2013) Enhanced Seasonal Exchange of CO2 by 2005 Northern Ecosystems Since 1960. Science, 341, 1085–1089. 2006 Guay KC, Beck PSA, Berner LT, Goetz SJ, Baccini A, Buermann W (2014) Vegetation 2007 productivity patterns at high northern latitudes: a multi-sensor satellite data assessment. 2008 Global Change Biology, 20, 3147–3158. 2009 Halsey LA, Vitt DH, Zoltai SC (1995) Disequilibrium response of permafrost in boreal 2010 continental western Canada to climate change. Climatic Change, 30, 57–73. 2011 Harden JW, Trumbore SE, Stocks BJ, Hirsch A, Gower ST, O'neill KP, Kasischke ES (2000) 2012 The role of fire in the boreal carbon budget. Global Change Biology, 6, 174–184. 2013 Harms TK, Abbott BW, Jones JB (2014) Thermo-erosion gullies increase nitrogen available for 2014 hydrologic export. Biogeochemistry, 117, 299–311. 2015 Hayes DJ, Mcguire AD, Kicklighter DW, Gurney KR, Burnside TJ, Melillo JM (2011) Is the 2016 northern high-latitude land-based CO 2sink weakening? 25, n/a–n/a. 2017 Heijmans MM, Arp WJ, Chapin FS (2004a) Controls on moss evaporation in a boreal black

69 2018 spruce forest. 18. 2019 Heijmans MMPD, Arp WJ, Chapin FS III (2004b) Carbon dioxide and water vapour exchange 2020 from understory species in boreal forest. Agricultural and Forest Meteorology, 123, 135– 2021 147. 2022 Helbig M, Pappas C, Sonnentag O (2016a) Permafrost thaw and wildfire: Equally important 2023 drivers of boreal tree cover changes in the Taiga Plains, Canada. Geophysical Research 2024 Letters. 2025 Helbig M, Wischnewski K, Kljun N, Chasmer LE, Quinton WL, Detto M, Sonnentag O (2016b) 2026 Regional atmospheric cooling and wetting effect of permafrost thaw-induced boreal forest 2027 loss. Global Change Biology, 22, 4048–4066. 2028 Hinkel KM, Nelson FE (2003) Spatial and temporal patterns of active layer thickness at 2029 Circumpolar Active Layer Monitoring (CALM) sites in northern Alaska, 1995–2000. 2030 Journal of Geophysical Research, 108. 2031 Hinkel KM, Outcalt SI (1994) Identification of heat-transfer processes during soil cooling, 2032 freezing, and thaw in central Alaska. Permafrost and Periglacial Processes, 5, 217–235. 2033 Hinkel KM, Paetzold F, Nelson FE, Bockheim JG (2001) Patterns of soil temperature and 2034 moisture in the active layer and upper permafrost at Barrow, Alaska: 1993–1999. Global and 2035 Planetary Change, 29, 293–309. 2036 Hinzman LD, Kane DL, Gieck RE, Everett KR (1991) Hydrologic and thermal properties of the 2037 active layer in the Alaskan Arctic. Cold Regions Science and Technology, 19, 95–110. 2038 Hobbie S (1996) Temperature and Plant Species Control Over Litter Decomposition in Alaskan 2039 Tundra. Ecological Monographs, 66, 503–522. 2040 Hobbie SE, Gough L (2004) Litter decomposition in moist acidic and non-acidic tundra with 2041 different glacial histories. Oecologia, 140, 113–124. 2042 Hu FS, Higuera PE, Walsh JE, Chapman WL, Duffy PA, Brubaker LB, Chipman ML (2010) 2043 Tundra burning in Alaska: Linkages to climatic change and sea ice retreat. Journal of 2044 Geophysical Research, 115, G04002. 2045 Huntington H, Arnbom T, Danielson F et al. (2013) Disturbance, feedbacks and conservation. In: 2046 Arctic Biodiversity Assessment Status and trends in Arctic biodiversity. Akureyri: 2047 Conservation of Arctic Flora and Fauna. 2048 Iijima Y, Fedorov AN, Park H, Suzuki K, Yabuki H, Maximov TC, Ohata T (2010) Abrupt 2049 increases in soil temperatures following increased precipitation in a permafrost region, 2050 central Lena River basin, Russia. Permafrost and Periglacial Processes, 21, 30–41. 2051 Iijima Y, Ohta T, Kotani A, Fedorov AN, Kodama Y, Maximov TC (2014) Sap flow changes in 2052 relation to permafrost degradation under increasing precipitation in an eastern Siberian larch 2053 forest. Ecohydrology, 7, 177–187. 2054 Iversen CM, Sloan VL, Sullivan PF et al. (2015) The unseen iceberg: plant roots in arctic tundra. 2055 New Phytologist, 205, 34–58. 2056 Iwahana G, Machimura T, Kobayashi Y (2005) Influence of forest clear‐cutting on the thermal 2057 and hydrological regime of the active layer near Yakutsk, eastern Siberia. Journal of 2058 Geophysical Research: Earth Surface. 2059 Jafarov EE, Romanovsky VE, Genet H, David McGuire A, Marchenko SS (2013) The effects of 2060 fire on the thermal stability of permafrost in lowland and upland black spruce forests of 2061 interior Alaska in a changing climate. Environmental Research Letters, 8, 035030. 2062 Jean M, Payette S (2014a) Dynamics of active layer in wooded palsas of northern Quebec. 2063 Geomorphology, 206, 87–96.

70 2064 Jean M, Payette S (2014b) Effect of Vegetation Cover on the Ground Thermal Regime of 2065 Wooded and Non-Wooded Palsas. Permafrost and Periglacial Processes, 25, 281–294. 2066 Jia G, Epstein H, Walker D (2003) Greening of arctic Alaska, 1981–2001. Geophysical Research 2067 Letters, 30, 2067. 2068 Jiang Y, Rocha AV, O'Donnell JA, Drysdale JA, Rastetter EB, Shaver GR, Zhuang Q (2015) 2069 Contrasting soil thermal responses to fire in Alaskan tundra and boreal forest. Journal of 2070 Geophysical Research: Earth Surface, 120, 363–378. 2071 Jin Y, Randerson JT, Goetz SJ, Beck PSA, Loranty MM, Goulden ML (2012) The influence of 2072 burn severity on postfire vegetation recovery and albedo change during early succession in 2073 North American boreal forests. Journal of Geophysical Research, 117. 2074 Jin Y, SCHAAF C, Gao F, Li X, STRAHLER A, Zeng X, Dickinson R (2002) How does snow 2075 impact the albedo of vegetated land surfaces as analyzed with MODIS data? Geophysical 2076 Research Letters, 29, 12–11. 2077 Johansen O (1977) Thermal conductivity of soils. 2078 Johansson T, Malmer N, Crill PM, Friborg T, ÅKERMAN JH, Mastepanov M, Christensen TR 2079 (2006) Decadal vegetation changes in a northern peatland, greenhouse gas fluxes and net 2080 radiative forcing. Global Change Biology, 12, 2352–2369. 2081 Johnstone JF, Hollingsworth TN, Chapin FS III, Mack MC (2010) Changes in fire regime break 2082 the legacy lock on successional trajectories in Alaskan boreal forest. Global Change Biology, 2083 16, 1281–1295. 2084 Jones BM, Breen AL, Gaglioti BV et al. (2013) Identification of unrecognized tundra fire events 2085 on the north slope of Alaska. Journal of Geophysical Research Biogeosciences, 118, 1334– 2086 1344. 2087 Jones BM, Grosse G, Arp CD, Miller E, Liu L, Hayes DJ, Larsen CF (2015) Recent Arctic 2088 tundra fire initiates widespread thermokarst development. Scientific Reports, 5, 15865. 2089 Jones BM, Kolden CA, Jandt R, Abatzoglou JT, Urban F, Arp CD (2009) Fire Behavior, 2090 Weather, and Burn Severity of the 2007 Anaktuvuk River Tundra Fire, North Slope, Alaska. 2091 Arctic, Antarctic, and Alpine Research, 41, 309–316. 2092 Jones MC, Grosse G, Jones BM, Walter Anthony K (2012) Peat accumulation in drained 2093 thermokarst lake basins in continuous, ice-rich permafrost, northern Seward Peninsula, 2094 Alaska. Journal of Geophysical Research, 117, n/a–n/a. 2095 Jorgensen CJ, Johansen KML, Westergaard-Nielsen A, Elberling B (2015) Net regional methane 2096 sink in High Arctic soils of northeast Greenland. Nature Geoscience, 8, 20–23. 2097 Jorgenson MT, Osterkamp TE (2005) Response of boreal ecosystems to varying modes of 2098 permafrost degradation. Canadian Journal of Forest Research, 35, 2100–2111. 2099 Jorgenson MT, HARDEN J, Kanevskiy M (2013) Reorganization of vegetation, hydrology and 2100 soil carbon after permafrost degradation across heterogeneous boreal landscapes. 2101 Environmental Research Letters. 2102 Jorgenson MT, Racine CH, Walters JC, Osterkamp TE (2001) Permafrost degradation and 2103 ecological changes associated with a warmingclimate in central Alaska. Climatic Change, 2104 48, 551–579. 2105 Jorgenson MT, Shur YL, Pullman ER (2006) Abrupt increase in permafrost degradation in 2106 Arctic Alaska. Geophysical Research Letters, 33, L02503. 2107 Jorgenson, Romanovsky V, Harden J et al. (2010) Resilience and vulnerability of permafrost to 2108 climate change. Candian Journal of Forest Research, 40, 1219–1236. 2109 Juday GP, Alix C, Grant TA III (2015) Spatial coherence and change of opposite white spruce

71 2110 temperature sensitivities on floodplains in Alaska confirms early-stage boreal biome shift. 2111 Forest Ecology And Management, 350, 46–61. 2112 Juszak I, Eugster W, Heijmans MMPD, Schaepman-Strub G (2016) Contrasting radiation and 2113 soil heat fluxes in Arctic shrub and wet sedge tundra. Biogeosciences, 13, 4049–4064. 2114 Kane DL, Hinkel KM, Goering DJ, Hinzman LD, Outcalt SI (2001) Non-conductive heat 2115 transfer associated with frozen soils. Global and Planetary Change, 29, 275–292. 2116 Kane DL, Hinzman LD, Benson CS, Everett KR (1989) Hydrology of Imnavait Creek, an arctic 2117 watershed. Ecography, 12, 262–269. 2118 Kane ES, Kasischke ES, Valentine DW, Turetsky MR, Mcguire AD (2007) Topographic 2119 influences on wildfire consumption of soil organic carbon in interior Alaska: Implications 2120 for black carbon accumulation. Journal of Geophysical Research, 112, n/a–n/a. 2121 Kasischke E, Johnstone J (2005) Variation in postfire organic layer thickness in a black spruce 2122 forest complex in interior Alaska and its effects on soil temperature and moisture. Canadian 2123 Journal of Forest Research, 35, 2164–2177. 2124 Kasischke E, Turetsky M (2006) Recent changes in the fire regime across the North American 2125 boreal region-spatial and temporal patterns of burning across Canada and Alaska. 2126 Geophysical Research Letters, 33, L09703. 2127 Kasurinen V, Alfredsen K, Kolari P et al. (2014) Latent heat exchange in the boreal and arctic 2128 biomes. Global Change Biology, 20, 3439–3456. 2129 Kelly R, Chipman ML, Higuera PE, Stefanova I, Brubaker LB, Hu FS (2013) Recent burning of 2130 boreal forests exceeds fire regime limits of the past 10,000 years. Proceedings of the 2131 National Academy of Sciences, 110, 13055–13060. 2132 Kershaw GP (2001) Snowpack Characteristics Following Wildfire on a Simulated Transport 2133 Corridor and Adjacent Subarctic Forest, Tulita, N.W.T., Canada. Arctic, Antarctic, and 2134 Alpine Research, 33, 131. 2135 Kershaw GP, McCulloch J (2007) Midwinter Snowpack Variation Across the Arctic Treeline, 2136 Churchill, Manitoba, Canada. Arctic, Antarctic, and Alpine Research, 39, 9–15. 2137 Keuper F, Bodegom PM, Dorrepaal E, Weedon JT, Hal J, Logtestijn RSP, Aerts R (2012) A 2138 frozen feast: thawing permafrost increases plant-available nitrogen in subarctic peatlands. 2139 Global Change Biology, 18, 1998–2007. 2140 Kharuk VI, Dvinskaya ML, Ranson KJ (2013) Fire return intervals within the northern boundary 2141 of the larch forest in Central Siberia. International Journal of Wildland Fire, 22, 207–6. 2142 Kharuk VI, Ranson KJ, Dvinskaya ML (2008) Wildfires dynamic in the larch dominance zone. 2143 Geophysical Research Letters, 35, L01402–6. 2144 Kholodov A, Gilichinsky D, Ostroumov V, Sorokovikov VA, Abramov AA, Davydov S, 2145 Romanovsky V (2012) Regional and local variability of modern natural changes in 2146 permafrost temperature in the Yakutian coastal lowlands, Northeastern Siberia. 2147 Kling GW, Kipphut GW, Miller MC (1991) Arctic lakes and streams as gas conduits to the 2148 atmosphere: implications for tundra carbon budgets. Science, 251, 298–301. 2149 Kokelj SV, Jorgenson MT (2013) Advances in Thermokarst Research. Permafrost and 2150 Periglacial Processes, 24, 108–119. 2151 Koven CD, Ringeval B, Friedlingstein P et al. (2011) Permafrost carbon-climate feedbacks 2152 accelerate global warming. Proceedings of the National Academy of Sciences, 108, 14769– 2153 14774. 2154 Kropp H, Loranty M, Alexander HD, Berner LT, Natali SM, Spawn SA (2017) Environmental 2155 constraints on transpiration and stomatal conductance in a Siberian Arctic boreal forest.

72 2156 Journal of Geophysical Research Biogeosciences, 209, 41–11. 2157 Kukavskaya EA, Soja AJ, Petkov AP, Ponomarev EI, Ivanova GA, Conard SG (2012) Fire 2158 emissions estimates in Siberia: evaluation of uncertainties in area burned, land cover, and 2159 fuel consumption. Canadian Journal of Forest Research, 43, 493–506. 2160 Landhäusser SM, Wein RW (1993) Postfire Vegetation Recovery and Tree Establishment at the 2161 Arctic Treeline: Climate-Change-Vegetation-Response Hypotheses. The Journal of Ecology, 2162 81, 665. 2163 Langer M, Westermann S, Muster S, Piel K, Boike J (2011a) The surface energy balance of a 2164 polygonal tundra site in northern Siberia – Part 1: Spring to fall. The Cryosphere, 5, 151– 2165 171. 2166 Langer M, Westermann S, Muster S, Piel K, Boike J (2011b) The surface energy balance of a 2167 polygonal tundra site in northern Siberia - Part 2: Winter. The Cryosphere, 5, 509–524. 2168 Lantz TC, Kokelj SV (2008) Increasing rates of retrogressive thaw slump activity in the 2169 Mackenzie Delta region, N.W.T., Canada. Geophysical Research Letters, 35, L06502. 2170 Lantz TC, Marsh P, Kokelj SV (2013) Recent shrub proliferation in the Mackenzie Delta uplands 2171 and microclimatic implications. Ecosystems, 16, 47–59. 2172 Lara MJ, Genet H, McGuire AD et al. (2016) Thermokarst rates intensify due to climate change 2173 and forest fragmentation in an Alaskan boreal forest lowland. Global Change Biology, 22, 2174 816–829. 2175 Lawrence DM, Swenson SC (2011) Permafrost response to increasing Arctic shrub abundance 2176 depends on the relative influence of shrubs on local soil cooling versus large-scale climate 2177 warming. Environmental Research Letters, 6, 045504. 2178 Lee X, Goulden ML, Hollinger DY, Barr A, Black TA (2011) Observed increase in local cooling 2179 effect of deforestation at higher latitudes. Nature Communications. 2180 Leibman M, Khomutov A, Kizyakov A (2014) Cryogenic landslides in the West-Siberian plain 2181 of Russia: classification, mechanisms, and landforms. In: Landslides in Cold Regions in the 2182 Context of Climate Change (eds Shan W, Guo Y, Mauri H, Strom A), pp. 143–162. Springer. 2183 Li Y, Zhao M, Motesharrei S, Mu Q, Kalnay E, Li S (2015) Local cooling and warming effects 2184 of forests based on satellite observations. Nature Communications, 6, 1–8. 2185 Liljedahl A, Hinzman L, Busey R, Yoshikawa K (2007) Physical short-term changes after a 2186 tussock tundra fire, Seward Peninsula, Alaska. Journal of Geophysical Research, 112, 165– 2187 13. 2188 Liljedahl AK, Boike J, Daanen RP et al. (2016) Pan-Arctic ice-wedge degradation in warming 2189 permafrost and its influence on tundra hydrology. Nature Geoscience, 9, 312–318. 2190 Liston GE, McFadden J, Sturm M, Pielke R (2002) Modelled changes in arctic tundra snow, 2191 energy and moisture fluxes due to increased shrubs. Global Change Biology. 2192 Lloyd AH, Bunn AG, BERNER L (2010) A latitudinal gradient in tree growth response to 2193 climate warming in the Siberian taiga. Global Change Biology, 17, 1935–1945. 2194 Lopez C ML, Saito H, Kobayashi Y, Shirota T, Iwahana G, Maximov TC, Fukuda M (2007) 2195 Interannual environmental-soil thawing rate variation and its control on transpiration from 2196 Larix cajanderi, Central Yakutia, Eastern Siberia. Journal of Hydrology, 338, 251–260. 2197 Loranty MM, Berner LT, Goetz SJ, Jin Y, Randerson JT (2014a) Vegetation controls on northern 2198 high latitude snow-albedo feedback: observations and CMIP5 model simulations. Global 2199 Change Biology, 20, 594–606. 2200 Loranty MM, Berner LT, Taber ED et al. (2018) Understory vegetation mediates permafrost 2201 active layer dynamics and carbon dioxide fluxes in open-canopy larch forests of northeastern

73 2202 Siberia (ed Rinnan R). PLoS ONE, 13, e0194014–17. 2203 Loranty MM, Goetz SJ, Beck PSA (2011) Tundra vegetation effects on pan-Arctic albedo. 2204 Environmental Research Letters, 6, 024014. 2205 Loranty MM, Liberman-Cribbin W, Berner LT, Natali SM, Goetz SJ, Alexander HD, Kholodov 2206 AL (2016) Spatial variation in vegetation productivity trends, fire disturbance, and soil 2207 carbon across arctic-boreal permafrost ecosystems. Environmental Research Letters, 11, 1– 2208 13. 2209 Loranty MM, Natali SM, Berner LT et al. (2014b) Siberian tundra ecosystem vegetation and 2210 carbon stocks four decades after wildfire. Journal of Geophysical Research Biogeosciences, 2211 119, 2144–2154. 2212 Lynch LM, Machmuller MB, Cotrufo MF, Paul EA, Wallenstein MD (2018) Tracking the fate of 2213 fresh carbon in the Arctic tundra: Will shrub expansion alter responses of soil organic matter 2214 to warming? Soil Biology and Biochemistry, 120, 134–144. 2215 Lyons EA, Jin Y, Randerson JT (2008) Changes in surface albedo after fire in boreal forest 2216 ecosystems of interior Alaska assessed using MODIS satellite observations. Journal of 2217 Geophysical Research, 113, 1–15. 2218 Macias-Fauria M, Forbes BC, Zetterberg P, Kumpula T (2012) Eurasian Arctic greening reveals 2219 teleconnections and the potential for structurally novel ecosystems. Nature Climate Change, 2220 2, 1–6. 2221 Mack MC, Bret-Harte MS, Hollingsworth TN, Jandt RR, Schuur EAG, Shaver GR, Verbyla DL 2222 (2011) Carbon loss from an unprecedented Arctic tundra wildfire. Nature Communications, 2223 475, 489–492. 2224 Mack MC, Treseder KK, Manies KL et al. (2008) Recovery of Aboveground Plant Biomass and 2225 Productivity After Fire in Mesic and Dry Black Spruce Forests of Interior Alaska. 2226 Ecosystems, 11, 209–225. 2227 Malhotra A, Roulet NT (2015) Environmental correlates of peatland carbon fluxes in a thawing 2228 landscape: do transitional thaw stages matter? Biogeosciences, 12, 3119–3130. 2229 Malmer N, Johansson T, Olsrud M, Christensen TR (2005) Vegetation, climatic changes and net 2230 carbon sequestration in a North-Scandinavian subarctic mire over 30 years. Global Change 2231 Biology, 11, 1895–1909. 2232 Mamet SD, Kershaw GP (2013) Multi-scale Analysis of Environmental Conditions and Conifer 2233 Seedling Distribution Across the Treeline Ecotone of Northern Manitoba, Canada. 2234 Ecosystems, 16, 295–309. 2235 Mamet SD, Chun KP, Kershaw GGL, Loranty MM, Peter Kershaw G (2017) Recent Increases in 2236 Permafrost Thaw Rates and Areal Loss of Palsas in the Western Northwest Territories, 2237 Canada. Permafrost and Periglacial Processes, 82, 45–15. 2238 Marsh P, Bartlett P, MackKay M, Pohl S, Lantz T (2010) Snowmelt energetics at a shrub tundra 2239 site in the western Canadian Arctic. Hydrological Processes, 24, 3603–3620. 2240 Ménard CB, Essery R, Pomeroy J (2014) Modelled sensitivity of the snow regime to topography, 2241 shrub fraction and shrub height. Hydrology and Earth System Sciences, 18, 2375–2392. 2242 Morse PD, Wolfe SA, Kokelj SV, Gaanderse AJR (2015) The Occurrence and Thermal 2243 Disequilibrium State of Permafrost in Forest Ecotopes of the Great Slave Region, Northwest 2244 Territories, Canada. Permafrost and Periglacial Processes, n/a–n/a. 2245 Mu CC, Abbott BW, Zhao Q et al. (2017) Permafrost collapse shifts alpine tundra to a carbon 2246 source but reduces N2O and CH4 release on the northern Qinghai-Tibetan Plateau. 2247 Geophysical Research Letters, 44, 8945–8952.

74 2248 Myers-Smith IH, Hik DS (2013) Shrub canopies influence soil temperatures but not nutrient 2249 dynamics: An experimental test of tundra snow-shrub interactions. Ecology and Evolution, 3, 2250 3683–3700. 2251 Myers-Smith IH, Arnesen BK, Thompson RM, Chapin FSI (2006) Cumulative impacts on 2252 Alaskan arctic tundra of a quarter century of road dust. Ecoscience, 13, 503–510. 2253 Myers-Smith IH, Elmendorf SC, Beck PSA et al. (2015) Climate sensitivity of shrub growth 2254 across the tundra biome. Nature Climate Change, 5, 887–891. 2255 Myneni R, Keeling C, Tucker C, Asrar G, Nemani R (1997) Increased plant growth in the 2256 northern high latitudes from 1981 to 1991. Nature Communications, 386, 698–701. 2257 Natali SM, Schuur EAG, Mauritz M et al. (2015) Permafrost thaw and soil moisture driving CO 2258 2 and CH 4 release from upland tundra. Journal of Geophysical Research Biogeosciences, 2259 120, 525–537. 2260 Natali SM, Schuur EAG, Trucco C, Hicks Pries CE, Crummer KG, Baron Lopez AF (2011) 2261 Effects of experimental warming of air, soil and permafrost on carbon balance in Alaskan 2262 tundra. Global Change Biology, 17, 1394–1407. 2263 Nauta AL, Heijmans MMPD, Blok D et al. (2014) Permafrost collapse after shrub removal shifts 2264 tundra ecosystem to a methane source. Nature Climate Change, 5, 67–70. 2265 Nauta AL, Heijmans MMPD, Blok D et al. (2015) Permafrost collapse after shrub removal shifts 2266 tundra ecosystem to a methane source. Nature Climate Change, 5, 67–70. 2267 Nossov DR, Torre Jorgenson M, Kielland K, Kanevskiy MZ (2013) Edaphic and microclimatic 2268 controls over permafrost response to fire in interior Alaska. Environmental Research Letters, 2269 8, 035013–13. 2270 O'Donnell JA, Harden JW, Mcguire AD, Romanovsky VE (2011a) Exploring the sensitivity of 2271 soil carbon dynamics to climate change, fire disturbance and permafrost thaw in a black 2272 spruce ecosystem. Biogeosciences, 8, 1367–1382. 2273 O'Donnell JA, Harden JW, Mcguire AD, Kanevskiy MZ, Jorgenson MT, Xu X (2011b) The 2274 effect of fire and permafrost interactions on soil carbon accumulation in an upland black 2275 spruce ecosystem of interior Alaska: implications for post‐thaw carbon loss. Global Change 2276 Biology, 17, 1461–1474. 2277 O'Donnell JA, Jorgenson MT, Harden JW, Mcguire AD, Kanevskiy MZ, Wickland KP (2012) 2278 The effects of permafrost thaw on soil hydrologic, thermal, and carbon dynamics in an 2279 Alaskan peatland. Ecosystems, 15, 213–229. 2280 O'Donnell JA, Romanovsky VE, Harden JW, Mcguire AD (2009a) The Effect of Moisture 2281 Content on the Thermal Conductivity of Moss and Organic Soil Horizons From Black 2282 Spruce Ecosystems in Interior Alaska. Soil Science, 174, 646–651. 2283 O'Donnell JA, Turetsky M, Harden J, Manies K, Pruett L, Shetler G, Neff J (2009b) Interactive 2284 Effects of Fire, Soil Climate, and Moss on CO 2 Fluxes in Black Spruce Ecosystems of 2285 Interior Alaska. Ecosystems, 12, 57–72. 2286 Olefeldt D, Goswami S, Grosse G, Hayes D (2016) Circumpolar distribution and carbon storage 2287 of thermokarst landscapes. Nature Communications, 7, 13043. 2288 Olefeldt D, Turetsky MR, Crill PM, Mcguire AD (2012) Environmental and physical controls on 2289 northern terrestrial methane emissions across permafrost zones. Global Change Biology, 19, 2290 589–603. 2291 Olofsson J (2006) Short- and long-term effects of changes in reindeer grazing pressure on tundra 2292 heath vegetation. Journal of Ecology, 94, 431–440. 2293 Olofsson J, Hulme PE, Oksanen L, Suominen O (2004a) Importance of large and small

75 2294 mammalian herbivores for the plant community structure in the forest tundra ecotone. Oikos, 2295 106, 324–334. 2296 Olofsson J, Kitti H, Rautiainen P, Stark S (2001) Effects of summer grazing by reindeer on 2297 composition of vegetation, productivity and nitrogen cycling. Ecography, 24, 13–24. 2298 Olofsson J, Oksanen L, Callaghan T, Hulme PE, Oksanen T, Suominen O (2009) Herbivores 2299 inhibit climate-driven shrub expansion on the tundra. Global Change Biology, 15, 2681– 2300 2693. 2301 Olofsson J, Stark S, Oksanen L (2004b) Reindeer influence on ecosystem processes in the 2302 tundra. Oikos, 105, 386–396. 2303 Osterkamp TE, Jorgenson MT, Schuur EAG, Shur YL, Kanevskiy MZ, Vogel JG, Tumskoy VE 2304 (2009) Physical and ecological changes associated with warming permafrost and thermokarst 2305 in Interior Alaska. Permafrost and Periglacial Processes, 20, 235–256. 2306 Osterkamp TE, Viereck L, Shur Y, Jorgenson MT, Racine C, Doyle A, Boone RD (2000) 2307 Observations of thermokarst and its impact on boreal forests in Alaska, USA. Arctic, 2308 Antarctic, and Alpine Research, 303–315. 2309 Outcalt SI, Nelson FE, Hinkel KM (1990) The zero‐curtain effect: Heat and mass transfer across 2310 an isothermal region in freezing soil. Water Resources Research, 26, 1509–1516. 2311 Park H, Walsh J, Fedorov AN, Sherstiukov AB, Iijima Y, Ohata T (2013) The influence of 2312 climate and hydrological variables on opposite anomaly in active-layer thickness between 2313 Eurasian and North American watersheds. The Cryosphere, 7, 631–645. 2314 Pearson RG, Phillips SJ, Loranty MM, Beck PSA, Damoulas T, Knight SJ, Goetz SJ (2013) 2315 Shifts in Arctic vegetation and associated feedbacks under climate change. Nature Climate 2316 Change, 3, 673–677. 2317 Peng C, Ma Z, Lei X et al. (2011) A drought-induced pervasive increase in tree mortality across 2318 Canada's boreal forests. Nature Climate Change, 1, 467–471. 2319 Phoenix GK, Bjerke JW (2016) Arctic browning: extreme events and trends reversing arctic 2320 greening. Global Change Biology, 22, 2960–2962. 2321 Plante S, Champagne E, Ropars P, Boudreau S, Lévesque E, Tremblay B, Tremblay J-P (2014) 2322 Shrub cover in northern Nunavik: can herbivores limit shrub expansion? Polar Biology, 37, 2323 611–619. 2324 Pomeroy JW, Bewley DS, Essery RLH et al. (2006) Shrub tundra snowmelt. Hydrological 2325 Processes, 20, 923–941. 2326 Ponomarev EI, Kharuk VI, Ranson KJ (2016) Wildfires dynamics in siberian larch forests. 2327 Forests, 7, 125. 2328 Poorter H, Niklas KJ, Reich PB, Oleksyn J, Poot P, Mommer L (2012) Biomass allocation to 2329 leaves, stems and roots: meta‐analyses of interspecific variation and environmental control. 2330 New Phytologist, 193, 30–50. 2331 Racine C, Jandt R, Meyers C, Dennis J (2004) Tundra fire and vegetation change along a 2332 hillslope on the Seward Peninsula, Alaska, USA. Arctic, Antarctic, and Alpine Research, 36, 2333 1–10. 2334 Radville L, McCormack ML, Post E, Eissenstat DM (2016) Root phenology in a changing 2335 climate. Journal Of Experimental Botany, erw062–12. 2336 Randerson JT, Liu H, Flanner MG et al. (2006) The Impact of Boreal Forest Fire on Climate 2337 Warming. Science, 314, 1130–1132. 2338 Rasmus S, Lundell R, Saarinen T (2011) Interactions between snow, canopy, and vegetation in a 2339 boreal coniferous forest. Plant Ecology & Diversity, 4, 55–65.

76 2340 Raynolds MK, Walker DA, Ambrosius KJ et al. (2014) Cumulative geoecological effects of 2341 62 years of infrastructure and climate change in ice-rich permafrost landscapes, Prudhoe Bay 2342 Oilfield, Alaska. Global Change Biology, 20, 1211–1224. 2343 Robinson SD, Moore TR (2000) The influence of permafrost and fire upon carbon accumulation 2344 in high boreal peatlands, Northwest Territories, Canada. Arctic, Antarctic, and Alpine 2345 Research, 32, 155. 2346 Rocha AV, Shaver GR (2011) Postfire energy exchange in arctic tundra: the importance and 2347 climatic implications of burn severity. Global Change Biology, 17, 2831–2841. 2348 Rocha AV, Loranty MM, Higuera PE et al. (2012) The footprint of Alaskan tundra fires during 2349 the past half-century: implications for surface properties and radiative forcing. 2350 Environmental Research Letters, 7, 044039. 2351 Romanovsky VE, Osterkamp TE (1995) Interannual variations of the thermal regime of the 2352 active layer and near‐surface permafrost in northern Alaska. Permafrost and Periglacial …. 2353 Romanovsky VE, Osterkamp TE (1997) Thawing of the active layer on the coastal plain of the 2354 Alaskan Arctic. Permafrost and Periglacial …. 2355 Romanovsky VE, Osterkamp TE (2000) Effects of unfrozen water on heat and mass transport 2356 processes in the active layer and permafrost. Permafrost and Periglacial Processes, 11, 219– 2357 239. 2358 Romanovsky VE, Smith SL, Christiansen HH (2010) Permafrost thermal state in the polar 2359 Northern Hemisphere during the international polar year 2007-2009: a synthesis. Permafrost 2360 and Periglacial Processes, 21, 106–116. 2361 Roy-Léveillée P, Burn CR, McDonald ID (2014) Vegetation-Permafrost Relations within the 2362 Forest-Tundra Ecotone near Old Crow, Northern Yukon, Canada. Permafrost and 2363 Periglacial Processes, 25, 127–135. 2364 Rydén BE, Kostov L (1980) Thawing and freezing in tundra soils. Ecological Bulletins. 2365 Schulze ED, Wirth C, Mollicone D et al. (2012) Factors promoting larch dominance in central 2366 Siberia: fire versus growth performance and implications for carbon dynamics at the 2367 boundary of evergreen and deciduous conifers. Biogeosciences, 9, 1405–1421. 2368 Schuur EAG, Abbott BW, Bowden WB et al. (2013) Expert assessment of vulnerability of 2369 permafrost carbon to climate change. Climatic Change, 119, 359–374. 2370 Schuur EAG, Crummer KG, Vogel JG, Mack MC (2007) Plant Species Composition and 2371 Productivity following Permafrost Thaw and Thermokarst in Alaskan Tundra. Ecosystems, 2372 10, 280–292. 2373 Schuur EAG, Mcguire AD, Schädel C et al. (2015) Climate change and the permafrost carbon 2374 feedback. Nature Communications, 520, 171–179. 2375 Shiklomanov NI, Streletskiy DA, Nelson FE et al. (2010) Decadal variations of active-layer 2376 thickness in moisture-controlled landscapes, Barrow, Alaska. Journal of Geophysical 2377 Research, 115, G00I04. 2378 Shur YL, Jorgenson MT (2007) Patterns of permafrost formation and degradation in relation to 2379 climate and ecosystems. Permafrost and Periglacial Processes, 18, 7–19. 2380 Sjöberg Y, Coon E, K Sannel AB et al. (2016) Thermal effects of groundwater flow through 2381 subarctic fens: A case study based on field observations and numerical modeling. Water 2382 Resources Research, 52, 1591–1606. 2383 Slater AG, Lawrence DM (2013) Diagnosing Present and Future Permafrost from Climate 2384 Models. Journal of Climate, 26, 5608–5623. 2385 Smith MW (1975) Microclimatic Influences on Ground Temperatures and Permafrost

77 2386 Distribution, Mackenzie Delta, Northwest Territories. Canadian Journal of Earth Sciences, 2387 12, 1421–1438. 2388 Sofronov M, Volokitina A (2010) Wildfire Ecology in Continuous Permafrost Zone. In: 2389 Permafrost Ecosystems Siberian Larch Forests (eds Osawa A, Zyryanova OA, Matsuura Y, 2390 Kajimoto T, Wein RW). Springer, New York. 2391 Soudzilovskaia NA, Van Bodegom PM, Cornelissen JHC (2013) Dominant bryophyte control 2392 over high-latitude soil temperature fluctuations predicted by heat transfer traits, field 2393 moisture regime and laws of thermal insulation (ed Schweitzer J). Functional Ecology, 27, 2394 1442–1454. 2395 Stiegler C, Johansson M, Christensen TR, Mastepanov M, Lindroth A (2016a) Tundra 2396 permafrost thaw causes significant shifts in energy partitioning. Tellus Series B-Chemical 2397 And Physical Meteorology, 68, 1–11. 2398 Stiegler C, Lund M, Christensen TR, Mastepanov M, Lindroth A (2016b) Two years with 2399 extreme and little snowfall: effects on energy partitioning and surface energy exchange in a 2400 high-Arctic tundra ecosystem. The Cryosphere, 10, 1395–1413. 2401 Stieglitz M (2003) The role of snow cover in the warming of arctic permafrost. Geophysical 2402 Research Letters, 30, 1–4. 2403 Stoy PC, Street LE, Johnson AV, Prieto-Blanco A, Ewing SA (2012) Temperature, Heat Flux, 2404 and Reflectance of Common Subarctic Mosses and Lichens under Field Conditions: Might 2405 Changes to Community Composition Impact Climate-Relevant Surface Fluxes? Arctic, 2406 Antarctic, and Alpine Research, 44, 500–508. 2407 Sturm M, Douglas T, Racine C, Liston GE (2005) Changing snow and shrub conditions affect 2408 albedo with global implications. Journal of Geophysical Research, 110, G01004. 2409 Sturm M, Holmgren J, Liston GE (1995) A seasonal snow cover classification system for local to 2410 global applications. Journal of Climate, 8, 1261–1283. 2411 Sturm M, McFadden J, Liston GE, Chapin FS III (2001) Snow–Shrub Interactions in Arctic 2412 Tundra: A Hypothesis with Climatic Implications. Journal of Climate. 2413 Swann AL, Fung IY, Levis S, Bonan GB, Doney SC (2010) Changes in Arctic vegetation 2414 amplify high-latitude warming through the greenhouse effect. Proceedings of the National 2415 Academy of Sciences, 107, 1295–1300. 2416 Tape K, Sturm M, Racine C (2006) The evidence for shrub expansion in Northern Alaska and 2417 the Pan-Arctic. Global Change Biology, 12, 686–702. 2418 Tchebakova N, Parfenova E, Soja A (2009) The effects of climate, permafrost and fire on 2419 vegetation change in Siberia in a changing climate. Environmental Research Letters, 4, 2420 045013. 2421 Turetsky MR, Kane ES, Harden JW, Ottmar RD, Manies KL, Hoy E, Kasischke ES (2011) 2422 Recent acceleration of biomass burning and carbon losses in Alaskan forests and peatlands. 2423 Nature Geoscience, 4, 27–31. 2424 Urban M, Forkel M, Schmullius C, Hese S, Hüttich C, Herold M (2013) Identification of land 2425 surface temperature and albedo trends in AVHRR Pathfinder data from 1982 to 2005 for 2426 northern Siberia. International Journal of Remote Sensing, 34, 4491–4507. 2427 van der Wal R, van Lieshout SMJ, Loonen MJJE (2001) Herbivore impact on moss depth, soil 2428 temperature and arctic plant growth. Polar Biology, 24, 29–32. 2429 Vavrek MC, Fetcher N, McGraw JB, Shaver GR, Chapin FS III, Bovard B (1999) Recovery of 2430 productivity and species diversity in tussock tundra following disturbance. Arctic, Antarctic, 2431 and Alpine Research, 254–258.

78 2432 Väisänen M, Ylänne H, Kaarlejärvi E, Sjögersten S, Olofsson J, Crout N, Stark S (2014) 2433 Consequences of warming on tundra carbon balance determined by reindeer grazing history. 2434 Nature Climate Change, 4, 384–388. 2435 Viereck LA, Werdin-Pfisterer NR, Adams PC, Yoshikawa K (2008) Effect of wildfire and 2436 fireline construction on the annual depth of thaw in a black spruce permafrost forest in 2437 interior Alaska: a 36-year record of recovery., pp. 1845–1850. 2438 Voigt C, Marushchak ME, Lamprecht RE et al. (2017) Increased nitrous oxide emissions from 2439 Arctic peatlands after permafrost thaw. Proceedings Of The National Academy Of Sciences 2440 Of The United States Of America, 114, 6238–6243. 2441 Walker D, Everett K (1991) Loess ecosystems of northern Alaska: regional gradient and 2442 toposequence at Prudhoe Bay. Ecological Monographs, 437–464. 2443 Walker DA, Everett KR (1987) Road dust and its environmental impact on Alaskan taiga and 2444 tundra. Arctic and Alpine Research. 2445 Walker DA, Billings WD, De Molenaar JG (2001) Snow–vegetation interactions in tundra 2446 environments. Snow ecology: an interdisciplinary examination of snow-covered ecosystems, 2447 266–324. 2448 Walker DA, Webber PJ, Binnian EF, Everett KR, Lederer ND, Nordstrand EA, Walker MD 2449 (1987) Cumulative impacts of oil fields on northern Alaskan landscapes. Science, 238, 757– 2450 761. 2451 Walker M, Wahren C, Hollister R et al. (2006) Plant community responses to experimental 2452 warming across the tundra biome. Proceedings of the National Academy of Sciences, 103, 2453 1342–1346. 2454 Walker X, Johnstone JF (2014) Widespread negative correlations between black spruce growth 2455 and temperature across topographic moisture gradients in the boreal forest. 1–10. 2456 Walker XJ, Mack MC, Johnstone JF (2015) Stable carbon isotope analysis reveals widespread 2457 drought stress in boreal black spruce forests. Global Change Biology, 21, 3102–3113. 2458 Walter KM, Chanton JP, Chapin FS, Schuur EAG, Zimov SA (2008) Methane production and 2459 bubble emissions from arctic lakes: Isotopic implications for source pathways and ages. 2460 Journal of Geophysical Research, 113, G00A08. 2461 Walter KM, Smith LC, Stuart Chapin F (2007) Methane bubbling from northern lakes: present 2462 and future contributions to the global methane budget. Phiosophical Transactions of the 2463 Royal Society A, 365, 1657–1676. 2464 Webb EE, Heard K, Natali SM et al. (2017) Variability in Above and Belowground Carbon 2465 Stocks in a Siberian Larch Watershed. Biogeosciences Discussions, 1–39. 2466 Webster C, Rutter N, Zahner F, Jonas T (2016) Measurement of Incoming Radiation below 2467 Forest Canopies: A Comparison of Different Radiometer Configurations. Journal of 2468 Hydrometeorology, 17, 853–864. 2469 Welp LR, Patra PK, RÖDENBECK C, Nemani R, Bi J, Piper SC, Keeling RF (2016) Increasing 2470 summer net CO₂ uptake in high northern ecosystems inferred from atmospheric 2471 inversions and comparisons to remote-sensing NDVI. Atmospheric Chemistry and Physics, 2472 16, 9047–9066. 2473 Williamson SN, Barrio IC, Hik DS, Gamon JA (2016) Phenology and species determine 2474 growing-season albedo increase at the altitudinal limit of shrub growth in the sub-Arctic. 2475 Global Change Biology, 1–11. 2476 Woo M (1990) Consequences of climatic change for hydrology in permafrost zones. Journal of 2477 cold regions engineering, 4, 15–20.

79 2478 Woo M-K, Mollinga M, Smith SL (2007) Climate warming and active layer thaw in the boreal 2479 and tundra environments of the Mackenzie Valley. Canadian Journal of Earth Sciences, 44, 2480 733–743. 2481 Xue X, Peng F, You Q, Xu M, Dong S (2015) Belowground carbon responses to experimental 2482 warming regulated by soil moisture change in an alpine ecosystem of the Qinghai–Tibet 2483 Plateau. Ecology and Evolution, 5, 4063–4078. 2484 Yi S, Mcguire AD, Harden J et al. (2009) Interactions between soil thermal and hydrological 2485 dynamics in the response of Alaska ecosystems to fire disturbance. Journal of Geophysical 2486 Research, 114, 1–20. 2487 Yoshikawa K, Hinzman LD (2003) Shrinking thermokarst ponds and groundwater dynamics in 2488 discontinuous permafrost near council, Alaska. Permafrost and Periglacial Processes, 14, 2489 151–160. 2490 Yoshikawa K, Bolton WR, Romanovsky VE, Fukuda M, Hinzman LD (2003) Impacts of 2491 wildfire on the permafrost in the boreal forests of Interior Alaska. Journal of Geophysical 2492 Research, 108, 8148. 2493 Zamin TJ, Grogan P (2012) Birch shrub growth in the low Arctic: the relative importance of 2494 experimental warming, enhanced nutrient availability, snow depth and caribou exclusion. 2495 Environmental Research Letters, 7, 034027–10. 2496 Zeng Z, Piao S, Li LZX et al. (2017) Climate mitigation from vegetation biophysical feedbacks 2497 during the past three decades. Nature Climate Change, 351, 600–8. 2498 Zhang K, Kimball JS, Mu Q, Jones LA, Goetz SJ, Running SW (2009) Satellite based analysis of 2499 northern ET trends and associated changes in the regional water balance from 1983 to 2005. 2500 Journal of Hydrology, 379, 92–110. 2501 Zhang T (2005) Influence of the seasonal snow cover on the ground thermal regime: An 2502 overview. Reviews of Geophysics, 43, RG4002. 2503 Zhang T, Heginbottom JA, Barry RG, Brown J (2000) Further statistics on the distribution of 2504 permafrost and ground ice in the Northern Hemisphere. Polar Geography, 24, 126–131. 2505 Zimov SA, Zimov NS, Tikhonov AN, Chapin FS III (2012) Mammoth steppe: a high- 2506 productivity phenomenon. Quaternary Science Reviews, 57, 26–45. 2507 Arctic shrub effects on NDVI, summer albedo and soil shading (2014) Arctic shrub effects on 2508 NDVI, summer albedo and soil shading. 153, 79–89. 2509 2510

80