The Cryosphere, 11, 2975–2996, 2017 https://doi.org/10.5194/tc-11-2975-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Shifted energy fluxes, increased Bowen ratios, and reduced thaw depths linked with drainage-induced changes in permafrost ecosystem structure Mathias Göckede1, Fanny Kittler1, Min Jung Kwon1,a, Ina Burjack1, Martin Heimann1,2, Olaf Kolle1, Nikita Zimov3, and Sergey Zimov3 1Max Planck Institute for Biogeochemistry, Jena, Germany 2Division of Atmospheric Sciences, Department of Physics, University of Helsinki, Helsinki, Finland 3North-East Science Station, Pacific Institute for Geography, Far-Eastern Branch of Russian Academy of Science, Chersky, Republic of Sakha (Yakutia), Russia anow at: Division of Polar Life Sciences, Korea Polar Research Institute, Incheon, Republic of Korea Correspondence: Mathias Göckede ([email protected]) Received: 12 September 2016 – Discussion started: 25 October 2016 Revised: 27 September 2017 – Accepted: 23 October 2017 – Published: 15 December 2017 Abstract. Hydrologic conditions are a key factor in Arc- 1 Introduction tic ecosystems, with strong influences on ecosystem struc- ture and related effects on biogeophysical and biogeochemi- The current state and future evolution of Arctic permafrost, cal processes. With systematic changes in water availability particularly its interactions with the atmosphere, are among expected for large parts of the northern high-latitude region the largest uncertainties in our understanding of the Earth’s in the coming centuries, knowledge on shifts in ecosystem climate system (Schuur and Abbott, 2011; Stocker et al., functionality triggered by altered water levels is crucial for 2013). The Arctic is one of the most susceptible regions on reducing uncertainties in climate change predictions. Here, Earth to climate change (e.g., Serreze et al., 2000; Polyakov we present findings from paired ecosystem observations in et al., 2003; Fyfe et al., 2013; Pithan and Mauritsen, 2014), northeast Siberia comprising a drained and a control site. At and altered climate conditions may have enormous con- the drainage site, the water table has been artificially low- sequences for the sustainability of its natural environment ered by up to 30 cm in summer for more than a decade. This (Schuur et al., 2008; Arneth et al., 2010). Interactions be- sustained primary disturbance in hydrologic conditions has tween permafrost, climate, hydrology, and ecology have the triggered a suite of secondary shifts in ecosystem proper- potential to cause dramatic changes (e.g., McGuire et al., ties, including vegetation community structure, snow cover 2002; Hinzman et al., 2005) via mechanisms that are cur- dynamics, and radiation budget, all of which influence the rently poorly monitored and therefore highly unpredictable net effects of drainage. Reduced thermal conductivity in dry (Heimann and Reichstein, 2008; van Huissteden and Dol- organic soils was identified as the dominating drainage ef- man, 2012; Rawlins et al., 2015). The associated changes in fect on energy budget and soil thermal regime. Through this the energy transfer between surface and atmosphere (Eugster effect, reduced heat transfer into deeper soil layers leads to et al., 2000; Langer et al., 2011a, b) and the emission pat- shallower thaw depths, initially leading to a stabilization of terns of greenhouse gases (Koven et al., 2011) raise the need organic permafrost soils, while the long-term effects on per- for experimental studies in this region that address the key mafrost temperature trends still need to be assessed. At the uncertainties linked to the functioning of the Arctic in the same time, more energy is transferred back into the atmo- Earth’s climate system (Semiletov et al., 2012). sphere as sensible heat in the drained area, which may trigger Hydrologic conditions play a pivotal role in shaping Arctic a warming of the lower atmospheric surface layer. ecosystems. Lakes and rivers in the permafrost region repre- sent an important portion of regional net carbon exchange Published by Copernicus Publications on behalf of the European Geosciences Union. 2976 M. Göckede et al.: Drainage-induced changes in permafrost ecosystems with the atmosphere, but net emissions over different spa- and found no systematic short-term shifts immediately fol- tiotemporal scales remain highly uncertain to date (Walter- lowing a drainage disturbance (Merbold et al., 2009). Anthony et al., 2014; Rasilo et al., 2015). Moreover, the vast Our study presents observational evidence of shifts in carbon pool in northern permafrost regions, currently esti- ecosystem properties and energy fluxes within a wet tussock mated at 1330–1580 Pg of organic carbon (Schuur et al., tundra ecosystem in northeast Siberia following a decade- 2015), has accumulated over past millennia since a combi- long drainage disturbance, providing insight into the sustain- nation of low mean temperatures and anoxic conditions in ability of ice-rich permafrost under future climate change. As areas with flooded or water-saturated soils slowed down the a first major objective, we used paired observations within decomposition of organic matter (e.g., Ping et al., 2008). In a drainage area and a control area to quantify several sec- addition to warming, shifts in the water balance in this re- ondary disturbance effects linked to lower water tables, in- gion are expected to trigger profound changes in the per- cluding changes in vegetation community, radiation budget, mafrost carbon cycle (e.g., Chapin et al., 2005; Oberbauer and soil thermal regime over longer time frames. Our sec- et al., 2007). Hydrology can vary at small spatial scales and ond objective was to link these shifts in ecosystem proper- thus create fine-scale mosaics in surface conditions (Muster ties to year-round eddy-covariance measurements of energy- et al., 2012, 2013). Even minor differences in mean water exchange patterns between permafrost ecosystem and atmo- levels can impose strong effects on vegetation and microbial sphere, and to evaluate potential feedback effects on Arctic community structures or soil thermal regimes (Zona et al., warming. Finally, links between heat transfer into deeper lay- 2011a). It can thus be expected that flooding or draining a ers and summertime thaw depth dynamics were investigated. site will initiate profound shifts in ecosystem structure, and that the long-term impacts of the disturbance will look very different than the short-term changes that can be observed 2 Methods immediately after the event (e.g., Shaver et al., 1992). 2.1 Study site and drainage experiment Future shifts in hydrologic conditions in the Arctic can be triggered by altered precipitation patterns, where a gen- Our field experiments were conducted on a wet tussock tun- eral trend towards increased rainfall is predicted (Kattsov and dra site within the floodplain of the Kolyma River (68.61◦ N, Walsh, 2000); however, patterns will vary strongly by region 161.35◦ E), approximately 15 km south of the town of Cher- (Huntington, 2006; Bintanja and Selten, 2014); therefore, for sky in northeast Siberia. The area was formed by a relatively some areas also lower precipitation can be expected. Geo- recent shift of the main channel of the Kolyma (Corradi et al., morphological processes such as subsidence (Jorgenson et 2005); therefore the terrain is flat and no thermokarst lakes or al., 2006; O’Donnell et al., 2012) as well as the formation of natural drainage channels have yet formed in the vicinity of a system of connected troughs through the preferential degra- the observation sites. Still, the existing minor differences in dation of ice wedges in ice-rich permafrost (Serreze et al., terrain height have led to the formation of a fine-scale mosaic 2000; Liljedahl et al., 2016) can lead to a lateral redistribu- in micro-site conditions (Fig. 1), mostly triggered by redis- tion of water, and thus can create both wetter and drier micro- tribution of water from slightly elevated areas towards the sites within a formerly uniform ecosystem. Also, a deepen- minor depressions. ing of the active layer can trigger reductions in waterlogged Site hydrology is strongly influenced by a flooding period conditions and wetland extent in permafrost regions (Avis et in spring caused by melting of the local snow cover as well al., 2011). In-depth insight into the net effect resulting from as a northward moving dam of ice floes on the main channel long-term changes in hydrology therefore forms a key chal- of the Kolyma that delays melt water runoff. Depending on lenge for improved future predictions of biogeochemical cy- the timing of snowmelt in the tributary watersheds, this pro- cles in Arctic ecosystems (e.g., Lupascu et al., 2014). cess leads to standing water on the site (up to 50 cm above Over the past two decades, several studies have docu- ground level) in most years around late May to early June. mented the effect of an experimental manipulation of soil After the flood has mostly receded, stagnant water remains hydrologic conditions on biogeochemical cycles in Arctic in large parts of the area that only gradually decreases over ecosystems (e.g., Oechel et al., 1998; Olivas et al., 2010; Na- the course of the growing season. tali et al., 2015). Only a few of these studies examined long- Soils consist of an organic layer of 15–20 cm overlaying term effects of manipulated water tables (e.g., Christiansen alluvial mineral soils (silty clay), with some organic material et al., 2012; Lupascu et al., 2014; Kittler et al., 2016), and also present in deeper layers following cryoturbation. More none of them explicitly addressed the net impact of hydro- information on site climatology (Sect. 3.1), spatiotemporal logic disturbance on the energy budget; only shifts in thaw patterns in hydrology (Sect. 3.2), and vegetation character- depth (Zona et al., 2011a; Kim, 2015) and indirect effects of istics (Sect. 3.3) can be found in the Results section. Fur- heat fluxes on the carbon cycle (Turetsky et al., 2008) were ther details on the study site are presented by Corradi et reported.