127 ARTICLE Responses of the capillifolium and strictum to nitrogen deposition in a : growth, ground cover, and CO2 exchange Sari Juutinen, Tim R. Moore, Anna M. Laine, Jill L. Bubier, Eeva-Stiina Tuittila, Allison De Young, and Mandy Chong

Abstract: Previous studies have shown that atmospheric nitrogen (N) deposition is detrimental to sphagna, which are a group of mosses that are important for carbon cycling in northern peatlands. Little is known about species interac- tions, such as relative responses of tall Menzies ex Brid. and sphagna. We studied the effects

of N deposition on growth, abundance, and CO2 exchange of the moss species Sphagnum capillifolium (Ehrh.) Hedw. and Polytrichum strictum in an experiment at a temperate bog. Sphagnum growth and cover decreased significantly with high-dose N treatment (6.4 g N·m−2·year−1) in years 4 and 5 of treatment, whereas the same parameters increased for

Polytrichum compared with the control. Net CO2 exchange, gross (Pg), and dark respiration (R)inthe intact moss cores, which were measured in year 5 of treatment, were elevated in the cores that had been treated with the high-dose of N, compared with the control, and this was associated with increased abundance of Polytrichum. The moss cores where Polytrichum was removed, however, had increased mass-based R with the high-dose N treatment. Our results showed that S. capillifolium at Mer Bleue may be close to N saturation, as 5 years of high-dose N loading (6.4 g N·m−2·year−1 + background) was harmful to this species, possibly as a result of increased respiratory cost. Polytrichum strictum had a competitive advantage, at least in the short-term, through allocating excess N to growth. This change in moss layer composition deserves further attention, as a shift to more easily decomposable litter, without correspond- ing increases in production, could reduce the carbon sequestration of the bog. Key words: peatland, photosynthesis, chlorophyll fluorescence, respiration, vegetation change, moss. Résumé : Des études précédentes ont montré que le dépôt de N atmosphérique peut être dommageable a` la sphaigne, For personal use only. un genre important au recyclage de C dans les tourbières du nord. On sait peu de choses des interactions entre les espèces, telles les réponses relatives de la mousse Polytricum strictum Menzies ex Brid. et de la sphaigne. Les auteurs ont

étudié les effets du dépôt de N sur la croissance en hauteur, l’abondance et l’échange de CO2 des espèces de mousses Sphaghum capillifolium (Ehrh.) Hedw. et Polytricum strictum lors d’une expérience en tourbière tempérée. La croissance en hauteur de Sphaghum et son couvert diminuaient significativement en présence d’une forte concentration de N (6,4 g N·m−2·an−1)a` la quatrième et cinquième années de traitement, alors que ceux de Polytricum augmentaient,

relativement au contrôle. L’échange net de CO2, la photosynthèse brute (Pb) et la respiration mitochondriale (R) des mousses intactes, mesurés a` la cinquième année de traitement, étaient accrus par le traitement riche en N relativement aux contrôles, a` cause de l’abondance accrue de Polytricum. Les mousses dont on avait retiré Polytrichum présentaient cependant une R en fonction de la masse accrue par le traitement riche en N. Les résultats obtenus par les auteurs montrent que S. capillifolium de la tourbière de Mer Bleue peut être presqu’a` saturation de N, car 5 ans de charge élevée en N (6,4 g N·m−2·an−1 + niveau de base) ont été dommageables a` cette espèce, possiblement a` cause d’un coût Botany Downloaded from www.nrcresearchpress.com by HELSINKI UNIV on 05/02/16 respiratoire accru. Polytrichum strictum possédait un avantage compétitif, du moins a` court terme, réservant l’excès de N a` la croissance. Ce changement dans la composition du couvert de mousse mérite une plus grande attention car un déplacement vers une litière plus facilement décomposable, sans augmentations correspondantes de la production végétale, peut diminuer la séquestration du C par la tourbière. [Traduit par la Rédaction] Mots-clés : tourbière, photosynthèse, fluorescence de la chlorophylle, respiration, changement de la végétations, mousse.

Received 1 October 2015. Accepted 14 November 2015. S. Juutinen* and J.L. Bubier. Environmental Studies Department, Mount Holyoke College, 50 College Street, South Hadley, MA 01075, USA. T.R. Moore, A. De Young, and M. Chong. Department of Geography and Global Environmental & Climate Change Centre, McGill University, 805 Sherbrooke Street West, Montreal, QC H3A 0B9, Canada. A.M. Laine. Department of Forest Sciences, University of Helsinki, P.O. Box 27, FI-00014 Helsinki, Finland. E.S. Tuittila. School of Forest Sciences, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland. Corresponding author: Sari Juutinen (email: Sari.juutinen@helsinki.fi). *Present address: Department of Forest Sciences, University of Helsinki, P.O. Box 27, FI-00014 Helsinki, Finland.

Botany 94: 127–138 (2016) dx.doi.org/10.1139/cjb-2015-0183 Published at www.nrcresearchpress.com/cjb on 17 November 2015. 128 Botany Vol. 94, 2016

Introduction speaking, excess N from atmospheric deposition only The deposition of atmospheric nitrogen (N) drastically becomes available to vascular when the Sphagnum increased N availability in the industrialized world dur- layer becomes saturated with N, after which, vascular ing the Anthropocene (e.g., Galloway et al. 2008). Excess plants allocate N to increased growth and thus gain com- N from deposition threatens many natural ecosystems, petitive advantage over peat mosses (e.g., Malmer et al. by impacting species composition and biogeochemistry, 2003). Polytrichum strictum Menzies ex Brid. is a moss spe- and by increasing the competitive ability of some species cies that often co-occurs with sphagna, but is typically while making conditions unfavorable for others (e.g., more abundant in drier microhabitats, intermediate in Bobbink et al. 2010). Peat mosses (Sphagnum spp.) are key its N requirement, and found to benefit from low-level species in northern nutrient-poor peatlands, but they are addition of N and P compared with sphagna (Vitt 1990; sensitive to the deposition of atmospheric N (Limpens et al. Gunnarsson and Rydin 2000; Berendse et al. 2001; 2011). A decrease in the abundance of Sphagnum could mark- Mitchell et al. 2002; Bubier et al. 2007; Sottocornola et al. edly impact litter quality and decomposability, surface 2007; Bu et al. 2011). Polytrichum strictum is a pioneer spe- structure, and water retention capacity, all of which affect cies that is considered to facilitate the establishment of the important carbon (C) sequestration capacity of peat- sphagna at peatland restoration sites (e.g., Robert et al. lands (e.g., Moore et al. 2007; Straková et al. 2010; Larmola 1999), but can outcompete sphagna through increased et al. 2013). abundance (González et al. 2013). However, studies have Sphagna can increase their growth and production shown that the abundance of P. strictum declines under under increased nutrient availability, but the responses conditions of high N loading (Bubier et al. 2007; Bu et al. depend on species sensitivity, dose, and temporal scale 2011). Therefore, it is important to understand the effects of loading (e.g., Rochefort et al. 1990; Vitt et al. 2003; of N deposition on different peatland plants, and the Gunnarsson et al. 2004). Critical loading of atmospheric possible interactions between the species. N, associated with reduced growth, is considered to be In this study we examined the effects of simulated N ϳ0.5 to 1.5 g N·m−2·year−1 for sphagna (Gunnarsson and deposition on the growth and abundance of Sphagnum Rydin 2000; Vitt et al. 2003; Bragazza et al. 2004; Granath capillifolium (Ehrh.) Hedw. and P. strictum during years 1–5 et al. 2014), and is currently exceeded in parts of Europe, of a fertilization experiment at the temperate ombro- North America, southern China, and south and southeast- trophic peatland, Mer Bleue. We also examined the im- ern Asia (Bobbink et al. 2010). Recognized factors contrib- pact of excess N on the CO2 exchange capacity of moss uting to the negative responses of sphagna to increased after 5 years of fertilization. We hypothesized that N availability are the respiratory cost of storing excess N (i) height growth (hereinafter, growth) of S. capillifolium (Limpens and Berendse 2003; Manninen et al. 2011), com- would decrease, but P. strictum would increase with N For personal use only. petition for light and space with vascular plants (van der addition; (ii)CO2 exchange and chlorophyll fluorescence Heijden et al. 2000b; Berendse et al. 2001), increase in measurements would indicate negative impacts of N depo- parasitic infections (Limpens et al. 2003), decrease in sition on S. capillifolium and positive impacts on P. strictum. other elements, most importantly phosphorus (P) (Aerts Materials and methods et al. 1992; Jauhiainen et al. 1998), and greater sensitivity to drying (van der Heijden et al. 2000a; Manninen et al. Study site The study was conducted at the Mer Bleue peatland, 2011; Fritz et al. 2012). In addition, experiments have in- near Ottawa, Ontario, Canada (46°N, 75.5°W), which has dicated that Sphagnum populations adapted to higher a mean annual temperature of 6.6 °C and an average background N loading are less sensitive to additional N rainfall of 756 mm per year (Canadian Climate Normals input than mosses from areas with low background de- 1981–2010). Nitrogen was applied to randomly assigned

Botany Downloaded from www.nrcresearchpress.com by HELSINKI UNIV on 05/02/16 position (Wiedermann et al. 2009a; Granath et al. 2012). A triplicate3m×3mplots per treatment, separated by 1 m- recent meta-analysis on the responses of sphagna to ex- wide buffer zones. Nitrogen was administered as NH NO perimental N addition found that tissue N concentration 4 3 dissolved in seven applications, 2 mm deep, from May to exceeding ϳ1% predicted a decrease in Sphagnum produc- August 2005–2009. The N addition rates were 0, 3.2, and tion, but noted that temperature, wetness, species com- 6.4gN·m−2·year−1, termed 0N (control), 3.2N, and 6.4N; the position, and species interactions are likely site factors control plots only received distilled water, and the total modulating the responses of sphagna to N addition amounts of NH NO in the N treatments were 9.2 and (Limpens et al. 2011). 4 3 18.5 g·m−2·year−1. The background N deposition for the re- While many studies have measured Sphagnum responses gion is ϳ0.6–0.8 g N·m−2·year−1 (Turunen et al. 2004). to N deposition, less is known about the effects of compe- tition or facilitation from co-existing plant species. Plant Growth and species abundance species differ in their ability to use the excess nutrients and Moss growth was measured using the cranked wire maintain homeostatic stability, i.e., maintain tissue levels method (Clymo 1970). Plastic-covered metal rods, 2 mm of N relative to input (e.g., Shaver and Laundre 2007; in diameter, were positioned about 10 cm above the peat Wiedermann et al. 2007; Elser et al. 2010). Generally surface and extended downwards into the peat by about

Published by NRC Research Press Juutinen et al. 129

25 cm (see also Moore 1989). The distance between the CO2 concentrations were recorded every 15 s over a tips of the rod and the surrounding S. capillifolium and ϳ3 min period, using an infrared gas analyzer (EGM4; P. strictum was measured in early April and late October PP-Systems). PPFD in the chamber, temperature, and each year from 2005 to 2009, capturing most of the typ- sample weight were determined for each measurement.

ical growing season (daily mean temperature above 0 °C, The CO2 exchange rate was calculated from the linear

see also Moore et al. 2002). We acknowledge that moss change in CO2 concentration over the measurement pe- growth may occur outside of this period, but believe that riod, correcting flux for gas chamber volume and tem- these measurements can capture the relative effects perature. Each observation was checked for leaks or

from the treatments. The growth of S. capillifolium and saturation. Gross photosynthesis (Pg) at light levels of 500 P. strictum is synchronous according to our observations and 700 ␮mol·m−2·s−1 was estimated by summing NE un- at this site, and the cranked wire method should capture der light and dark conditions, the latter representing

the vertical height increments for both species (Moore respiration (R). The CO2 exchange rate was calculated per 1989 and references therein). There were 10 replicate unit ground area and per unit dry mass of moss.

rods in each plot (30 per treatment), and readings per To directly compare Pg and R of the two moss species,

species in each treatment varied from 4 (P. strictum in 0N, and to test whether Pg and R, and the quantum yield in 2008) to 29 (S. capillifolium in 0N, in 2009). efficiency of photochemistry (ratio between variable flu-

Frequency of the moss species was determined in orescence and maximum fluorescence, Fv/Fm) after a dark 60 cm × 60 cm gas flux measurement collars in each acclimation period were affected by the excess N, an- plot in mid-July of years 2007, 2008, and 2009. A point other moss sample was collected in April 2010 after snow intercept frame was set on top of the collar and hits to thaw. Chlorophyll fluorescence analysis was used to a pin were recorded in 61 grid points in each collar. study the efficiency of photosynthetic light reactions, Hits to the vascular plants were recorded similarly. and to gain information about the physiological state of a plant (Maxwell and Johnson 2000; Laine et al. 2011). CO exchange and chlorophyll fluorescence 2 Cores similar to those described above were collected To determine the effect of 5 years of treatments on pho- from the treatment plots (total of3×3cores), and the tosynthesis and respiration of the mosses, we measured moistened samples were shipped to the University of the CO exchange in moss cores (height ϳ2 cm, diameter 2 Helsinki, Finland, where the measurements were con- 7.2 cm) from which all vascular plants were removed and, ducted. Measurements were made using an open, fully separately, in Sphagnum capitula and Polytrichum shoots. controlled, flow-through gas exchange fluorescence mea- The moss cores were sampled in October 2009 when the surement system (GFS-3000; Walz). We used a standard co- plots had been treated with water or fertilizer for 5 years. nifer cuvette equipped with a net in the lower jaw. A

For personal use only. Two cores with diameter of 7.2 cm, containing the top uniform layer of S. capillifolium capitula or segments of the 2cmoftheSphagnum stems including capitula, and taller top2cmofP. strictum were placed on the net, so that am- P. strictum shoots growing between S. capillifolium, were bient air was allowed to flow freely above and below the sampled from each plot (3 treatments × 3 replicates × sample. CO exchange was measured at PPFD of 600 and 2 cores). One core was kept intact and only vascular plants 2 1300 ␮mol·m−2·s−1. The samples were allowed to adjust to and visible detritus were removed, whereas in the other each light level for 5 min before measurements. After this, core, shoots of P. strictum were also removed. Samples were the sample was placed in the dark and the respiration rate watered by spraying with distilled water and placed into a was recorded after 5 min. Chlorophyll fluorescence (F /F ) growth chamber under the following summer-like condi- v m was measured after 25 min in the dark. The other environ- tions: 16 h light with a photosynthetically active photon mental conditions in the cuvette were maintained at a flux density (PPFD) of ϳ500 ␮mol·m−2·s−1 at the moss sur- constant level during the measurement period: the CO2

Botany Downloaded from www.nrcresearchpress.com by HELSINKI UNIV on 05/02/16 face, temperature controlled at 18 °C, and8hofdarkness at concentration of the incoming air was 500 ppm and the 10 °C; CO concentration of 380 ppm; and relative humidity 2 flow rate was 600 ␮mol·s−1. The relative humidity inside the of 70%. cuvette was controlled and temperature was set at 20 °C. For each sample pot we measured net CO exchange (NE) 2 Samples were weighed before and after measurements. under light levels of 0, 500, and 700 PPFD ␮mol·m−2·s−1, Upon completion of the measurements, the moss sam- and the moss water content was ϳ1000%–2000% for ples were oven-dried (60 °C) to determine dry biomass S. capillifolium, which are the optimal light (Marschall and and moisture content. Subsamples from the moss sam- Proctor 2004) and moisture (Chong et al. 2012) conditions ples were ground and analyzed for C and N levels using for the photosynthesis of S. capillifolium. There were an elemental analyzer (model NC2500; Carlo Erba). six observations per treatment for the intact cores and the S. capillifolium-only cores (each core was measured twice). Homeostatic regulation coefficients The measurements were made using a small chamber We compared species ability to maintain tissue levels (10 cm diameter, 10 cm high) to cover the pot containing of N relative to input. Strength of N homeostasis was the mosses, which was set on a small platform with a assessed on the basis of homeostatic regulation coeffi- water-filled groove to ensure a gas-tight seal. Headspace cients (H) calculated on the basis of their tissue N concen-

Published by NRC Research Press 130 Botany Vol. 94, 2016

Fig. 1. (a and b) Seasonal growth (April–October) of treated Sphagnum capillifolium and Polytrichum strictum by comparison with the control (treatment means, n = 3); (c–d) frequency (hit counts) for treated S. capillifolium and P. strictum in 60 cm × 60 cm collars by comparison with the control (treatment means, n = 3) in the first 5 years of treatment (2005–2009). Treatment differences were assessed for the years with significant species × treatment interactions (Table 1) and p values are shown for the treatments significantly different from the controls. Grey symbols represent the 3.2N treatment; black symbols represent the 6.4N treatment; p values refer to year-wise treatment versus control comparisons.

a) S. capillifolium b) P. strictum 2.5

2.0

1.5

1.0 Treatment : Control Height (ratio) growth 0.5 p=0.002 p=0.03 0.0 c) S. capillifolium d) P. strictum 2.5 p=0.019 2.0

1.5 p=0.035 quency (ratio) quency tment : Control e a 1.0 Fr Tre 0.5 p=0.005 p=0.04 0.0 2005 2006 2007 2008 2009 2005 2006 2007 2008 2009 Year Year

tration and N inputs (atmospheric and fertilization) as were not equal, running analyses separately for intact described by Sterner and Elser (2002): cores and cores with only S. capillifolium. Impacts of moss For personal use only. species and N treatment on moss levels of N, Pg, R, and ϭ 1/H (1) y cx Fv/Fm were tested using 2-way ANOVA. Owing to signifi- cant species × treatment effects, the species were tested

where y is the N content (%) in the moss, x is the input of separately. Relationships between R, Pg, and Fv/Fm and N N (combined ambient atmospheric N deposition and fer- levels were examined using linear regression analyses. tilization (g·m−2·year−1)), c is a constant, and H is the ho- meostatic regulation coefficient. To compare the moss Results species against the dominant bog shrub species at our Growth and abundance site, we calculated H values for leaves of Chamaedaphne Nitrogen addition had no significant effect on moss calyculata, Rhododendron groenlandicum, and Vaccinium growth during the first 3 years (2005, 2006, and 2007; myrtilloides from the same experiment. Leaf levels of N Figs. 1a–1b; Table 1), when the seasonal growth varied Botany Downloaded from www.nrcresearchpress.com by HELSINKI UNIV on 05/02/16 for vascular plants are published in Bubier et al. (2011). from 7.9 to 18.3 mm in P. strictum and from 8.5 to 22.3 mm in S. capillifolium. Both species had the smallest height incre- Statistical analyses Treatment and species effects on moss growth and ment in the year 2007. Significant species and treatment cover were analyzed by 2-way ANOVA separately for each interaction appeared in the years 4 and 5 (2008 and 2009) year. Data were checked for normality and equality of when the growth of S. capillifolium was significantly smaller variances. When there was a significant species × treat- in the 6.4N plots than in the control plots (Fig. 1a), with a ment interaction, the species were analyzed separately. large difference between treatment means (ϳ15 mm). On Plot means for growth were used to balance the number an annual scale, N treatments had no significant effect on of observations per treatments (treatment n = 3). Vascu- the height of P. strictum, although there was a transient lar plant abundance was used as covariate for moss positive trend of higher mean values for the 3.2N and 6.4N frequency (hit count) for the years where data were avail- plots than in 0N plot in years 2, 3, and 4 (Figs. 1b and 1d). able (years 3, 4, and 5 of treatment).Treatment effects on The year-to-year variability in absolute growth rates

area and mass-based NE, Pg, and R were tested using should be treated with caution, because the length of the 1-way ANOVA, or a Kruskal–Wallis test when variances period between spring and autumn measurements was

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Table 1. Results from analysis of variance for moss growth and frequency (hit count) in 2005– 2009 (years 1–5 of treatment). 2 Dependent variable (year) Source MS FpRadj Model p Growth (1) Species 1 0.018 0.894 −0.14 0.707 Treatment 30 0.973 0.406 Species × treatment 15 0.497 0.620 Error 31 Growth (2) Species 3 0.024 0.879 0.00 0.452 Treatment 128 1.182 0.340 Species × treatment 145 1.338 0.299 Error 108 Growth (3) Species 46 2.669 0.128 0.36 0.061 Treatment 85 4.908 0.028 Species × treatment 17 0.993 0.399 Error 17 Growth (4) Species 980 19.072 0.001 0.64 0.003 Treatment 5 0.096 0.909 Species × treatment 396 7.706 0.007 Error 51 Growth (5) Species 216 18.172 0.001 0.73 0.001 Treatment 98 8.238 0.006 Species × treatment 90 7.549 0.008 Error 12 Frequency (1) Species 34672 1313.9 <0.001 0.99 <0.001 Treatment 17 0.632 0.549 Species × treatment 39 1.474 0.268 Error 26 Frequency (3) Species 174 0.637 0.440 0.01 0.437 Treatment 67 0.244 0.787 Species × treatment 559 2.045 0.172 Error 274 Frequency (4) Species 80 0.611 0.450 0.39 0.049 Treatment 188 1.433 0.277

For personal use only. Species × treatment 800 6.093 0.015 Error 131 Frequency (5) Species 47 0.181 0.678 0.28 0.150 Treatment 245 0.948 0.415 Species × treatment 1024 3.971 0.047 Error 258 Note: Degrees of freedom for species, treatment, species × treatment, and error were 1, 2, 2, and 12, respectively. For significant differences between treatments see Fig. 1.

not exactly the same each year. The 5 year total height when tested as a covariate, and the 6.4N plots seemed to increment was 15 mm higher for S. capillifolium than have a distinctively low S. capillifolium abundance relative

Botany Downloaded from www.nrcresearchpress.com by HELSINKI UNIV on 05/02/16 for P. strictum in the 0N treatment, whereas the cumu- to vascular plant abundance (Fig. 2). Vascular plant cover lative height increment for P. strictum exceeded that of was, however, significant for P. strictum frequency in year 4 S. capillifolium by 32 and 61 mm at the 3.2N and 6.4N (p = 0.04). Thus, there was no clear relationship between plots, respectively. vascular plants and moss abundance. Sphagnum capillifolium had significantly higher frequency than P. strictum in the initial year (Figs. 1e–1f; Table 1), but the Exchange of CO2 and chlorophyll fluorescence difference had diminished by year 3 (no data for the 2nd Net exchange of CO2 (NE) and its component fluxes year). Similar to the data for growth, significant species and dark respiration (R) and gross photosynthesis (Pg)in treatment interactions appeared in years 4 and 5 (2008 the intact moss cores were significantly higher in the and 2009, Fig. 1e). The frequency of S. capillifolium was 6.4N plots than in the 0N plots (NE 2.3 vs. 4.5, R 2.4 vs. −2 −1 significantly smaller in the 6.4N plots than in the control 3.9, and Pg 4.7 vs. 8.4 mmol·m ·h ) expressed per unit plots in year 4 (23 vs. 53 hits), and P. strictum abundance area (Figs. 3a–3c; Table 2). In the cores with S. capillifolium was greater with N treatment compared with the control only (P. strictum removed), R was significantly higher and (43 and 49 vs. 26 hits) in year 5. Vascular-plant abundance NE was significantly decreased in the 6.4N cores by had no significant effect on S. capillifolium frequency comparison with the control (R, 1.1 vs. 1.7 and NE, 1.6

Published by NRC Research Press 132 Botany Vol. 94, 2016

Fig. 2. Frequency of (a) Sphagnum capillifolium, and static regulation coefficient (H) was 3.56 and 6.80 for (b) Polytrichum strictum in relation to vascular plant S. capillifolium and P. strictum, respectively. The H values abundance in 60 cm × 60 cm collars within the treatment of the two moss species were not significantly different plots. Data points are the mean ± SE for point intercept (p > 0.05), but those were significantly smaller than for three hits (n = 3) for years 2007, 2008, and 2009, i.e., years 3, 4, shrub species (Fig. 4). The H values indicate that S. capillifolium and 5 of treatment. accumulates excess N more than P. strictum, which resem- a)a bled the shrubs growing in the same site with respect to the it #) change in tissue levels of N under changing N input.

40 Discussion 0N, -07

frequency (h 0N, -08 The simulated and background levels of N deposition 0N, -09 The background rate of N deposition at Mer Bleue falls 20

folium between the deposition values in central and northern 3.2N, -07 Europe (e.g., Berendse et al. 2001; Solberg et al. 2009), but 3.2N, -08 3.2N, -09 is close to1gN·m−2·year−1, which is considered harmful S. capilli 0 for Sphagnum growth and overall ecosystem biodiver- b) sity in a natural state (e.g., Bobbink et al. 2010, Granath et al. 2014). The lower dose of N applied in this study y(hit#) 40 (3.2 g N·m−2·year−1) is similar to highest deposition rates measured in parts of Western Europe, and the higher dose −2 −1

frequenc (6.4 g N·m ·year ) only corresponds to the observed depo- 20 sition around point sources (Galloway et al. 2008). These

6.4N, -07 doses of N are commonly used in peatland experiments 6.4N, -08 (e.g., Jauhiainen et al. 1998; Gunnarsson and Rydin 2000;

P. strictum 6.4N, -09 0 Manninen et al. 2011; Granath et al. 2012). 120 160 200 240 280 320 Moss growth and abundance Vascular plant frequency (hit #) As expected, adding N decreased the growth of S. capillifolium, but only in the 6.4N plots (Fig. 1), and vs. –0.32 mmol·m−2·h−1). There were no significant ef- there was a trend to increased growth for P. strictum, fects of treatment in the intact cores when the fluxes although this may be a transient phase, as the differ- For personal use only. were expressed per unit dry mass of moss. In turn, mass- ences between the controls and the N-treated mosses based R was significantly increased and mass-based NE decreased after 5 years. Moss abundance responded in significantly decreased in the 6.4N cores compared with a similar manner to growth (Figs. 1e–1f). This negative the control in the cores with S. capillifolium only (R, 4.2 vs. growth response to the administration of very high doses of −1.6 and NE, 2.7 vs. 5.1 ␮mol·m−2·h−1)(Figs. 3d–3f). As a N, and the lack of marked positive growth responses to the 2 result, mass-based Pg was negatively (R = 0.23, p = 0.026) lower doses of N indicate that S. capillifolium is not likely to related to the moss levels of N, whereas the relationship be N-limited at Mer Bleue, but excess N such as caused by N between the mass-based R and N levels was positive (R2 = saturation (Berendse et al. 2001) may impact the growth 0.63, p < 0.001) in the cores with S. capillifolium only. patterns when other nutrients are limited (Aerts et al. 1992; There were no effects from treatment on CO2 exchange Jauhiainen et al. 1998; Hoosbeek et al. 2002; Bragazza when S. capillifolium capitula and P. strictum shoots were et al. 2004). Co-limitation of P and N occurs in many Botany Downloaded from www.nrcresearchpress.com by HELSINKI UNIV on 05/02/16 measured separately (Fig. 3; Table 3). Polytrichum strictum plant functional groups at the Mer Bleue bog (Wang and had significantly higher Pg and R than S. capillifolium, and Moore 2014), and this finding is supported by the stron- thus had higher photosynthetic efficiency relative to tissue ger responses in vascular plant growth and moss cover in levels of N. Maximum quantum yield efficiency of PSII, areas receiving the N and P treatments than in the areas FV/FM, was low and similar for both species and among that were treated with N only (Larmola et al. 2013). treatments: the treatment means varied between 0.49 Similar to results published by Gunnarsson et al. (2004), and 0.65 in P. strictum and between 0.58 and 0.62 in the impact of N addition increased over time in the 6.4N S. capillifolium, and indicated that there was no stress plots (Figs. 1a and 1c). Both the small and the large doses related to N treatments. of N stimulated growth of Sphagnum in the short-term Response of tissue concentration to N treatment (Bonnett et al. 2010; Granath et al. 2012), and Sphagnum spe- The N treatments affected moss tissue N concentra- cies differ in their response (e.g., Gunnarsson et al. 2004). tion, with a doubling in S. capillifolium (0.7% to 1.4%), and Jauhiainen et al. (1994) found that adding N at the dose a 50% increase in P. strictum (0.8% to 1.2%), comparing the of 1–3 g N·m−2·year−1 increased growth of S. fuscum, but control with the 6.4N plots (Fig. 4). The resulting homeo- 10 g·m−2·year−1 led to an immediate reduction in bio-

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Fig. 3. CO2 exchange (mean ± SE, n = 6) for the intact cores and the Sphagnum capillifolium-only (Polytrichum strictum removed) cores, (a–c) expressed per unit area and (d–f) per unit dry mass. All vascular plants were removed from the cores. (g–i) Net exchange, respiration, and photosynthesis of S. capillifolium capitula and P. strictum shoots; values are the mean ± SE (n = 3). Treatments significantly different from control are indicated with p values (see Tables 2 and 3 for ANOVA results).

10 a) Net exchange b) Respiration c) Photosynthesis ) 8 -1 h 002 0.002 . =0.01 = 0 -2 p 6 p = m p 4 =0.024 p 0.012

2 = p 0 10 d) 0N e) f) 2

8 3.2N 5 )(mmol =0.028

6.4N .0 -1 p 0 h 6 = -1 hange 4 p exc 2 2 0.017 µmol g = ( p CO 0 -2 Intact S. capillifolium Intact S. capillifolium Intact S. capillifolium core core core core core core g) h) i)

) 80 -1 h 60 -1 40

(µmol g 20 0 P. strictum S. capillifolium P. strictum S. capillifolium P. strictum S. capillifolium shoot capitulum shoot capitulum shoot capitulum

Table 2. Results of 1-way ANOVAs for net CO2 exchange (NE), respiration (R), and gross photosynthesis (Pg) per unit area, and of Kruskal–Wallis test for CO2 fluxes per unit mass. For personal use only. Core type Dependent FpTreatment differences

Intact NE(area) 5.416 0.017 6.4N > 0N, p = 0.010 Intact R(area) 8.528 0.003 6.4N > 0N, p = 0.002

Intact Pg(area) 8.016 0.004 6.4N > 0N, p = 0.002 S. capillifolium NE(area) 5.275 0.018 6.4N < 0N, p = 0.012 S. capillifolium R(area) 4.3 0.033 6.4N > 0N, p = 0.024 S. capillifolium Pg(area) 3.307 0.065

Intact NE(mass) 3.661 0.160 Intact R(mass) 2.217 0.330 Intact Pg(mass) 3.661 0.160

S. capillifolium NE(mass) 8.035 0.018 6.4N < 0N, p = 0.017

Botany Downloaded from www.nrcresearchpress.com by HELSINKI UNIV on 05/02/16 S. capillifolium R(mass) 7.328 0.026 6.4N > 0N, p = 0.028 S. capillifolium Pg(mass) 6.222 0.045 6.4N < 0N, p = 0.052 Note: Intact, cores containing Sphagnum capillifolium and Polytrichum strictum; S. capillifolium, cores where P. strictum has been removed. Differences between treat- ments were assessed using Dunett’s test. Degrees of freedom for between, within, and total, were 2, 15, and 17, respectively.

mass production and elongation in a 4 month experi- net effect of competition between species and internal ment where there was background deposition of about changes in physiology. The increase in P. strictum fre- ϳ0.2 g N·m−2·year−1. Gunnarsson and Rydin (2000), however, quency in this study reached a level where it was found showed that even small deposition rates (<1 g N·m−2·year−1) to outcompete sphagna in a peatland restoration site. saturate sphagna in a few years, and lead to growth reduction. Competition for light could be a factor behind this re- It seems that both of the doses of N administered in our sponse (González et al. 2013). study caused reductions in growth and abundance, al- The enhanced N supply was used by P. strictum for though it was not statistically significant with the lower increased growth (Figs. 1b and 1d) in the short-term, dose. Changes in the growth and abundance can be a which agrees with an earlier finding that its distribution

Published by NRC Research Press 134 Botany Vol. 94, 2016

Table 3. Results of analysis of variance for nitrogen levels in moss [N], quantum efficiency of ␮ −2 −1 PSII (Fv/Fm), photosynthesis (Pg; at PAR 600 and 1300 mol·m ·s ), and dark respiration (R)in year 5 of the experiment. 2 Dependent variable Source MS FpRadj Model p [N] Species 3.416 239.792 <0.001 0.96 <0.001 Treatment 0.274 19.251 0.001 Species × treatment* 0.056 3.916 0.06 Error 0.014

Fv/Fm Species 0.002 0.426 0.53 0.13 0.308 Treatment 0.009 2.339 0.152 Species × treatment 0.005 1.295 0.32 Error 0.004

Pg600 Species 1951 34.782 <0.001 0.70 0.005 Treatment 165 2.942 0.104 Species × treatment 107 1.906 0.204 Error 56

Pg1300 Species 267 13.261 0.005 0.38 0.09 Treatment 13 0.623 0.558 Species × treatment 25 1.215 0.341 Error 20 R Species 2326 40.014 <0.001 0.73 0.003 Treatment 84 1.452 0.284 Species × treatment 126 2.172 0.17 Error 58 Note: Degrees of freedom for species, treatment, species × treatment, and error were 1, 2, 2, and 9, respectively. MS, mean square. *, Treatments affected [N] of Sphagnum capillifolium (Tukey HSD): 0N < 3.2N (p = 0.004); 0N < 6.4N (p < 0.001). Fig. 4. Concentration of N in Polytrichum strictum shoots and in peatlands is N-limited (Vitt 1990). Responses of P. strictum Sphagnum capillifolium capitula after 5 years of fertilization in tend to be positive in the initial phase, as significant relation to the annual N input (fertilization plus estimated increases in growth occurred with 3 g N·m−2·year−1 ap- −2 −1 0.6 g N·m ·year of ambient atmospheric deposition). Data plied over 3 growing seasons at a former cut-away peat- are the mean ± SE per treatment. The power relationship land where P. strictum and S. fallax co-existed (Mitchell between N concentration and N input is shown along with For personal use only. the homeostatic regulation coefficient (H). Nitrogen et al. 2002). On the other hand, ultimate reduction in growth concentration data for Vaccinium myrtilloides, Rhododendron was detected after 2 years where N was added at the dose of groenlandicum, and Chamaedaphne calyculata are from Bubier 2 g·m−2·year−1 in a fen (Bu et al. 2011), and 6.4 g·m−2·year−1 with et al. (2011). additional P and K drastically reduced abundance in the fertilization experiment at Mer Bleue (Bubier et al. 2007). 1.8 C. calyculata y = 1.08x0.104, R2 = 0.23, H = 9.30 Bu et al. (2011) found that P. strictum performed better in R. groenlandicum y = 1.12x0.099, R2 = 0.25, H = 10.10 the presence of sphagna, suggesting that the overall de- 0.105 2 1.6 V. myrtilloides y = 1.18x , R = 0.35, H = 9.53 crease in moss coverage may be harmful to P. strictum. V. myrtilloides C. calyculata R. groenlandicum 1.4 CO2 exchange We hypothesized that adding N would impact photo-

Botany Downloaded from www.nrcresearchpress.com by HELSINKI UNIV on 05/02/16 synthesis and dark respiration of these mosses. However, 1.2 we found no significant effects from the treatment on

CO2 exchange rates of separated Sphagnum capitula and 1.0 P. strictum shoots (Figs. 3g–3h). Moreover no differences in

P. strictum stress between the species or treatments were indicated Tissue N concentration (%) 0.8 by the measurements of maximum quantum efficiency S. capillifolium of PSII. However, in the Sphagnum-only cores, R measured per unit dry mass was increased in the 6.4N plots by com- 0.6 S. capillifolium y = 0.81x0.281 , R2 = 0.76, H = 3.56 parison with the control (Figs. 3e–3f), and R was positively P. strictum y = 0.90x0.147, R2 = 0.49, H = 6.80 related to tissue levels of N. These results for respiration 0.4 could be due to decay in the lower parts; moreover, the 02468 N input (g m-2 yr-1) removal of P. strictum likely loosened the cores, especially in the 6.4N plots, thus potentially decreasing its extracellular water transport capacity, and thereby affecting the distri- bution of water in the core (Schipperges and Rydin 1998;

Published by NRC Research Press Juutinen et al. 135

Marschall and Proctor 2004). There was also a slightly neg- changes in vascular plant canopy stayed moderate in this ative relationship between photosynthesis and levels of N N-only experiment. Decreased light penetration to the moss for sphagna in the cores where P. strictum was removed, but surface is an alternative hypothesis to explain the decrease in there was no significant differences between treatments. Sphagnum/moss coverage associated with increased vascular Increased respiration is a sign of building and main- plant coverage under conditions of enhanced N availability taining N-rich compounds to act as N storage, for exam- (e.g., Bubier et al. 2007; Wiedermann et al. 2009b). Drastic ple, the production of detoxifying amino-acids (Baxter shading caused by nets (PPFD < 40 ␮mol·m−2·s−1) was et al. 1992; Reich et al. 1998; Limpens and Berendse 2003; found to decrease the production of S. capillifolium (Bonnett Koranda et al. 2007). Increased dark respiration as a re- et al. 2010). The expansion of P. strictum, however, must sponse to fertilization with N has also been found in partly reduce the available irradiation to the surface of dwarf-shrub leaves (Heskel et al. 2012; Bui 2013). The re- S. capillifolium, and it is possible that shading by vascular sponse of photosynthesis to different tissue levels of N plants allow P. strictum to expand first, followed by a has been found to be negligible, positive, or negative, in reduction in the abundance of S. capillifolium. other studies. Granath et al. (2009a, 2009b, 2012) showed variation from positive to a unimodal response of photo- Nitrogen and ecosystem dynamics synthesis to N levels among species (S. balticum, S. fallax, The N treatments affected the moss levels of N (Fig. 4), and S. fuscum). Among others, Fritz et al. (2012) discussed and the range was similar to that found in studies with a the possibility that allocation of excess N into chlo- deposition gradient and simulated deposition of 0.1– rophyll can lead to photo-inhibition of photosynthesis. A 4.0 g N·m−2·year−1 (e.g., Aerts et al. 1992; Jauhiainen et al. decrease in photosynthesis of S. recurvum was detected 1998; Bragazza et al. 2005). Sphagnum capillifolium tissues when levels of N in the capitula reached about 1.5%, and accumulated N, whereas P. strictum better regulated its was associated with reduced water content and necrosis tissue levels of N under different deposition doses, thus (van der Heijden et al. 2000a). These factors may contrib- having stronger N homeostasis (H)(Fig. 4). The weak H of ute to the slight negative relationship between photo- S. capillifolium (3.6) in our study agrees with the global synthesis and tissue levels of N found in our data. Reduced water content can result from a change in growth data set that gives an H value of 3.2 for the capitula of pattern and lower the bulk density of the moss canopy due to hummock Sphagnum (Limpens et al. 2011). In turn, the excess N, which may be reflected as a decrease in photosyn- H value of P. strictum (6.80) is closer to that of the bog thetic capacity of Sphagnum mosses (e.g., Fritz et al. 2012). shrubs of Mer Bleue (9.3 to 10.1, Fig. 4). In a grassland Moreover, S. capillifolium exposed to ammonium-N with a study, Yu et al. (2010) found these high values of H are total load equaling our 6.4 g N·m−2·year−1 allocated the ex- typical for species that can tolerate changes in N input. For personal use only. cess N to cell-wall proteins, which resulted in decreased It seems that P. strictum avoided nutrient deficiency from cross-sectional area of hyaline cells and reduced water- our N treatments and took advantage of the increased avail- holding capacity (Manninen et al. 2011). It has been noted ability of N, initiating and expanding new photosynthetic that adding N produces a stronger negative impact on tissue. Our study suggests that its photosynthetic rate is Sphagnum performance during dry conditions (Gerdol not changing, but because of the increase in biomass et al. 2008; Limpens et al. 2011). We have observed that and higher photosynthetic capacity of P. strictum, com- S. capillifolium reappeared in patches in the 6.4N plots pared with S. capillifolium (Fig. 3), the photosynthetic ca- also treated with P and K since 2001, where the surface pacity of the moss layer in the 6.4N plots increased became wetter and the shrub canopy opened, giving sup- (Figs. 3a–3c). The increase in abundance and growth for port to the idea that the response of S. capillifolium to excess P. strictum following enhanced N supply seems to be tran-

Botany Downloaded from www.nrcresearchpress.com by HELSINKI UNIV on 05/02/16 N can be modulated by moisture in the environment. sient (Fig. 1b, Bubier et al. 2007; Juutinen et al. 2010; Maximum quantum efficiency of PSII (F /F ) is consid- v m Larmola et al. 2013). It is possibly related to the strong ered to be indicative of plant stress (e.g., Maxwell and increase in dwarf-shrub abundance and decrease in light Johnson 2000). The values measured in this study were reaching the moss canopy (Chong et al. 2012). In addi- low compared with what are considered to be nonstressed tion, the conditions associated with the overall decrease vascular plants (0.83) (e.g., Demmig and Björkman 1987) and measured for sphagna in shaded habitats (0.72–0.82) in moss cover may have inhibited its growth (see also Bu (Kangas et al. 2014), but in a similar range to several other et al. 2011). moss studies in open peatlands with no N treatments Changes in light and water availability may be important (Murray et al. 1993; Granath et al. 2009a; Hájek et al. 2009; factors for the growth of the moss layer and responses to Laine et al. 2011). excess N. The loss of Sphagnum mosses with their slow rates Our data gave no clear indication that vascular plant of decomposition, compared with P. strictum and shrub presence played a significant role in changes in growth leaves and stems (Moore and Basiliko 2006), means that the and abundance of S. capillifolium and P. strictum within the ability of peatlands to accumulate C can decrease without 5 years of this study. This may be due to the fact that the corresponding increases in C input.

Published by NRC Research Press 136 Botany Vol. 94, 2016

Acknowledgements Canadian Climate Normals. 1981–2010. National climate and infor- We thank Mike Dalva (Geography) and Mark Romer mation archive. Available from http://climate.weather.gc.ca/ (Biology) of McGill University for their help in the field and climate_normals/results_1981_2010_e.html?stnID=4789& autofwd=1. Phytotron, respectively. The National Science Foundation Chong, M., Humphreys, E., and Moore, T.R. 2012. Microclimatic (DEB1019523) and the Natural Sciences and Engineering Re- response to increasing shrub cover and its effect on Sphagnum

search Council provided funding to J.L.B. and T.R.M., re- CO2 exchange in a bog. Ecoscience, 19: 89–97. doi:10.2980/19- spectively, and the National Capital Commission gave 1-3489. Clymo, R.S. 1970. The growth of Sphagnum: methods of measure- access to Mer Bleue. S.J., A.M.L., and E.-S.T. were funded by ment. J. Ecol. 58: 13–49. doi:10.2307/2258168. the Academy of Finland (project 140863). Demmig, B., and Björkman, O. 1987. Comparison of the effect of excessive light and chlorophyll fluorescence (77k) and pho- References ton yield of O2 evolution in leaves of higher plants. Planta, Aerts, R., Wallen, B., and Malmer, N. 1992. Growth-limiting nu- 171: 171–184. doi:10.1007/BF00391092. PMID:24227324. trients in sphagnum dominated subject to low and high Elser, J.J., Fagan, W.F., Kerkhoff, A.J., Swenson, N.G., and atmospheric supply. J. Ecol. 80: 131–140. doi:10.2307/2261070. Enquist, B.J. 2010. Biological stoichiometry of plant produc- Baxter, R., Emes, M.J., and Lee, J.A. 1992. Effects of an experi- tion: metabolism, scaling and ecological response to global mentally applied increase in ammonium on growth and change. New Phytol. 186: 593–608. doi:10.1111/j.1469-8137. amino-acid metabolism of Sphagnum cuspidatum Ehrh. ex 2010.03214.x. PMID:20298486. Hoffm. from differently polluted areas. New Phytol. 120: Fritz, C., van Dijk, G., Smolders, A.J.P., Pancotto, V.A., 265–274. doi:10.1111/j.1469-8137.1992.tb05663.x. Elzenga, T.J.M.T., Roefols, J.G.M., and Grootjans, A.P. 2012. Berendse, F., van Breemen, N., Rydin, H., Buttler, A., Hejmans, M., Nutrient additions in pristine Patagonian Sphagnum bog Hoosbeek, M.R., Lee, J.A., Mitchell, E., Saarinen, T., Vasander, H., vegetation: can phosphorus addition alleviate (the effects of) and Wallen, B. 2001. Raised atmospheric CO2 levels and in- increased nitrogen loads. Plant Biol. 14: 491–499. doi:10.1111/ creased N deposition cause shifts in plant species composi- j.1438-8677.2011.00527.x. PMID:22221295. tion and production in Sphagnum bogs. Global Change Biol. 7: Galloway, J.N., Townsend, A.R., Erisman, J.W., Bekunda, M., 591–598. doi:10.1046/j.1365-2486.2001.00433.x. Cai, Z., Freney, J.R., Martinelli, L.A., Seitzinger, S.P., and Bobbink, R., Hicks, K., Galloway, J., Spranger, T., Alkemade, R., Sutton, M.A. 2008. Transformation of the nitrogen cycle: re- Ashmore, M., Bustamante, M., Cinderby, S., Davidson, E., cent trends, questions, and potential solutions. Science, 320: Dentener, F., Emmett, B., Erisman, J.-W., Fenn, M., Gilliam, F., 889–892. doi:10.1126/science.1136674. PMID:18487183. Nordin, A., Pardo, L., and De Vries, W. 2010. Global assessment Gerdol, R., Bragazza, L., and Brancaleoni, L. 2008. Heatwave of nitrogen deposition effects on terrestrial plant diversity: a 2003: High summer temperature, rather than experimental

synthesis. Ecol. Appl. 20: 30–59. doi:10.1890/08-1140.1. PMID: fertilization, affects vegetation and CO2 exchange in an al- 20349829. pine bog. New Phytol. 179: 142–154. doi:10.1111/j.1469-8137. Bonnett, S.A.F., Ostle, N., and Freeman, C. 2010. Short-term ef- 2008.02429.x. PMID:18373651. fect of deep shade and enhanced nitrogen supply on Sphagnum González, E., Rochefort, L., Bourdeau, S., Hugron, S., and capillifolium morphophysiology. Plant Ecol. 207: 347–358. doi: Poulin, M. 2013. Can indicator species predict restoration

For personal use only. 10.1007/s11258-009-9678-0. outcomes early in the monitoring process? A case study with Bragazza, L., Tahvanainen, T., Kutnar, L., Rydin, H., Limpens, J., peatlands. Ecol. Indic. 32: 232–238. doi:10.1016/j.ecolind.2013. Hájek, M., Grosvernier, P., Hájek, T., Hajkova, P., Hansen, I., 03.019. Iacumin, P., and Gerdol, R. 2004. Nutritional constraints in Granath, G., Wiedermann, M.M., and Strengbom, J. 2009a. Phys- ombrotrophic Sphagnum plants under increasing atmospheric iological responses to nitrogen and sulphur addition and nitrogen deposition in Europe. New Phytol. 163: 609–616. raised temperature in Sphagnum balticum. Oecologia, 161: doi:10.1111/j.1469-8137.2004.01154.x. 481–490. doi:10.1007/s00442-009-1406-x. PMID:19593588. Bragazza, L., Limpens, J., Gerdol, R., Grosvenier, P., Hájek, M., Granath, G., Strengbom, J., Breeuwer, A., Heijmans, M.M.P.D., Hájek, T., hajkova, P., Hansen, I., Iacumin, P., Kutnar, L., Berendse, F., and Rydin, H. 2009b. Photosynthetic perfor- Rydin, H., and Tahvanainen, T. 2005. Nitrogen concentration mance in Sphagnum transplanted along a latitudinal nitrogen and ␦15N signature of ombrotrophic Sphagnum mosses at dif- deposition gradient. Oecologia, 159: 705–715. doi:10.1007/ ferent N deposition levels in Europe. Global Change Biol. 11: s00442-008-1261-1. PMID:19137328. 106–114. doi:10.1111/j.1365-2486.2004.00886.x. Granath, G., Strengbom, J., and Rydin, H. 2012. Direct physio- Bu, Z.J., Rydin, H., and Chen, X. 2011. Direct and interaction- logical effects of nitrogen on Sphagnum: a greenhouse exper- Botany Downloaded from www.nrcresearchpress.com by HELSINKI UNIV on 05/02/16 mediated effects of environmental changes on peatland iment. Funct. Ecol. 26: 353–364. doi:10.1111/j.1365-2435.2011. bryophytes. Oecologia, 166: 555–563. doi:10.1007/s00442-010- 01948.x. 1880-1. PMID:21170747. Granath, G., Limpens, J., Posch, M., Mücher, S., and de Vries, W. Bubier, J.L., Moore, T.R., and Bledzki, L. 2007. Effects of nutrient 2014. Spatio-temporal trends of nitrogen deposition and cli- addition on vegetation and carbon cycling in an ombro- mate effects on Sphagnum productivity in European peat- trophic bog. Global Change Biol. 13: 1168–1186. doi:10.1111/j. lands. Environ. Pollut. 187: 73–80. doi:10.1016/j.envpol.2013. 1365-2486.2007.01346.x. 12.023. PMID:24457298. Bubier, J.L., Smith, R., Juutinen, S., Moore, T.R., Minocha, R., Gunnarsson, U., and Rydin, H. 2000. Nitrogen fertilization re- Long, S., and Minocha, S. 2011. Effects of nutrient addition on duces Sphagnum production in Swedish bogs. New Phytol. leaf chemistry, morphology, and photosynthetic capacity of 147: 527–537. doi:10.1046/j.1469-8137.2000.00717.x. three bog shrubs. Oecologia, 167(2): 355–368. doi:10.1007/ Gunnarsson, U., Granberg, G., and Nilsson, M. 2004. Growth, s00442-011-1998-9. PMID:21544572. production and interspecific competition in Sphagnum ef- Bui, V.N.T. 2013. Photosynthetic performance of Chamaedaphne fects of temperature, nitrogen and sulphur treatments on a calyculata after twelve years of nutrient fertilization at Mer boreal mire. New Phytol. 163: 349–359. doi:10.1111/j.1469- Bleue bog, Ontario. B.Sc. Honours thesis, Department of En- 8137.2004.01108.x. vironmental Sciences, Mount Holyoke College, South Had- Hájek, T., Tuittila, E.-S., Ilomets, M., and Laiho, R. 2009. Light ley, Mass. responses of mire mosses — a key to survival after water-

Published by NRC Research Press Juutinen et al. 137

level drawdown? Oikos, 118: 240–250. doi:10.1111/j.1600-0706. open grown) Sphagnum capillifolium. Environ. Exp. Bot. 72: 2008.16528.x. 140–148. doi:10.1016/j.envexpbot.2011.02.015. Heskel, M.A., Anderson, O.R., Atkin, O.K., Turnbull, M.H., and Marschall, M., and Proctor, M.C.F. 2004. Are bryophytes shade Griffin, K.L. 2012. Leaf- and cell-level carbon cycling re- plants? Photosynthetic light responses and proportions of sponses to a nitrogen and phosphorus gradient in two Arctic chlorophyll a, chlorophyll b, and total carotenoids. Ann. Bot. species. Am. J. Bot. 99: 1702–1714. doi:10.3732/ajb. 94: 593–603. PMID:15319230. 1200251. PMID:22984095. Maxwell, K., and Johnson, G.N. 2000. Chlorophyll fluores- Hoosbeek, M.R., van Breemen, N., Vasander, H., Buttler, A., and cence — a practical guide. J. Exp. Bot. 51: 659–668. doi:10. Berendse, F. 2002. Potassium limits potential growth of bog 1093/jexbot/51.345.659. PMID:10938857.

vegetation under elevated atmospheric CO2 and N deposi- Mitchell, E.A.D., Buttler, A., Grosvernier, P., Rydin, H., tion. Global Change Biol. 8: 1130–1138. doi:10.1046/j.1365- Siegenthaler, A., and Gobat, J.-M. 2002. Contrasted effects of 2486.2002.00535.x. increased N and CO2 supply on two keystone species in peat- Jauhiainen, J., Vasander, H., and Silvola, J. 1994. Response of land restoration and implications for global change. J. Ecol. 90: 529–533. doi:10.1046/j.1365-2745.2002.00679.x. Sphagnum fuscum to N deposition and increased CO2. J. Bryol. 18: 83–95. doi:10.1179/jbr.1994.18.1.83. Moore, T.R. 1989. Growth and net production of Sphagnum at Jauhiainen, J., Vasander, H., and Silvola, J. 1998. Nutrient con- five fen sites, subarctic eastern Canada. Can. J. Bot. 67(4): centration in Sphagna at increased N-deposition rates and 1203–1207. doi:10.1139/b89-156. Moore, T., and Basiliko, N. 2006. Decomposition. In Boreal peat- raised atmospheric CO2 concentrations. Plant Ecol. 138: 149– 160. doi:10.1023/A:1009750702010. land ecosystems. Edited by R.K. Wieder and D.H. Vitt. Ecol. Juutinen, S., Bubier, J.L., and Moore, T.R. 2010. Responses of Stud. 188: 126–143. Springer-Verlag. Moore, T., Bubier, J., Lafleur, P., Frolking, S., and Roulet, N. 2002. vegetation and ecosystem CO2 exchange to nine years of nu- trient addition at Mer Bleue bog. Ecosystems, 13: 874–887. Plant biomass, production and CO2 exchange in an ombro- doi:10.1007/s10021-010-9361-2. trophic bog. J. Ecol. 90: 25–36. doi:10.1046/j.0022-0477.2001. Kangas, L., Maanavilja, L., Hájek, T., Juurola, E., Chimner, R., 00633.x. Moore, T.R., Bubier, J.L., and Bledzki, L. 2007. Litter decomposi- Mehtätalo, L., and Tuittila, E.-S. 2014. Photosynthetic traits of tion in temperate peatland ecosystems: the effect of substrate Sphagnum and feather moss species in undrained, drainaed and site. Ecosystems, 10: 949–963. doi:10.1007/s10021-007- and rewetted boreal spruce swamp forests. Ecol. Evol. 4: 381– 9064-5. 396. doi:10.1002/ece3.939. PMID:24634723. Murray, K.J., Tenhunen, J.D., and Nowak, R.S. 1993. Photoinhibition Koranda, M., Kerschbaum, S., Wanek, W., Zechmeister, H., and as a control on photosynthesis and production of Sphagnum Richter, A. 2007. Physiological responses of bryophytes mosses. Oecologia, 96: 200–207. doi:10.1007/BF00317733. Thuidium tamariscinum and Hylocomium splendens to increased Reich, R., Ellsworth, D.S., and Walters, M.B. 1998. Leaf structure nitrogen deposition. Ann. Bot. 99: 161–169. doi:10.1093/aob/ (specific leaf area) modulates photosynthesis–nitrogen rela- mcl239. PMID:17101638. tions: evidence from within and across species and functional Laine, A.M., Juurola, E., Hájek, T., and Tuittila, E.-S. 2011. Sphag- groups. Funct. Ecol. 12: 948–958. doi:10.1046/j.1365-2435.1998. num growth and ecophysiology during mire succession. 00274.x. Oecologia, 167: 1115–1125. doi:10.1007/s00442-011-2039-4. PMID: Robert, E.C., Rochefort, L., and Garneau, M. 1999. Natural vege- 21656299. For personal use only. tation of two block-cut mined peatlands in eastern Canada. Larmola, T., Bubier, J.L., Kobyljanec, C., Basiliko, N., Can. J. Bot. 77(3): 447–459. doi:10.1139/b99-019. Juutinen, S., Humphreys, E., Preston, M., and Moore, T.R. Rochefort, L., Vitt, D.H., and Bayley, S. 1990. Growth, produc- 2013. Vegetation feedbacks of nutrient addition lead to a tion, and decomposition dynamics of Sphagnum under natu- weaker carbon sink in an ombrotrophic bog. Global Change ral and experimentally acidified conditions. Ecology, 71: Biol. 19: 3729–3739. doi:10.1111/gcb.12328. 1986–2000. doi:10.2307/1937607. Limpens, J., and Berendse, F. 2003. Growth reduction of Sphagnum Schipperges, B., and Rydin, H. 1998. Response of photosynthesis magellanicum subjected to high nitrogen deposition: the role of of Sphagnum species from contrasting microhabitats to tissue amino acid nitrogen concentration. Oecologia, 135: 339–345. water content and repeated desiccation. New Phytol. 140: doi:10.1007/s00442-003-1224-5. PMID:12721822. 677–684. doi:10.1046/j.1469-8137.1998.00311.x. Limpens, J., Raymakers, J.T.A.G., Baar, J., Berendse, F., and Shaver, G.R., and Laundre, J. 2007. Exertion, elongation, and Zijlstra, J.D. 2003. The interaction between epiphytic algae, a senescence of leaves of Eriophorum vaginatum and Carex parasitic and Sphagnum as affected by N and P. Oikos, bigelowii in northern Alaska. Global Change Biol. 3: 146–157. 103: 59–68. doi:10.1034/j.1600-0706.2003.12580.x. Solberg, S., Dobbertin, M., Reinds, G.J., Lange, H., Andreassen, K., Botany Downloaded from www.nrcresearchpress.com by HELSINKI UNIV on 05/02/16 Limpens, J., Granath, G., Gunnarsson, U., Aerts, R., Bayley, S., Fernandez, P.G., Hildingsson, A., and de Vries, W. 2009. Analy- Bragazza, L., Bubier, J., Buttler, A., van den Berg, L.J.L., ses of the impact of changes in atmospheric deposition and Francez, A.-J., Gerdol, R., Grosvernier, P., Heijmans, M.M.P.D., climate on forest growth in European monitoring plots: a Hoosbeek, M.R., Hotes, S., Ilomets, M., Leith, I., Mitchell, E.A.D., stand growth approach. For. Ecol. Manage. 258: 1735–1750. Moore, T., Nilsson, M.B., Nordbakken, J.-F., Rochefort, L., doi:10.1016/j.foreco.2008.09.057. Rydin, H., Sheppard, L.M., Thormann, M., Wiedermann, M.M., Sottocornola, M., Boudreau, S., and Rochefort, L. 2007. Peat bog Williams, B.L., and Xu, B. 2011. Climatic modifiers of the re- restoration: effect of phosphorus on plant re-establishment. sponse to nitrogen deposition in peat-forming Sphagnum Ecol. Eng. 31: 29–40. doi:10.1016/j.ecoleng.2007.05.001. mosses: a meta-analysis. New Phytol. 191: 496–507. doi:10. Sterner, R.W., and Elser, J.J. 2002. Ecological stoichiometry: the 1111/j.1469-8137.2011.03680.x. PMID:21434930. biology of elements from molecules to the biosphere. Prince- Malmer, N., Albinson, C., Svensson, B.M., and Wallén, B. 2003. ton University Press, Princeton, N.J. Interferences between Sphagnum and vascular plants: ef- Straková, P., Anttila, J., Spetz, P., Kitunen, V., Tapanila, T., and fects on plant community structure and peat formation. Laiho, R. 2010. Litter quality and its response to water level Oikos, 100: 469–482. doi:10.1034/j.1600-0706.2003.12170.x. drawdown in boreal peatlands at plant species and com- Manninen, S., Woods, C., Leith, I.D., and Sheppard, L.C. 2011. munity level. Plant Soil, 335: 501–520. doi:10.1007/s11104- Physiological and morphological effects of long-term ammo- 010-0447-6. nium or nitrate deposition on the green and red (shade and Turunen, J., Roulet, N., Moore, T.R., and Richard, P. 2004. Nitro-

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gen deposition and increased carbon accumulation in om- 106: 235–245. doi:10.1639/0007-2745(2003)106[0235:ROSFTN]2.0. brotrophic peatlands in eastern Canada. Global Biogeochem. CO;2. Cycles, 18(3): GB3002. doi:10.1029/2003GB002154. Wang, M., and Moore, T.R. 2014. Carbon, nitrogen, phosphorus, van der Heijden, E., Verbeek, S.K., and Kuiper, P.J.C. 2000a. and potassium stoichiometry in an ombrotrophic peatland reflects plant functional type. Ecosystems, 17: 673–684. doi: Elevated atmospheric CO and increased nitrogen deposi- 2 10.1007/s10021-014-9752-x. tion: effects on C and N metabolism and growth of the peat Wiedermann, M.M., Nordin, A., Gunnarsson, U., Nilsson, M.B., moss Sphagnum recurvum P. Beauv. var. mucronatum (Russ.) and Ericson, L. 2007. Global change shifts vegetation and Warnst. Global Change Biol. 6: 201–212. doi:10.1046/j.1365- plant-parasite interactions in a boreal mire. Ecology, 88: 2486.2000.00303.x. 454–464. doi:10.1890/05-1823. PMID:17479763. van der Heijden, E., Jauhiainen, J., Silvola, J., Vasander, H., and Wiedermann, M.M., Gunnarsson, U., Ericson, L., and Nordin, A.

Kuiper, P.J.C. 2000b. Effects of elevated atmospheric CO2 con- 2009a. Ecophysiological adjustment of two Sphagnum species centration and increased nitrogen deposition on growth and in response to anthropogenic nitrogen deposition. New Phy- chemical composition of ombrotrophic Sphagnum balticum tol. 181: 208–217. doi:10.1111/j.1469-8137.2008.02628.x. PMID: and oligo-mesotrophic Sphagnum papillosum. J. Bryol. 22: 175– 18811618. 182. doi:10.1179/jbr.2000.22.3.175. Wiedermann, M.M., Gunnarsson, U., Nilsson, M.B., Nordin, A., and Ericson, L. 2009b. Can small-scale experiments predict Vitt, D. 1990. Growth and production dynamics of boreal mosses ecosystem responses? An example from peatlands. Oikos, over climatic, chemical and topographic gradients. Bot. J. 118: 449–456. doi:10.1111/j.1600-0706.2008.17129.x. Linn. Soc. 104: 35–59. doi:10.1111/j.1095-8339.1990.tb02210.x. Yu, Q., Chen, Q., Elser, J.J., He, N., Wu, H., Zhang, G., Wu, J., Vitt, D.H., Wieder, K., Halsey, L.A., and Turetsky, M. 2003. Re- Bai, Y., and Han, X. 2010. Linking stoichiometric homoeostasis sponse of Sphagnum fuscum to nitrogen deposition: a case with ecosystem structure, functioning and stability. Ecol. Lett. 13: study of ombrogenous peatlands in Alberta, Canada. Bryologist, 1390–1399. doi:10.1111/j.1461-0248.2010.01532.x. PMID:20849443. For personal use only. Botany Downloaded from www.nrcresearchpress.com by HELSINKI UNIV on 05/02/16

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