Thermo-erosion gullies boost the transition from wet to mesic vegetation

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Introduction
Warming in the Arctic is occurring twice as fast as the global average (USGCRP, 2009; New et al., 2011;NOAA, 2014).Perennially frozen ground (permafrost) has consequently warmed by 2 • C over the last 20 to 30 years (Christiansen et al., 2010;Romanovsky et al., 2010), and there is now evidence of a decrease in both permafrost area extent across the Northern Hemisphere and permafrost thickness at the local scale (Beilman and Robinson, 2003;Payette et al., 2004;Couture and Pollard, 2007;Smith, 2011) Permafrost is tightly associated with biophysical components such as air temperatures, soil conditions, surface water, groundwater, snow cover and vegetation (Jorgenson et al., 2010;Sjöberg, 2015).For instance, permafrost impedes water to drain to deeper soil layers maintaining a perched water table and saturated soils across the Arctic (Woo, 2012;Natali et al., 2015).These wet habitats are dominated by highly productive graminoid species which offer abundant high quality food for arctic herbivores (Manseau and Gauthier, 2003;Doiron, 2014) while significantly contributing to methane emission (Brummel et al., 2012;Bouchard et al., 2014).Permafrost degradation that would increase subsurface drainage and reduce the extent of lakes and wetlands at high latitudes (Avis et al., 2011;Jorgenson et al., 2013;Beck et al., 2015) would have major consequences on ecosystem structure and function (Collins et al., 2013;Jorgenson et al., 2013).This would also strongly influence variations of active layer depths (Wright et al., 2009;Shiklomanov et al., 2010;Gangodagamage et al., 2014) as illustrated by long-term monitoring sites throughout the circumpolar North (Tarnocai et al., 2004;Nelson et al., 2008;Smith et al., 2009;Shiklomanov et al., 2010).
Several forms of ground and massive ice can be found within permafrost (Rowland et al., 2010), especially ice-wedges in regions where winter temperatures enable thermal contraction cracking (Fortier and Allard, 2005;Ewertowski, 2009;Kokelj et al., 2014).Continuous permafrost zones with well-developed polygonal ice-wedge networks are particularly vulnerable to climate change because ice-wedges are usually found near the top of permafrost (Smith et al., 2005;Jorgenson et al., 2006;Woo et al., 2008;Vonk et al., 2013).In these regions, thawing permafrost can result in ground ice erosion and displacement of sediments, carbon and nutrients by drainage (Rowland et al., 2010;Godin et al., 2014;Harms et al., 2014).This thermo-erosion process has especially been observed across North-America (Grosse et al., 2011), in Siberia (Günther et al., 2013) and in the Antarctic Dry Valleys (Levy et al., 2008).On Bylot Island in Nunavut, thermo-mechanical erosion by water has initiated internal tunneling and the development of gully networks in both aeolian and organic depositional environments which cover about 20 000 m 2 (Fortier et al., 2007;Godin andFortier, 2010, 2012a).Introduction

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Full A fine-scale spatio-temporal monitoring study of the six largest gullies showed that their development rate ranges from 14 to 25 m yr −1 , going up to 80 m yr −1 during their inception (Godin and Fortier, 2012b), leading to substantial changes in the drainage network and increased eroded area throughout the valley (Godin et al., 2014).The extent to which vegetation is affected by thermo-erosion gullying is currently unknown, and contrasts with the extensive literature that has documented increased plant abundance, growth and biomass as well as shrub expansion northward in response to climate warming (Hudson and Henry, 2010;Epstein et al., 2013;Boulanger-Lapointe et al., 2014;Naito and Cairns, 2015).This information is urgently needed as vegetation plays an important role in structuring Arctic ecosystems and regulating permafrost response to climate change (Jorgenson et al., 2010;Gauthier et al., 2011;Legagneux et al., 2012).Wetlands, which serve as preferred grounds for Arctic herbivores (Gauthier et al., 1996;Massé et al., 2001;Doiron et al., 2014), are expected to produce more methane compared to shrub-dominated areas (Olefeldt et al., 2013;Nauta et al., 2015;Treat et al., 2015), which on the other hand may reduce active layer thickness and permafrost thaw rate (Blok et al., 2010).The present study aimed at examining plant community patterns following thermoerosion processes.Bylot Island, where geomorphological and ecological processes in response to climate change have been monitored for over two decades (Gauthier et al., 1995;Allard, 1996;Fortier and Allard, 2004), offered a unique opportunity to specifically assess the response of wetlands to gullying.The following questions were addressed: ( 1) to what extent thermo-erosion gullying modifies environmental conditions of low-centered wetland polygons?(2) how do plant communities cope with these geomorphological changes, i.e. do we observe shifts in plant diversity, abundance and productivity?Introduction

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Study area
This study took place in the Qarlikturvik valley of Bylot Island, Nunavut, Canada (73 • 09 N, 79 • 57 W; Fig. 1a).Bound to the North and South by plateaus < 500 m a.s.l., it connects C-79 and C-93 glaciers to the Navy Board Inlet sea via a proglacial river that passes through the plain.The valley (ca.50 km 2 ) is characterized by a polygonpatterned ladnscape that results from ice-wedge development during the late Holocene (Ellis and Rochefort, 2004;Fortier and Allard, 2004;Ellis et al., 2008).It is defined by elevated ridges that enclose a central depression usually flooded by snowmelt water (Woo and Young, 2006;Minke et al., 2007).

Field sites
Our work was conducted along three gullies that were selected among the 36 identified in the valley (Fig. 1b; Godin and Fortier, 2012b).All three were located in the lowcentered polygon complex of the Qarlikturvik valley.Gullies A and B, respectively 835 and 717 m long, are characterized by substantial thermo-erosion processes (Fortier Introduction

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Full  et al., 2007;Godin and Fortier, 2012b) whilst gully C, 180 m long, has not been active in recent years.A total of 197 sampling sites (i.e.polygons) were randomly selected along the three gullies (Fig. 1b; n = 95, 79 and 21 for gully A, B and C, respectively).They were classified into one of four polygon categories (referred hereafter as habitats) that were defined by the following characteristics: polygon rim integrity, drainage observation and polygon location along the gully (Fortier et al., 2007;Godin and Fortier, 2012b).The habitats were sorted as follows: (i ) undisturbed wet polygons (n = 62), encompassing sites that were not affected by gullying, i.e. with intact rims and non-apparent ice wedges; less than five year disturbed polygons (n = 44), characterizing sites located along the gully most recent sections and that have been recently perturbed, i.e. with partially degraded rims, ongoing drainage and apparent ice wedges; more than five year disturbed polygons (n = 43), representing sites where rims are heavily degraded and drainage is almost complete; mesic environments (n = 48), characterizing sites located along the oldest gully sections and where rims have entirely collapsed.

Environmental condition monitoring
Daily precipitation was recorded with a manual rain gauge throughout summer 2010 at the base camp, located 700 m west of the gully A (Gauthier et al., 2010).Soil (top 10 cm) moisture content was recorded at the center of each polygon using ECH 2 O EC-5 moisture sensors connected to Em5b dataloggers (Decagon Devices, WA, USA).Measurements were carried out in 2010, both 5 July (early season) and 30 July (late season) in polygons located along gullies A and B, and 30 July only in gully C polygons.Thaw front depth was recorded at the center of each polygon using a steel rod graduated in centimetre and driven in the active layer.This was done late July 2009 and 2010 in polygons located along gullies A and B, and mid-July 2010 in those situated along gully C. One measure was taken per polygon, except for the mesic sites where three measurements were randomly conducted.In this case, means per site were calculated prior to analyses.Introduction

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Plant community characterization
Species richness and abundance were determined in each polygon in July 2009 or 2010 using randomly placed 70 cm × 70 cm quadrats within areas that were representative of habitats.Abundance of vascular plants, lichens, mosses, Nostoc, fungi, cryptogamic crust, bare ground, litter, standing dead, standing water, sings of grubbing, and geese feces was evaluated as cover percentages using photography analyses.Three quadrats were vertically photographed at each site at ca 1.3 m from the ground (see detailed protocols in Chen et al., 2010 and the IPY CiCAT, 2012).Daubenmire cover class method (Daubenmire, 1959) was then applied on each quadrat picture to evaluate species cover as the projection on the ground of all species above-ground parts.
Five polygons per habitat were also randomly selected to measure above-ground biomass of graminoid species.At each site, an exclosure of 1 m × 1 m was made of chicken wire 30 cm high and supported by wooden stakes at each corner (see Gauthier et al., 1995).Exclosures were set up in early July 2012 to avoid any significant grazing by geese.Above-ground biomass was harvested inside the exclosures near peak production in early August 2012, using imaginary grids of 25 cm × 25 cm for wet and recently disturbed polygons and of 50 cm × 20 cm for long-time disturbed polygons and mesic environments.All vegetation present in the exclosures was cut to a standard height, i.e. at an average of 1 cm below the moss surface (Gauthier et al., 1995).

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Statistical analyses
Differences in soil moisture content and thaw front depth between habitats were tested with a generalized linear model (procedure GLM in SAS, version 9.4, SAS Institute, Cary, NC, USA).Soil moisture content, thaw front depth as well as date or year of measurements and the interaction terms were the factors included in the model.Type III sums of squares were used for the calculation of F statistics.SNK post hoc contrasts were performed to ascertain specific differences at alpha < 0.05.Differences in graminoid above-ground biomass between habitats were tested using a one-way analysis of variance (procedure ANOVA).Distributions of variables were tested for normality prior to analyses using Shapiro-Wilk test (procedure UNIVARIATE).Contrary to graminoid above-ground biomass (W = 0.95, P = 0.42), soil moisture content (W = 0.96, P < 0.001) and thaw front depth (W = 0.98, P = 0.002) did not meet normal distribution assumptions, therefore analyses were performed on log-transformed data for these two variables (Tabachnik and Fidell, 1996;Quinn and Keough, 2002).Canonical Correspondence Analyses (CCA) were conducted to test unimodal relationships between eco-terrain sites and environmental variables (ter Braak, 1986;Zuur et al., 2007) using the "vegan" package in R 3.1.3.Two matrices were elaborated: one of 197 sites × 66 species using species mean cover per site (i.e.polygon), and another of 197 sites × 8 environmental characteristics comprising the following continuous variables: soil moisture content, thaw front depth, litter cover, bare ground, grubbing, standing dead, standing water and geese feces.Soil moisture measurements obtained on 30 July 2010 were used in the analyses.Introduction

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Environmental conditions
In 2009, above-average spring temperatures led to a rapid snowmelt (16 June) while summer was one of the driest ones on record (Gauthier et al., 2009).In 2010, despite a relatively warm spring (0.26 • C above normal), the high snow pack at the end of the winter (41.6 cm on 31 May) delayed the snowmelt in the lowlands, which was a week later than normal (28 June).Summer was characterized by warm and sunny conditions as well as below-average precipitations (cumulative rain of 84 mm vs. long-time average of 92 mm; Gauthier et al., 2010).However, the 36 mm received in only five days in mid-July were an important water input for the polygons and explain the significant difference observed in soil moisture between the two monitoring dates (df = 1, F = 55.50,P < 0.001; Fig. 2).There was also a significant difference in soil moisture content among habitats (df = 3, F = 38.02,P < 0.001; Fig. 2), which was driven both in early and late July by differences between wet polygons and the other habitats (early July: F = 18.72,P < 0.001, late July: F = 19.34,P < 0.001; Table 1).Throughout the summer, soil moisture was approximately 40 % higher in wet sites compared to disturbed and mesic ones.There was no significant habitat x date interaction (df = 3, F = 0.06, P = 0.98).We found a significant difference in thaw front depth among habitats (df = 3, F = 51.51,P < 0.001; Fig. 3), which was also driven by significant differences between wet polygons and the other habitats (2009: F = 20.51,P < 0.001, 2010: F = 33.09,P < 0.001; Table 1).Thaw front depth of wet polygons was approximately 10 cm deeper than in disturbed and mesic sites (35-36 vs. 25-27 cm).There was no significant difference between years of thaw front depth monitoring (df = 1, F = 0.46, P = 0.50; Fig. 3) and no significant habitat x year interaction (df = 3, F = 0.14, P = 0.94).Introduction

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Plant community characterization
A total of 18 vascular plant families encompassing 59 species were sampled throughout the study (Table A1).The greatest species richness was found in polygons that were disturbed 5 years ago and where both hydrophilic and mesic species were present (Table 2).The transition from wet polygons to mesic environments was accompanied by significant changes in plant community composition, especially with the decline in Cyperaceae and Poaceae cover and the emergence of Salicaceae species (Table 2).
Carex aquatilis and Dupontia fisheri were respectively present in 100 and 93 % of the wet polygons sampled, where they accounted for 52 and 26 % of the total vascular plant cover.They were found in only 47 and 16 % of mesic sites and accounted for 9 and 0.45 % of the total vascular plant cover.In contrast, Salix arctica and Arctagrostis latifolia, which were found in approximately half of the wet polygons accounting for 3 % of the total plant cover, were present in 98 % of the mesic sites where they accounted for 50 and 14 % of the total plant cover.Vegetation changes were also observed through the decline of graminoid above-ground biomass which varied significantly among habitats (df = 3, F = 11.59,P < 0.001; Table 1; Fig. 4a).Graminoid biomass was nearly five times greater in wet than in mesic sites (29.2 vs. 5.9 g m −2 ; Figs. 4a and B1).Differences were mainly driven by the decline of hydrophilic Carex aquitilis, Eriophorum scheuchzeri, Anthoxanthum arcticum and Dupontia fisheri from wet polygons to mesic environments (28.8, 19.7, 3.6 and 2.5 g m −2 in wet, recently disturbed, disturbed 5 years ago and mesic polygons, respectively; Fig. 4b).Above-ground biomass of Luzula spp., Arctagrostis latifolia and Festuca brachyfylla was contrastingly fifteen times greater in mesic than in wet habitats (4.45 vs. 0.29 g m −2 ).

Relationships between plant communities and environmental variables
The first two axes of the Canonical Correspondence Analysis retained 14 % of the vegetation data variance and 80 % of the vegetation-environment relationship variance (Table 3).Five of the eight environmental variables tested were significant within the Introduction

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Full canonical model (P < 0.05, 999 permutations), but only three -litter cover, thaw front depth and soil moisture -showed high correlations with the canonical axes (Table 4).Thaw front depth and soil moisture were strongly related to the first axis, while litter cover was mainly associated with the second axis (Table 4).Altogether, these variables discriminated well the eco-terrain units defined in the polygon-patterned landscape of the Qarlikturvik valley.Wet polygons were mainly related to high soil moisture content and substantial thaw front depth (i.e.active layer thickness) whilst mesic environments were associated with greater litter cover (Fig. 5).A transition from less to more than five year disturbed polygons was also observed along the soil moisture gradient (Fig. 5).

Discussion
Sustainability of wetlands at high latitudes relies on perennial frozen ground that prevents drainage and allows wet soil moisture conditions (Woo and Young, 2006;Ellis et al., 2008).However, snowmelt water run-off through ice-wedge polygon landscapes can initiate permafrost thermo-erosion and the development of gullies (Fortier et al., 2007;Godin et al., 2014).We show here that thermo-erosion significantly alters wetlands by rapidly decreasing soil moisture content and thaw front depth of polygons adjacent to gullies.More importantly, vegetation was sensitive to this process, and mesic habitat plant species replaced gradually hydrophilic ones within a five to ten year span.This vegetation turn-over has substantial consequences on wildlife biology, permafrost stabilization and ecosystem-level greenhouse gas emissions (Blok et al., 2010;Doiron et al., 2014;McEwing et al., 2015).

Transition in environmental conditions
Thermo-erosion gullying led to a significant decrease of soil moisture content of disturbed polygons, which is consistent with what has been previously observed in gullied areas (Seppälä 1997;Poesen et al., 2003;Godin and Fortier, 2012a;Godin et al., Introduction

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Full 2014; Harms et al., 2014).While intact wet sites store important quantities of water during spring snowmelt and summer rainfalls, gully formation rapidly reduces water conservation capacity of breached polygons due to polygon ridge erosion and water runoff through frost cracks leading to increased hydrological connectivity (Fortier et al., 2007;Godin et al., 2014).However, disturbed polygons still responded positively to summer rainfalls, and their summer volumetric water content was much more dependent of these rainfalls than in wet sites (data not shown).This is in line with the work of Godin et al. (2015) at the polygon scale, who found that soil moisture content variability in disturbed sites is significantly greater than in undisturbed polygons.
Decreasing soil moisture content in disturbed polygons came with decreasing thaw front depth.While this result was expected given that active layer thickness is closely related to local hydrological processes (Nelson et al., 1999;Hinzman et al., 2005;Minke et al., 2009;Wright et al., 2009;Gangodagamage et al., 2014), it contrasts with the active layer thickening generally observed in response to climate change (Tarnocai et al., 2004;Woo et al., 2007;Akerman and Johansson, 2008;Smith et al., 2009;Nauta et al., 2015).In addition, the values of thaw front depth obtained at our sampled sites are of similar order of magnitude than those measured by Godin et al. (2015) in their intact and disturbed polygons.Within five years of drainage, thaw front depth in disturbed polygons decreased by 37 % compared to that in intact wet sites.This is mainly explained by the greater heat transfer from water and higher thermal conduction rates in wetter polygons that provide substantial melt energy to the frost table (Nelson et al., 1997;Hinzman et al., 2005;Wright et al., 2009;Romanovsky et al., 2010).This effect is also sharpened by the low thermal conductivity of moss carpets in disturbed sites (Wright et al., 2009).

Vegetation changes
Gully development in the Qarlikturvik valley and subsequent drainage of adjacent lowcentered polygons has led to a radical change in plant communities, thus illustrating the standard plant succession following climatic and/or geomorphic variations (Billings and Introduction

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Full  , 1980;Ellis and Rochefort, 2006;Woo and Young, 2006).In the canonical ordination analysis, soil moisture gradient discriminated wet polygons from the other sites as well as recently disturbed from long-time disturbed habitats.The influence of soil moisture content in separating plant community types at high latitudes has been previously well documented (Hinzman et al., 2005;Daniëls and de Molenaar, 2011;Daniëls et al., 2011;Sandvik and Odland, 2014).This is particularly the case of Duclos (2002), who surveyed mesic and dried habitats of Bylot Island, and Mann et al. (2002), who reported that vegetation community changes in Alaska over the past 8-13 000 years mostly relied on soil moisture changes.Four other variables significantly influenced the distinction among habitats: (i) thaw front depth discriminated habitat types in the same direction than soil moisture, which was expected since these two factors are closely related to each other (see Sect. 4.1), (ii) litter cover separated mesic polygons from the others, which can be explained by increased organic matter related to greater shrub abundance in mesic environments (Zamin et al., 2014), (iii) standing dead separated wet and recently disturbed polygons from the other habitats, which can be explained by the greater abundance and biomass of Cyperaceae species at these sites (Fig. 4), (iv) geese feces were mainly associated with long-time disturbed and mesic sites.While this may suggest a higher use of these habitats by geese, the slower degradation of feces in dryer habitats cannot be ruled out; this has yet to be tested.Overall, the floristic composition of our sampled sites is in line with previous field surveys conducted in the same area (Gauthier et al., 1996;Duclos, 2002;Doiron, 2014).The presence of Carex aquatilis, Eriophorum scheuchzeri and Dupontia fisheri also characterizes well the typical vegetation of Arctic wetlands (Jorgenson et al., 2013;Sandvik and Odland, 2014;Lara et al., 2015).While the highest in disturbed polygons, species richness increased from wet and mesic environments, which is concordant with previous work showing changes in species richness in response to climate change (Britton et al., 2009;Pauli et al., 2012;Ross et al., 2012;Sandvik and Odland, 2014).More interestingly, we highlight here the rapidity of vegetation transition in response to gully development, which can be surprising given that Arctic plant

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Full communities can be highly resistant to disturbance (Hollister et al., 2005;Jonsdottir et al., 2005;Hudson and Henry, 2010).Mesic habitat species such as Luzula and Salix spp.have gradually established following the decreasein soil moisture and thaw front depth, and completely replaced hydrophilic Cyperaceae and Poaceae within ten years after gullying initiation.Plant species turn-over was also observed in moss species (data not shown), with mesic habitat Aulacomnium spp.replacing wetland-dominant Drepanochladus spp.(Ellis and Rochefort, 2004;Pouliot et al., 2009).Given that previous work has mainly characterized long-term sensitivity of wetlands to climate and geomorphologic changes (Woo and Young, 2006;Myers-Smith et al., 2008), our results offer a new perspective on vegetation response to ongoing global change.
The shift in vegetation composition was accompanied by significant changes in biomass.Above-ground biomass of graminoids was the greatest in wet polygons, which is concordant with the fact that wetlands are the most productive habitats in the Arctic (Sheard and Geale, 1983;Duclos, 2002;Doiron, 2014).Effects of thermoerosion gullying were however opposite to that of climate warming, which tends to increase plant biomass across continents and latitudes (Baldwin et al., 2014)

Impacts on ecosystems
It is likely that the replacement of hydrophilic plants by mesic vegetation will severely impact wildlife biology.The Qarlikturvik valley of Bylot Island represents an important summer habitat for greater snow geese (Legagneux et al., 2012).It is well documented Introduction

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Full that this species mostly relies on wetlands for food resources (Gauthier et al., 1995(Gauthier et al., , 2011)), especially because graminoids are easily digested thanks to their low fiber concentration and rich nutritive elements (Sedinger and Raveling, 1989;Manseau and Gauthier, 1993;Audet et al., 2007).For instance, geese removed respectively 40 and 31 % of the total annual production of Dupontia fisheri and Eriophorum scheuchzeri during the period 1990-2007(Cadieux et al., 2008)).Thus, the degradation of wetland habitat following thermo-erosion gullying will probably limit the availability of highquality food to geese since mesic habitats have a lower carrying capacity than wetlands (Massé et al., 2001.Moreover, climate warming can speed up the seasonal decline in nutritive quality of forage plants, eventually impacting goose gosling size and conditions (Doiron, 2014).In addition, the presence of ponds in wetlands provides geese refuges from predators such as arctic fox (Hughes et al., 1994;Lecomte et al., 2009), and their disappearance might also change predator-prey interactions.Effects of gullying-induced vegetation changes may finally be visible on variations of greenhouse gas emissions.There is evidence for a strong vegetation control on methane emission from wetlands (Olefeldt et al., 2013;McEwing et al., 2015;Tveit et al., 2015).In wet polygonal tundra of Northern Siberia, Kutzbach et al. (2004) found for instance that dense Carex aquatilis stands emitted more methane than sites with low Carex densities.Overall, wetlands and lakes expansion are thought to increase methane emission but also carbon storage (Myers-Smith, 2005;Nauta et al., 2015;Treat et al., 2015;Bouchard et al., 2015).We can therefore expect that the reverse transition from wet to mesic environments observed within our low-centered polygon landscape would lead to reduced methane emission and greater carbon release through enhanced decomposition.However, no general pattern on ecosystem responses to decreased water table position and subsequent gas emissions has emerged to date (see Grosse et al., 2011 for review).It will thus be crucial to determine in the near future the specific evolution of Salix and Luzula spp.primary production in mesic sites in order to accurately predict the effects of wetland retreat on methane and soil organic carbon cycles.Introduction

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Conclusions
This study illustrates that changes the shift in the hydrological and thermal regimes following the induction of thermo-erosion boost the transition of wet to mesic plant communities within low-centered polygon landscapes, providing evidence that permafrost degradation is a critical component of vegetation modifications at high latitudes.More importantly, gullying effects such as increasing drainage and wetland degradation go in the opposite direction to what is generally observed in response to climate warming.Ecological studies should consequently start using an approach that integrates disturbed permafrost monitoring if one wants to more efficiently document climate change effects on arctic terrestrial ecosystems.In addition, while surface warming is expected to alter the ground's thermal regime (Couture and Pollard, 2007), more work should be devoted to the understanding of the feedbacks plant communities and vegetation succession may have on the thermal and mechanical stabilization dynamics of permafrost terrains disturbed by thermo-erosion gullying.This is especially needed since plant community differences between disturbed and undisturbed sites can last several centuries (Cray and Pollard, 2015).
The Supplement related to this article is available online at doi:10.5194/bgd-12-12191-2015-supplement.Introduction

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Full Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3 Results Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . In our study, graminoid biomass was 35 % lower than what Cadieux et al. (2008) found via a long-term plant monitoring on Bylot Island (45.2 g m −2 ), and 62.7 % lower than what Gauthier et al. (2012) measured in the most productive ungrazed wetlands of the Qarlikturvik Valley (78.4 ± 10.5 g m −2 ).These contrasts may be explained by earlier plant harvesting in our case but more probably by varying species composition.Indeed, while we focused on wet polygons dominated by Carex aquatilis, Cadieux et al. (2008) and Gauthier et al. (2012) used wet sites dominated by Dupontia fisheri and Eriophorum scheuchzeri.