Arctic Tundra Ecosystems Research Articles

Quantifying Muskox Plant Biomass Removal and Spatial Relocation of Nitrogen in a High Arctic Tundra Ecosystem

ABSTRACTThe muskox (Ovibos moschatus), a key species in the arctic tundra, is the only large-bodied herbivore in Northeast Greenland. Here, we quantify the biomass removal and fecal deposition by muskoxen during the snow-free period in the years 1996 to 2013 in the high arctic tundra ecosystem at Zackenberg, Northeast Greenland. We show that despite high densities, muskoxen removed only 0.17% and 0.04% of the available forage in graminoid-dominated areas and in Salix snowbeds (including Salix dominated heaths), respectively, during the main plant growing season (from mid-June to end of August). Into the autumn, the biomass removal increased to ∼4.6% and 0.19% in the graminoid-dominated areas and Salix snowbeds, respectively. Muskoxen forage mainly in the graminoid-dominated areas, but defecate primarily in Salix snowbeds, resulting in net nutrient transfers from the nitrogen-rich wet habitats to the nitrogen-poor, drier habitats, corresponding to an addition of 0.016 g m−2 of nitrogen in the Salix snowbeds per year. This nitrogen addition is of same magnitude as the dissolved inorganic nitrogen pool in similar arctic soils. Hence, while the quantitative impact of muskox biomass removal seems negligible, the nitrogen relocation may be important for the arctic vegetation and associated biota in the tundra ecosystem.

The snow and the willows: earlier spring snowmelt reduces performance in the low‐lying alpine shrub Salix herbacea

Summary Current changes in shrub abundance in alpine and arctic tundra ecosystems are primarily driven by climate change. However, while taller shrub communities are expanding, dwarf shrub communities show reductions under climate warming, and the mechanisms driving the latter (such as warming temperatures or accelerated spring snowmelt) may be complex. To determine and disentangle the response of a widespread arctic–alpine prostrate dwarf shrub to both climate warming and changes in snowmelt time, we investigated phenology, clonal and sexual reproduction, leaf size, wood tissue carbon balance and leaf damage in 480 patches of Salix herbacea, along its elevational and snowmelt microhabitat range over 3 years in a space‐for‐time substitution. Earlier snowmelt was associated with longer phenological development periods, an increased likelihood of herbivory and fungal damage, lower stem density, smaller leaves and lower end‐of‐season wood reserve carbohydrates. Furthermore, while early snowmelt was associated with an increased proportion of flowering stems, the proportion of fruiting stems was not, as fruit set decreased significantly with earlier snowmelt. Warmer temperatures at lower elevations were associated with lower stem numbers and larger leaves. Synthesis. Our study indicates that phenology, fitness proxies and fungal/insect damage of the dwarf shrub S. herbacea are strongly influenced by snowmelt timing, and that earlier spring snowmelt reduced performance in S. herbacea. The likely mechanisms for many of the observed patterns are related to adverse temperature conditions in the early growing season. Reductions in clonal (stem number) and sexual reproduction (reduced fruit set) under earlier snowmelt, in addition to increasing damage probability, will likely lead to lower fitness and poorer performance, particularly in shrubs growing in early‐exposure microhabitats. Further, we saw few concurrent benefits of higher temperatures for S. herbacea, particularly as warming was associated with lower clonal growth. As growing seasons become warmer and longer in arctic and alpine tundra ecosystems, early snowmelt is a critical mechanism reducing fitness and performance in a widespread dwarf shrub and may ultimately reduce dwarf shrub communities in tundra biomes.

The effect of a permafrost disturbance on growing-season carbon-dioxide fluxes in a high Arctic tundra ecosystem

Abstract. Soil carbon stored in high-latitude permafrost landscapes is threatened by warming and could contribute significant amounts of carbon to the atmosphere and hydrosphere as permafrost thaws. Thermokarst and permafrost disturbances, especially active layer detachments and retrogressive thaw slumps, are present across the Fosheim Peninsula, Ellesmere Island, Canada. To determine the effects of retrogressive thaw slumps on net ecosystem exchange (NEE) of CO2 in high Arctic tundra, we used two eddy covariance (EC) tower systems to simultaneously and continuously measure CO2 fluxes from a disturbed site and the surrounding undisturbed tundra. During the 32-day measurement period in the 2014 growing season, the undisturbed tundra was a small net sink (NEE = −0.1 g C m−2 d−1); however, the disturbed terrain of the retrogressive thaw slump was a net source (NEE = +0.4 g C m−2 d−1). Over the measurement period, the undisturbed tundra sequestered 3.8 g C m−2, while the disturbed tundra released 12.5 g C m−2. Before full leaf-out in early July, the undisturbed tundra was a small source of CO2 but shifted to a sink for the remainder of the sampling season (July), whereas the disturbed tundra remained a source of CO2 throughout the season. A static chamber system was also used to measure daytime fluxes in the footprints of the two towers, in both disturbed and undisturbed tundra, and fluxes were partitioned into ecosystem respiration (Re) and gross primary production (GPP). Average GPP and Re found in disturbed tundra were smaller (+0.40 µmol m−2 s−1 and +0.55 µmol m−2 s−1, respectively) than those found in undisturbed tundra (+1.19 µmol m−2 s−1 and +1.04 µmol m−2 s−1, respectively). Our measurements indicated clearly that the permafrost disturbance changed the high Arctic tundra system from a sink to a source for CO2 during the majority of the growing season (late June and July).

Nitrogen accumulation and partitioning in a High Arctic tundra ecosystem from extreme atmospheric N deposition events

Arctic ecosystems are threatened by pollution from recently detected extreme atmospheric nitrogen (N) deposition events in which up to 90% of the annual N deposition can occur in just a few days. We undertook the first assessment of the fate of N from extreme deposition in High Arctic tundra and are presenting the results from the whole ecosystem 15N labelling experiment. In 2010, we simulated N depositions at rates of 0, 0.04, 0.4 and 1.2gNm−2yr−1, applied as 15NH415NO3 in Svalbard (79°N), during the summer. Separate applications of 15NO3− and 15NH4+ were also made to determine the importance of N form in their retention.More than 95% of the total 15N applied was recovered after one growing season (~90% after two), demonstrating a considerable capacity of Arctic tundra to retain N from these deposition events. Important sinks for the deposited N, regardless of its application rate or form, were non-vascular plants>vascular plants>organic soil>litter>mineral soil, suggesting that non-vascular plants could be the primary component of this ecosystem to undergo measurable changes due to N enrichment from extreme deposition events. Substantial retention of N by soil microbial biomass (70% and 39% of 15N in organic and mineral horizon, respectively) during the initial partitioning demonstrated their capacity to act as effective buffers for N leaching. Between the two N forms, vascular plants (Salix polaris) in particular showed difference in their N recovery, incorporating four times greater 15NO3− than 15NH4+, suggesting deposition rich in nitrate will impact them more. Overall, these findings show that despite the deposition rates being extreme in statistical terms, biologically they do not exceed the capacity of tundra to sequester pollutant N during the growing season. Therefore, current and future extreme events may represent a major source of eutrophication.

Plant functional type affects nitrogen use efficiency in high-Arctic tundra

To unravel the potential effects of climate warming on soil N availability in a high Arctic tundra ecosystem we studied temperature effects on soil mineralization, and N uptake from different soil depths (−3, −10 and −30 cm) by tundra plants. Uptake was assessed using 15N tracer injected directly into mineral soil as 15NH4Cl solution to specifically mimic altered N availability from enhanced mineralization. Net N mineralization rates were very low, suggesting that N is strongly limiting in this system. There was no apparent temperature effect (−2 °C, 5 °C, 10 °C) on mineralization, but net nitrification was strongly limited by temperature – under the −2 °C treatment no nitrification occurred. As a consequence of ongoing mineralization and limited nitrification under freezing conditions, mineral NH4 may accumulate during the winter season and be available for plant uptake without risk of loss via NO3− leaching immediately after snowmelt. Nitrogen uptake niches were clearly stratified by depth. Graminoids (Carex misandra and Luzula arctica) were most effective at taking up N from deep soil horizons, and recovery in graminoid biomass after one year was independent of 15N injection depth. Recovery of N by the dwarf shrub Salix polaris was significantly higher following shallow application (−3 cm) compared to deeper treatments (−10 and −30 cm). Lichens and mosses also showed a decline in N uptake with application depth, and very little N was recovered by lichens and mosses even from −3 cm, in contrast to the strong uptake that has been observed in mosses when N is applied to the vegetation surface. The ability of graminoids to access nutrients from deeper mineral soil may give them an advantage over mosses and dwarf shrubs in warmer high Arctic tundra in acquiring limited available nutrient resources.

Non-growing season soil CO2 flux and its contribution to annual soil CO2 emissions in two typical grasslands in the permafrost region of the Qinghai-Tibet Plateau

Non-growing season soil CO2 emissions are very important part of the annual C balance in the Arctic tundra ecosystems, but very limited information is available for the permafrost region of the Qinghai-Tibet Plateau, which hold great quantities of C. We conducted a full year measurement of soil CO2 flux in an alpine meadow and swamp meadow to quantify seasonal dynamics of non-growing season (include winter and initial thaw and freeze period (TFP)) soil CO2 flux and its contribution to annual soil CO2 emissions. Mean soil CO2 flux was higher in TFP (0.38–0.54 μmol m−2 s−1) than in winter (0.11–0.23 μmol m−2 s−1), with significant higher in swamp meadow than alpine meadow in the non-growing season. Soil temperature explained 76–85% of the yearly variation in the soil CO2 flux, with higher temperature sensitivity (Q10) in the TFP (5.67–9.43) than in other seasons (2.65–2.99). The cumulative non-growing season soil CO2 emission was 228–358 g CO2 m−2, accounted for 25–26% of annual emissions. The integrated contribution of non-growing season to annual emission based on season-specific, annual and growing season Q10 model were 26–27%, 32–34% and 44–45%, respectively, season-specific model may improve the accuracy of estimating the soil CO2 emissions. Our results indicate that non-growing season contributes a large portion of the annual soil CO2 emission in the permafrost region of the Qinghai-Tibet Plateau. Hence, estimating of annual C balance must consider the non-growing season soil CO2 emission, and should not neglect the soil CO2 emission of swamp meadow.

Sensitivity analysis of ecosystem CO2 exchange to climate change in High Arctic tundra using an ecological process-based model

Arctic terrestrial ecosystems are extremely vulnerable to climate change. A major concern is how the carbon balance of these ecosystems will respond to climate change. In this study, we constructed a simple ecological process-based model to assess how the carbon balance will be altered by ongoing climate change in High Arctic tundra ecosystems using in situ observations of carbon cycle processes. In particular, we simulated stand-level photosynthesis, root respiration, heterotrophic respiration, and hence net ecosystem production (NEP) of a plant community dominated by vascular plants and mosses. Analyses were carried out for current and future temperature and precipitation conditions. Our results showed that the tundra ecosystem was a CO2 sink (NEP of 2.3–18.9 gC m−2 growing season−1) under present temperature conditions. Under rising temperature (2–6 °C), carbon gain is significantly reduced, but a few days’ extension of the foliage period caused by their higher temperatures compensated for the negative effect of temperature on NEP. Precipitation is the major environmental factor driving photosynthetic productivity of mosses, but it had a minor influence on community-level NEP. However, NEP decreased by a maximum 15.3 gC m−2 growing season−1 under a 30-day prolongation of the moss-growing season, suggesting that growing season extension had a negative effect on ecosystem carbon gain, because of poorer light conditions in autumn. Because the growing season creates a weak CO2 sink at present, lengthening of the snow-free season coupled with rising temperature could seriously affect the future carbon balance of this Arctic tundra ecosystem.

Arctic tundra fires: natural variability and responses to climate change

Anthropogenic climate change may result in novel disturbances to Arctic tundra ecosystems. Understanding the natural variability of tundra‐fire regimes and their linkages to climate is essential in evaluating whether tundra burning has increased in recent years. Historical observations and charcoal records from lake sediments reveal a wide range of fire regimes in Arctic tundra, with fire‐return intervals varying from decades to millennia. Analysis of historical data shows strong climate–fire relationships, with threshold effects of summer temperature and precipitation. Projections based on 21st‐century climate scenarios suggest that annual area burned will approximately double in Alaskan tundra by the end of the century. Fires can release ancient carbon from tundra ecosystems and catalyze other biogeochemical and biophysical changes, with local to global consequences. Given the increased likelihood of tundra burning in coming decades, land managers and policy makers need to consider the ecological and socioeconomic impacts of fire in the Far North.

Microbial activity is not always limited by nitrogen in Arctic tundra soils

Both primary productivity and decomposition appear to be limited by low soil nitrogen (N) availability throughout much of the Arctic tundra active growing season, making these ecosystems among the most N-limited in the world. Climate warming may potentially stimulate microbial activities such as N mineralization, which could have cascading long-term effects on Arctic tundra ecosystems. Despite previous evidence of N limitation to microbial decomposition in Arctic tundra, N may not limit microbial activity throughout the entire active season. Labile carbon (C) may be co-limiting for portions of the active season when there is relatively high inorganic N and/or low labile C availability. To assess seasonal variation in the controls on microbial activity, we conducted a series of laboratory incubations with soils collected at the beginning and peak of the active season in two years to examine intra-seasonal and annual variability of soil microbial N limitation in an Arctic moist acidic tundra (MAT) soil. The soil incubations were set-up in a factorial design with treatments of added N or DI water as a control and incubation temperatures of 5 °C and 15 °C. We measured chloroform-labile C and N (as a proxy for microbial biomass), extractable nutrients, C-mineralization and potential enzyme activities. In contrast to previous studies, we found that these metrics of microbial activity were not consistently stimulated by N additions; rather, added N was primarily immobilized by microbes resulting in decreased chloroform-labile C:N ratios. Stimulation of C mineralization by N addition was short-lived and variable between our two sampling dates within a single active season. Additionally, there were differences in temperature sensitivities of C mineralization and contrasting effects of N amendment on enzyme activities between the two study years. These findings suggest that, at times, other factors co-limit microbial activities in MAT soils. The current dogma of universal N-limitation to microbial activity may need to be refined in light of these results, to more accurately predict the fate of the large amounts of C currently sequestered in Arctic tundra soils.

Isotopic identification of soil and permafrost nitrate sources in an Arctic tundra ecosystem

AbstractThe nitrate (NO3−) dual isotope approach was applied to snowmelt, tundra active layer pore waters, and underlying permafrost in Barrow, Alaska, USA, to distinguish between NO3− derived from atmospheric deposition versus that derived from microbial nitrification. Snowmelt had an atmospheric NO3− signal with δ15N averaging −4.8 ± 1.0‰ (standard error of the mean) and δ18O averaging 70.2 ± 1.7‰. In active layer pore waters, NO3− primarily occurred at concentrations suitable for isotopic analysis in the relatively dry and oxic centers of high‐centered polygons. The average δ15N and δ18O of NO3− from high‐centered polygons were 0.5 ± 1.1‰ and −4.1 ± 0.6‰, respectively. When compared to the δ15N of reduced nitrogen (N) sources, and the δ18O of soil pore waters, it was evident that NO3− in high‐centered polygons was primarily from microbial nitrification. Permafrost NO3− had δ15N ranging from approximately −6‰ to 10‰, similar to atmospheric and microbial NO3−, and highly variable δ18O ranging from approximately −2‰ to 38‰. Permafrost ice wedges contained a significant atmospheric component of NO3−, while permafrost textural ice contained a greater proportion of microbially derived NO3−. Large‐scale permafrost thaw in this environment would release NO3− with a δ18O signature intermediate to that of atmospheric and microbial NO3. Consequently, while atmospheric and microbial sources can be readily distinguished by the NO3− dual isotope technique in tundra environments, attribution of NO3− from thawing permafrost will not be straightforward. The NO3− isotopic signature, however, appears useful in identifying NO3− sources in extant permafrost ice.

Identifying multiscale zonation and assessing the relative importance of polygon geomorphology on carbon fluxes in an Arctic tundra ecosystem

AbstractWe develop a multiscale zonation approach to characterize the spatial variability of Arctic polygonal ground geomorphology and to assess the relative controls of these elements on land surface and subsurface properties and carbon fluxes. Working within an ice wedge polygonal region near Barrow, Alaska, we consider two scales of zonation: polygon features (troughs, centers, and rims of polygons) that are nested within different polygon types (high, flat, and low centered). In this study, we first delineated polygons using a digital elevation map and clustered the polygons into four types along two transects, using geophysical and kite‐based landscape‐imaging data sets. We extrapolated those data‐defined polygon types to all the polygons over the study site, using the polygon statistics extracted from the digital elevation map. Based on the point measurements, we characterized the distribution of vegetation, hydrological, thermal, and geochemical properties, as well as carbon fluxes, all as a function of polygon types and polygon features. Results show that nested polygon geomorphic zonation—polygon types and polygon features—can be used to represent distinct distributions of carbon fluxes and associated properties, as well as covariability among those properties. Importantly, the results indicate that polygon types have more power to explain the variations in those properties than polygon features. The approach is expected to be useful for improved system understanding, site characterization, and parameterization of numerical models aimed at predicting ecosystem feedbacks to the climate.

Trends in NDVI and Tundra Community Composition in the Arctic of NE Alaska Between 1984 and 2009

As Arctic ecosystems experience increases in surface air temperatures, plot-level analyses of tundra vegetation composition suggest that there are important changes occurring in tundra communities that are typified by increases in shrubs and declines in non-vascular species. At the same time analyses of NDVI indicate that the Arctic tundra is greening. Few studies have combined plot-level trends in species composition and cover with remote sensing measurements to understand the linkages between tundra vegetation dynamics and NDVI over time. This study reports on trends in species composition for field plots in the Arctic National Wildlife Refuge in NE Alaska from 1984 to 2009 and links these trends to the trends in NDVI at fine and coarse scales. Over this time frame there were few changes in plant community composition. None of the five tundra types that were measured had increases in total vegetative cover, and deciduous shrub cover did not show the large increases reported elsewhere. Surface-(plot) measured NDVI was positively correlated to deciduous and evergreen shrub composition suggesting that these functional groups had a strong influence on NDVI values. Modeled values of NDVI, derived from measures of deciduous and evergreen shrub composition over time, decreased slightly for tussock tundra but did not change for other tundra types. This result suggests that surface NDVI did not change over time on these tundra types. Fine-scale (30-m pixels) Landsat NDVI also did not show any changes for the pixels located at the permanent plots (1992–2009). However, coarse-scale (8-km pixels) AVHRR NDVI across the study area did increase (1988–2007). Furthermore, aggregate values of Landsat pixels matching the same area as AVHRR pixels also did not show significant changes over time. Although Landsat NDVI was consistent with surface-measured NDVI, AVHRR NDVI was not. AVHRR NDVI values showed increases that were in neither the field nor Landsat data. This result suggests that AVHRR may be demonstrating increasing trends in NDVI that are not occurring on the ground in some Arctic tundra ecosystems. These results highlight the need to combine remote sensing with on-the-ground measurements of plant community composition and NDVI in the analysis of the responses of Arctic tundra ecosystems to climate change.

Estimating aboveground biomass and leaf area of low-stature Arctic shrubs with terrestrial LiDAR

Arctic tundra ecosystems are responding to effects of climatic warming via shifts in vegetation composition and increased woody biomass. In this sensitive ecosystem, minor increases in woody plant biomass may induce significant changes in ecosystem structure and function. However, establishing methods for quantifying and potentially scaling woody plant biomass in low-stature biomes is challenging. In this study, we investigated the potential to use terrestrial laser scanning (TLS) to remotely estimate harvested biomass and leaf area of two dominant low-stature (<1.5m tall) Arctic shrub species in 0.64m2 subplots established in northern Alaskan tundra. We explored two biomass estimation approaches (volumetric surface differencing and voxel counting) applied to point clouds obtained from close-range (2m) and variable-range (<50m) terrestrial laser scans. Relationships between harvested biomass and TLS metrics were strong for all combinations of approaches, with voxel counting giving a marginally better result than surface differencing for close-range data (R2=0.94 vs 0.92; RMSE=102g vs 117g) and surface differencing proving stronger than voxel counting for variable-range data (R2=0.91 vs 0.82; RMSE=119g vs 169g). Strong relationships between total harvested biomass and total leaf dry mass (R2=0.93; RMSE=13.4g), and between leaf dry mass and leaf wet area (R2=0.99; RMSE=9.01cm2) justify estimation of shrub leaf area from TLS-derived shrub biomass. Our results show that rapidly acquired, repeatable terrestrial laser scans taken at multiple ranges can be processed using simple biomass estimation approaches to yield aboveground biomass and leaf area estimates for low-stature shrubs at fine spatial scales (sub-meter to ~50 meters) with the fidelity required to monitor small but ecologically meaningful changes in tundra structure. Further, these data may be employed as ground reference data for broader scale remote sensing data collection, such as airborne LiDAR scanning, that would enable shrub biomass and leaf-area estimates at fine spatial resolution over large spatial extents.

Environmental and vegetation controls on the spatial variability of CH4 emission from wet-sedge and tussock tundra ecosystems in the Arctic.

AimsDespite multiple studies investigating the environmental controls on CH4 fluxes from arctic tundra ecosystems, the high spatial variability of CH4 emissions is not fully understood. This makes the upscaling of CH4 fluxes from plot to regional scale, particularly challenging. The goal of this study is to refine our knowledge of the spatial variability and controls on CH4 emission from tundra ecosystems.MethodsCH4 fluxes were measured in four sites across a variety of wet-sedge and tussock tundra ecosystems in Alaska using chambers and a Los Gatos CO2 and CH4 gas analyser.ResultsAll sites were found to be sources of CH4, with northern sites (in Barrow) showing similar CH4 emission rates to the southernmost site (ca. 300 km south, Ivotuk). Gross primary productivity (GPP), water level and soil temperature were the most important environmental controls on CH4 emission. Greater vascular plant cover was linked with higher CH4 emission, but this increased emission with increased vascular plant cover was much higher (86 %) in the drier sites, than the wettest sites (30 %), suggesting that transport and/or substrate availability were crucial limiting factors for CH4 emission in these tundra ecosystems.ConclusionsOverall, this study provides an increased understanding of the fine scale spatial controls on CH4 flux, in particular the key role that plant cover and GPP play in enhancing CH4 emissions from tundra soils.

Evidence for Toxoplasma gondii in migratory vs. nonmigratory herbivores in a terrestrial arctic ecosystem

It is currently unclear how Toxoplasma gondii (Nicolle and Manceaux, 1908) persists in arctic tundra ecosystems in the absence of felid definitive hosts. To investigate potential transmission routes of T. gondii in a terrestrial arctic food web, we collected samples from two migratory herbivores, Ross’s Geese (Chen rossi (Cassin, 1861)) and Lesser Snow Geese (Chen caerulescens (L., 1758)), and from two resident herbivores, Nearctic brown lemmings (Lemmus trimucronatus (Richardson, 1825)) and collared lemmings (Dicrostonyx groenlandicus (Traill, 1823)), trapped at Karrak Lake, Nunavut, Canada. Antibodies were detected in 76 of 234 (32.4%) serum samples from Ross’s Geese and 66 of 233 (28.3%) serum samples from Lesser Snow Geese. We did not detect T. gondii antibodies in filter-paper eluate tested from thoracic fluid samples collected from 84 lemmings. We did not detect T. gondii DNA in brain tissue from these lemmings. Although a small sample size, our findings suggest that lemmings in this terrestrial arctic ecosystem are not exposed to, or infected with, the parasite. This suggests that oocysts are not introduced into the terrestrial arctic ecosystem at Karrak Lake via freshwater runoff from temperate regions. This study demonstrated that live adult arctic-nesting geese are exposed to T. gondii and therefore migratory herbivorous hosts are potential sources of T. gondii infection for predators in terrestrial arctic ecosystems.

Permafrost collapse after shrub removal shifts tundra ecosystem to a methane source

Arctic tundra ecosystems are warming almost twice as fast as the global average. Permafrost thaw and the resulting release of greenhouse gases from decomposing soil organic carbon have the potential to accelerate climate warming. In recent decades, Arctic tundra ecosystems have changed rapidly, including expansion of woody vegetation, in response to changing climate conditions. How such vegetation changes contribute to stabilization or destabilization of the permafrost is unknown. Here we present six years of field observations in a shrub removal experiment at a Siberian tundra site. Removing the shrub part of the vegetation initiated thawing of ice-rich permafrost, resulting in collapse of the originally elevated shrub patches into waterlogged depressions within five years. This thaw pond development shifted the plots from a methane sink into a methane source. The results of our field experiment demonstrate the importance of the vegetation cover for protection of the massive carbon reservoirs stored in the permafrost and illustrate the strong vulnerability of these tundra ecosystems to perturbations. If permafrost thawing can more frequently trigger such local permafrost collapse, methane-emitting wet depressions could become more abundant in the lowland tundra landscape, at the cost of permafrost-stabilizing low shrub vegetation.

Delayed responses of an Arctic ecosystem to an extreme summer: impacts on net ecosystem exchange and vegetation functioning

Abstract. The importance and consequences of extreme events on the global carbon budget are inadequately understood. This includes the differential impact of extreme events on various ecosystem components, lag effects, recovery times, and compensatory processes. In the summer of 2007 in Barrow, Arctic Alaska, there were unusually high air temperatures (the fifth warmest summer over a 65-year period) and record low precipitation (the lowest over a 65-year period). These abnormal conditions were associated with substantial desiccation of the Sphagnum layer and a reduced net Sphagnum CO2 sink but did not affect net ecosystem exchange (NEE) from this wet-sedge arctic tundra ecosystem. Microbial biomass, NH4+ availability, gross primary production (GPP), and ecosystem respiration (Reco) were generally greater during this extreme summer. The cumulative ecosystem CO2 sink in 2007 was similar to the previous summers, suggesting that vascular plants were able to compensate for Sphagnum CO2 uptake, despite the impact on other functions and structure such as desiccation of the Sphagnum layer. Surprisingly, the lowest ecosystem CO2 sink over a five summer record (2005–2009) was observed during the 2008 summer (~70% lower), directly following the unusually warm and dry summer, rather than during the extreme summer. This sink reduction cannot solely be attributed to the potential damage to mosses, which typically contribute ~40% of the entire ecosystem CO2 sink. Importantly, the return to a substantial cumulative CO2 sink occurred two summers after the extreme event, which suggests a substantial resilience of this tundra ecosystem to at least an isolated extreme event. Overall, these results show a complex response of the CO2 sink and its sub-components to atypically warm and dry conditions. The impact of multiple extreme events requires further investigation.

Non‐growing‐season soil respiration is controlled by freezing and thawing processes in the summer monsoon‐dominated Tibetan alpine grassland

AbstractThe Tibetan alpine grasslands, sharing many features with arctic tundra ecosystems, have a unique non‐growing‐season climate that is usually dry and without persistent snow cover. Pronounced winter warming recently observed in this ecosystem may significantly alter the non‐growing‐season carbon cycle processes such as soil respiration (Rs), but detailed measurements to assess the patterns, drivers of, and potential feedbacks on Rs have not been made yet. We conducted a 4 year study on Rs using a unique Rs measuring system, composed of an automated soil CO2 flux sampling system and a custom‐made container, to facilitate measurements in this extreme environment. We found that in the nongrowing season, (1) cumulative Rs was 82–89 g C m−2, accounting for 11.8–13.2% of the annual total Rs; (2) surface soil freezing controlled the diurnal pattern of Rs and bulk soil freezing induced lower reference respiration rate (R0) and temperature sensitivity (Q10) than those in the growing season (0.40–0.53 versus 0.84–1.32 µmol CO2 m−2 s−1 for R0 and 2.5–2.9 versus 2.9–5.6 for Q10); and (3) the intraannual variation in cumulative Rs was controlled by accumulated surface soil temperature. We found that in the summer monsoon‐dominated Tibetan alpine grassland, surface soil freezing, bulk soil freezing, and accumulated surface soil temperature are the day‐, season‐, and year‐scale drivers of the non‐growing‐season Rs, respectively. Our results suggest that warmer winters can trigger carbon loss from this ecosystem because of higher Q10 of thawed than frozen soils.

Bacillariophyta водных экосистем арктических тундр западного Ямала (бассейн р. Харасавэйяха, Россия)

Bacillariophyta водных экосистем арктических тундр западного Ямала (бассейн р. Харасавэйяха, Россия)

Long‐term experimental warming and nutrient additions increase productivity in tall deciduous shrub tundra

Warming Arctic temperatures can drive changes in vegetation structure and function directly by stimulating plant growth or indirectly by stimulating microbial decomposition of organic matter and releasing more nutrients for plant uptake and growth. The arctic biome is currently increasing in deciduous shrub cover and this increase is expected to continue with climate warming. However, little is known how current deciduous shrub communities will respond to future climate induced warming and nutrient increase. We examined the plant and ecosystem response to a long‐term (18 years) nutrient addition and warming experiment in an Alaskan arctic tall deciduous shrub tundra ecosystem to understand controls over plant productivity and carbon (C) and nitrogen (N) storage in shrub tundra ecosystems. In addition, we used a meta‐analysis approach to compare the treatment effect size for aboveground biomass among seven long‐term studies conducted across multiple plant community types within the Arctic. We found that biomass, productivity, and aboveground N pools increased with nutrient additions and warming, while species diversity decreased. Both nutrient additions and warming caused the dominant functional group, deciduous shrubs, to increase biomass and proportional C and N allocation to aboveground stems but decreased allocation to belowground stems. For all response variables except soil C and N pools, effects of nutrients plus warming were largest. Soil C and N pools were highly variable and we could not detect any response to the treatments. The biomass response to warming and fertilization in tall deciduous shrub tundra was greater than moist acidic and moist non‐acidic tundra and more similar to the biomass response of wet sedge tundra. Our data suggest that in a warmer and more nutrient‐rich Arctic, tall deciduous shrub tundra will have greater total deciduous shrub biomass and a higher proportion of woody tissue that has a longer residence time, with a lower proportion of C and N allocated to belowground stems.