Rapid carbon turnover beneath shrub and tree vegetation is associated with low soil carbon stocks at a subarctic treeline

Climate warming at high northern latitudes has caused substantial increases in plant productivity of tundra vegetation and an expansion of the range of deciduous shrub species. However significant the increase in carbon (C) contained within above‐ground shrub biomass, it is modest in comparison with the amount of C stored in the soil in tundra ecosystems. Here, we use a ‘space‐for‐time’ approach to test the hypothesis that a shift from lower‐productivity tundra heath to higher‐productivity deciduous shrub vegetation in the sub‐Arctic may lead to a loss of soil C that out‐weighs the increase in above‐ground shrub biomass. We further hypothesize that a shift from ericoid to ectomycorrhizal systems coincident with this vegetation change provides a mechanism for the loss of soil C. We sampled soil C stocks, soil surface CO₂ flux rates and fungal growth rates along replicated natural transitions from birch forest (Betula pubescens), through deciduous shrub tundra (Betula nana) to tundra heaths (Empetrum nigrum) near Abisko, Swedish Lapland. We demonstrate that organic horizon soil organic C (SOC_org) is significantly lower at shrub (2.98 ± 0.48 kg m^?2) and forest (2.04 ± 0.25 kg m^?2) plots than at heath plots (7.03 ± 0.79 kg m^?2). Shrub vegetation had the highest respiration rates, suggesting that despite higher rates of C assimilation, C turnover was also very high and less C is sequestered in the ecosystem. Growth rates of fungal hyphae increased across the transition from heath to shrub, suggesting that the action of ectomycorrhizal symbionts in the scavenging of organically bound nutrients is an important pathway by which soil C is made available to microbial degradation. The expansion of deciduous shrubs onto potentially vulnerable arctic soils with large stores of C could therefore represent a significant positive feedback to the climate system.     

Predicting ecosystem carbon balance in a warming Arctic: the importance of long‐term thermal acclimation potential and inhibitory effects of light on respiration

The carbon balance of Arctic ecosystems is particularly sensitive to global environmental change. Leaf respiration (R), a temperature‐dependent key process in determining the carbon balance, is not well‐understood in Arctic plants. The potential for plants to acclimate to warmer conditions could strongly impact future global carbon balance. Two key unanswered questions are (1) whether short‐term temperature responses can predict long‐term respiratory responses to growth in elevated temperatures and (2) to what extent the constant daylight conditions of the Arctic growing season inhibit leaf respiration. In two dominant Arctic species E riophorum vaginatum (tussock grass) and B etula nana (woody shrub), we assessed the extent of respiratory inhibition in the light (R _L/R _D), respiratory response to short‐term temperature change, and respiratory acclimation to long‐term warming treatments. We found that R of both species is strongly inhibited by light (averaging 35% across all measurement temperatures). In E . vaginatum both R _L and R _D acclimated to the long‐term warming treatment, reducing the magnitude of respiratory response relative to the short‐term response to temperature increase. In B . nana, both R _L and R _D responded to short‐term temperature increase but showed no acclimation to the long‐term warming. The ability to predict plant respiratory response to global warming with short‐term temperature responses will depend on species‐specific acclimation potential and the differential response of R _L and R _D to temperature. With projected woody shrub encroachment in Arctic tundra and continued warming, changing species dominance between these two functional groups, may impact ecosystem respiratory response and carbon balance.     

Long‐term experimental manipulation of climate alters the ectomycorrhizal community of Betula nana in Arctic tundra

Climate warming is leading to shrub expansion in Arctic tundra. Shrubs form ectomycorrhizal (ECM) associations with soil fungi that are central to ecosystem carbon balance as determinants of plant community structure and as decomposers of soil organic matter. To assess potential climate change impacts on ECM communities, we analysed fungal internal transcribed spacer sequences from ECM root tips of the dominant tundra shrub Betula nana growing in treatments plots that had received long‐term warming by greenhouses and/or fertilization as part of the Arctic Long‐Term Ecological Research experiment at Toolik Lake Alaska, USA. We demonstrate opposing effects of long‐term warming and fertilization treatments on ECM fungal diversity; with warming increasing and fertilization reducing the diversity of ECM communities. We show that warming leads to a significant increase in high biomass fungi with proteolytic capacity, especially Cortinarius spp., and a reduction of fungi with high affinities for labile N, especially Russula spp. In contrast, fertilization treatments led to relatively small changes in the composition of the ECM community, but increased the abundance of saprotrophs. Our data suggest that warming profoundly alters nutrient cycling in tundra, and may facilitate the expansion of B. nana through the formation of mycorrhizal networks of larger size.     

Effects of grass species and grass growth on atmospheric nitrogen deposition to a bog ecosystem surrounded by intensive agricultural land use

We applied a ¹⁵N dilution technique called “Integrated Total Nitrogen Input” (ITNI) to quantify annual atmospheric N input into a peatland surrounded by intensive agricultural practices over a 2-year period. Grass species and grass growth effects on atmospheric N deposition were investigated using Lolium multiflorum and Eriophorum vaginatum and different levels of added N resulting in increased biomass production. Plant biomass production was positively correlated with atmospheric N uptake (up to 102.7 mg N pot⁻¹) when using Lolium multiflorum. In contrast, atmospheric N deposition to Eriophorum vaginatum did not show a clear dependency to produced biomass and ranged from 81.9 to 138.2 mg N pot⁻¹. Both species revealed a relationship between atmospheric N input and total biomass N contents. Airborne N deposition varied from about 24 to 55 kg N ha⁻¹ yr⁻¹. Partitioning of airborne N within the monitor system differed such that most of the deposited N was found in roots of Eriophorum vaginatum while the highest share was allocated in aboveground biomass of Lolium multiflorum. Compared to other approaches determining atmospheric N deposition, ITNI showed highest airborne N input and an up to fivefold exceedance of the ecosystem-specific critical load of 5–10 kg N ha⁻¹ yr⁻¹.     

Climate and species affect fine root production with long-term fertilization in acidic tussock tundra near Toolik Lake,Alaska

Long-term fertilization of acidic tussock tundra has led to changes in plant species composition, increases in aboveground production and biomass and substantial losses of soil organic carbon (SOC). Root litter is an important input to SOC pools, although little is known about fine root demography in tussock tundra. In this study, we examined the response of fine root production and live standing fine root biomass to short- and long-term fertilization, as changes in fine root demography may contribute to observed declines in SOC. Live standing fine root biomass increased with long-term fertilization, while fine root production declined, reflecting replacement of the annual fine root system of Eriophorum vaginatum, with the long-lived fine roots of Betula nana. Fine root production increased in fertilized plots during an unusually warm growing season, but remained unchanged in control plots, consistent with observations that B. nana shows a positive response to climate warming. Calculations based on a few simple assumptions suggest changes in fine root demography with long-term fertilization and species replacement could account for between 20 and 39% of the observed declines in SOC stocks.     

Contrasting effects of long term versus short-term nitrogen addition on photosynthesis and respiration in the Arctic

We examined the effects of short (<1–4 years) and long-term (22 years) nitrogen (N) and/or phosphorus (P) addition on the foliar CO₂ exchange parameters of the Arctic species Betula nana and Eriophorum vaginatum in northern Alaska. Measured variables included: the carboxylation efficiency of Rubisco (V_cmax), electron transport capacity (J_max), dark respiration (R_d), chlorophyll a and b content (Chl), and total foliar N (N). For both B. nana and E. vaginatum, foliar N increased by 20–50 % as a consequence of 1–22 years of fertilisation, respectively, and for B. nana foliar N increase was consistent throughout the whole canopy. However, despite this large increase in foliar N, no significant changes in V_cmax and J_max were observed. In contrast, R_d was significantly higher (>25 %) in both species after 22 years of N addition, but not in the shorter-term treatments. Surprisingly, Chl only increased in both species the first year of fertilisation (i.e. the first season of nutrients applied), but not in the longer-term treatments. These results imply that: (1) under current (low) N availability, these Arctic species either already optimize their photosynthetic capacity per leaf area, or are limited by other nutrients; (2) observed increases in Arctic NEE and GPP with increased nutrient availability are caused by structural changes like increased leaf area index, rather than increased foliar photosynthetic capacity and (3) short-term effects (1–4 years) of nutrient addition cannot always be extrapolated to a larger time scale, which emphasizes the importance of long-term ecological experiments.     

Nitrogen‐dependent recovery of subarctic tundra vegetation after simulation of extreme winter warming damage to Empetrum hermaphroditum

Vast areas of (sub)arctic tundra are dominated by the ericoid dwarf shrub Empetrum hermaphroditum. Recent experimental and observational data have shown that Empetrum can be damaged heavily by recurrent extreme winter warming. In addition, summer warming leads to increased soil N availability in tundra ecosystems. In a 7‐year experiment, I investigated the recovery of subarctic Empetrum‐dominated tundra vegetation using a factorial combination of various degrees of aboveground Empetrum removal (simulating the damaging effects of extreme winter warming) and N addition (simulating one of the effects of summer warming). After 7 years no new species had established in the plots. The growth of planted Betula nana seedlings was stimulated by Empetrum removal and reduced by N addition. This Empetrum‐dominated tundra ecosystem was resilient against severe disturbances. Only when Empetrum was 100% removed did it fail to recover, and only in combination with high N supply the subordinate species (notably Eriophorum vaginatum and Rubus chamaemorus, a graminoid and a forb) could benefit. In the 50% removal treatment Empetrum recovered in 7 years when no N was supplied and the cover of the subordinate species did not change. However, when N was added Empetrum recovered faster (in 4 years) and the subordinates decreased. When Empetrum was not removed and N was added, Empetrum even increased in abundance at the expense of the subordinate species. Thus, profound changes in tundra ecosystems can only be expected when Empetrum is very heavily damaged as a result of recurrent extreme winter warming and when soil N availability is increased as a result of summer warming. These changes in species composition upon extreme disturbance events may lead to a wide variety of ecosystem feedbacks and cascade processes as this tundra system is relatively species‐poor, and can be hypothesized to have low functional redundancy.     

Winter warming as an important co‐driver for Betula nana growth in western Greenland during the past century

Growing season conditions are widely recognized as the main driver for tundra shrub radial growth, but the effects of winter warming and snow remain an open question. Here, we present a more than 100 years long Betula nana ring‐width chronology from Disko Island in western Greenland that demonstrates a highly significant and positive growth response to both summer and winter air temperatures during the past century. The importance of winter temperatures for Betula nana growth is especially pronounced during the periods from 1910–1930 to 1990–2011 that were dominated by significant winter warming. To explain the strong winter importance on growth, we assessed the importance of different environmental factors using site‐specific measurements from 1991 to 2011 of soil temperatures, sea ice coverage, precipitation and snow depths. The results show a strong positive growth response to the amount of thawing and growing degree‐days as well as to winter and spring soil temperatures. In addition to these direct effects, a strong negative growth response to sea ice extent was identified, indicating a possible link between local sea ice conditions, local climate variations and Betula nana growth rates. Data also reveal a clear shift within the last 20 years from a period with thick snow depths (1991–1996) and a positive effect on Betula nana radial growth, to a period (1997–2011) with generally very shallow snow depths and no significant growth response towards snow. During this period, winter and spring soil temperatures have increased significantly suggesting that the most recent increase in Betula nana radial growth is primarily triggered by warmer winter and spring air temperatures causing earlier snowmelt that allows the soils to drain and warm quicker. The presented results may help to explain the recently observed ‘greening of the Arctic’ which may further accelerate in future years due to both direct and indirect effects of winter warming.     

Interactions among shrub cover and the soil microclimate may determine future Arctic carbon budgets

Arctic and Boreal terrestrial ecosystems are important components of the climate system because they contain vast amounts of soil carbon (C). Evidence suggests that deciduous shrubs are increasing in abundance, but the implications for ecosystem C budgets remain uncertain. Using midsummer CO₂ flux data from 21 sites spanning 16° of latitude in the Arctic and Boreal biomes, we show that air temperature explains c. one‐half of the variation in ecosystem respiration (ER) and that ER drives the pattern in net ecosystem CO₂ exchange across ecosystems. Woody sites were slightly stronger C sinks compared with herbaceous communities. However, woody sites with warm soils (> 10 °C) were net sources of CO₂, whereas woody sites with cold soils (< 10 °C) were strong sinks. Our results indicate that transition to a shrub‐dominated Arctic will increase the rate of C cycling, and may lead to net C loss if soil temperatures rise.     

Nitrogen supply differentially affects litter decomposition rates and nitrogen dynamics of sub-arctic bog species

High-latitude peatlands are important soil carbon sinks. In these ecosystems, the mineralization of carbon and nitrogen are constrained by low temperatures and low nutrient concentrations in plant litter and soil organic matter. Global warming is predicted to increase soil N availability for plants at high-latitude sites. We applied N fertilizer as an experimental analogue for this increase. In a three-year field experiment we studied N fertilization effects on leaf litter decomposition and N dynamics of the four dominant plant species (comprising >75% of total aboveground biomass) in a sub-arctic bog in northern Sweden. The species were Empetrum nigrum (evergreen shrub), Eriophorum vaginatum (graminoid), Betula nana (deciduous shrub) and Rubus chamaemorus (perennial forb). In the controls, litter mass loss rates increased in the order: Empetrum < Eriophorum < Betula < Rubus. Increased N availability had variable, species-specific effects: litter mass loss rates (expressed per unit litter mass) increased in Empetrum, did not change in Eriophorum and Betula and decreased in Rubus. In the leaf litter from the controls, we measured no or only slight net N mineralization even after three years. In the N-fertilized treatments we found strong net N immobilization, especially in Eriophorum and Betula. This suggests that, probably owing to substantial chemical and/or microbial immobilization, additional N supply does not increase the rate of N cycling for at least the first three years.     

Plastic and genetic responses of a common sedge to warming have contrasting effects on carbon cycle processes

Climate warming affects plant physiology through genetic adaptation and phenotypic plasticity, but little is known about how these mechanisms influence ecosystem processes. We used three elevation gradients and a reciprocal transplant experiment to show that temperature causes genetic change in the sedge Eriophorum vaginatum. We demonstrate that plants originating from warmer climate produce fewer secondary compounds, grow faster and accelerate carbon dioxide (CO₂) release to the atmosphere. However, warmer climate also caused plasticity in E. vaginatum, inhibiting nitrogen metabolism, photosynthesis and growth and slowing CO₂ release into the atmosphere. Genetic differentiation and plasticity in E. vaginatum thus had opposing effects on CO₂ fluxes, suggesting that warming over many generations may buffer, or reverse, the short‐term influence of this species over carbon cycle processes. Our findings demonstrate the capacity for plant evolution to impact ecosystem processes, and reveal a further mechanism through which plants will shape ecosystem responses to climate change.     

Warming effects on greenhouse gas fluxes in peatlands are modulated by vegetation composition

Understanding the effects of warming on greenhouse gas feedbacks to climate change represents a major global challenge. Most research has focused on direct effects of warming, without considering how concurrent changes in plant communities may alter such effects. Here, we combined vegetation manipulations with warming to investigate their interactive effects on greenhouse gas emissions from peatland. We found that although warming consistently increased respiration, the effect on net ecosystem CO₂ exchange depended on vegetation composition. The greatest increase in CO₂ sink strength after warming was when shrubs were present, and the greatest decrease when graminoids were present. CH₄ was more strongly controlled by vegetation composition than by warming, with largest emissions from graminoid communities. Our results show that plant community composition is a significant modulator of greenhouse gas emissions and their response to warming, and suggest that vegetation change could alter peatland carbon sink strength under future climate change.     

Thermal acclimation of shoot respiration in an Arctic woody plant species subjected to 22 years of warming and altered nutrient supply

Despite concern about the status of carbon (C) in the Arctic tundra, there is currently little information on how plant respiration varies in response to environmental change in this region. We quantified the impact of long‐term nitrogen (N) and phosphorus (P) treatments and greenhouse warming on the short‐term temperature (T) response and sensitivity of leaf respiration (R), the high‐T threshold of R, and associated traits in shoots of the Arctic shrub Betula nana in experimental plots at Toolik Lake, Alaska. Respiration only acclimated to greenhouse warming in plots provided with both N and P (resulting in a ~30% reduction in carbon efflux in shoots measured at 10 and 20 °C), suggesting a nutrient dependence of metabolic adjustment. Neither greenhouse nor N+P treatments impacted on the respiratory sensitivity to T (Q₁₀); overall, Q₁₀ values decreased with increasing measuring T, from ~3.0 at 5 °C to ~1.5 at 35 °C. New high‐resolution measurements of R across a range of measuring Ts (25–70 °C) yielded insights into the T at which maximal rates of R occurred (T_max). Although growth temperature did not affect T_max, N+P fertilization increased T_max values ~5 °C, from 53 to 58 °C. N+P fertilized shoots exhibited greater rates of R than nonfertilized shoots, with this effect diminishing under greenhouse warming. Collectively, our results highlight the nutrient dependence of thermal acclimation of leaf R in B. nana, suggesting that the metabolic efficiency allowed via thermal acclimation may be impaired at current levels of soil nutrient availability. This finding has important implications for predicting carbon fluxes in Arctic ecosystems, particularly if soil N and P become more abundant in the future as the tundra warms.     

Experimentally increased nutrient availability at the permafrost thaw front selectively enhances biomass production of deep‐rooting subarctic peatland species

Climate warming increases nitrogen (N) mineralization in superficial soil layers (the dominant rooting zone) of subarctic peatlands. Thawing and subsequent mineralization of permafrost increases plant‐available N around the thaw‐front. Because plant production in these peatlands is N‐limited, such changes may substantially affect net primary production and species composition. We aimed to identify the potential impact of increased N‐availability due to permafrost thawing on subarctic peatland plant production and species performance, relative to the impact of increased N‐availability in superficial organic layers. Therefore, we investigated whether plant roots are present at the thaw‐front (45 cm depth) and whether N‐uptake (¹⁵N‐tracer) at the thaw‐front occurs during maximum thaw‐depth, coinciding with the end of the growing season. Moreover, we performed a unique 3‐year belowground fertilization experiment with fully factorial combinations of deep‐ (thaw‐front) and shallow‐fertilization (10 cm depth) and controls. We found that certain species are present with roots at the thaw‐front (Rubus chamaemorus) and have the capacity (R. chamaemorus, Eriophorum vaginatum) for N‐uptake from the thaw‐front between autumn and spring when aboveground tissue is largely senescent. In response to 3‐year shallow‐belowground fertilization (S) both shallow‐ (Empetrum hermaphroditum) and deep‐rooting species increased aboveground biomass and N‐content, but only deep‐rooting species responded positively to enhanced nutrient supply at the thaw‐front (D). Moreover, the effects of shallow‐fertilization and thaw‐front fertilization on aboveground biomass production of the deep‐rooting species were similar in magnitude (S: 71%; D: 111% increase compared to control) and additive (S + D: 181% increase). Our results show that plant‐available N released from thawing permafrost can form a thus far overlooked additional N‐source for deep‐rooting subarctic plant species and increase their biomass production beyond the already established impact of warming‐driven enhanced shallow N‐mineralization. This may result in shifts in plant community composition and may partially counteract the increased carbon losses from thawing permafrost.     

Belowground carbon responses to experimental warming regulated by soil moisture change in an alpine ecosystem of the Qinghai–Tibet Plateau

Recent studies found that the largest uncertainties in the response of the terrestrial carbon cycle to climate change might come from changes in soil moisture under the elevation of temperature. Warming‐induced change in soil moisture and its level of influence on terrestrial ecosystems are mostly determined by climate, soil, and vegetation type and their sensitivity to temperature and moisture. Here, we present the results from a warming experiment of an alpine ecosystem conducted in the permafrost region of the Qinghai–Tibet Plateau using infrared heaters. Our results show that 3 years of warming treatments significantly elevated soil temperature at 0–100 cm depth, decreased soil moisture at 10 cm depth, and increased soil moisture at 40–100 cm depth. In contrast to the findings of previous research, experimental warming did not significantly affect NH ₄ ⁺‐N, NO ₃ ⁻‐N, and heterotrophic respiration, but stimulated the growth of plants and significantly increased root biomass at 30–50 cm depth. This led to increased soil organic carbon, total nitrogen, and liable carbon at 30–50 cm depth, and increased autotrophic respiration of plants. Analysis shows that experimental warming influenced deeper root production via redistributed soil moisture, which favors the accumulation of belowground carbon, but did not significantly affected the decomposition of soil organic carbon. Our findings suggest that future climate change studies need to take greater consideration of changes in the hydrological cycle and the local ecosystem characteristics. The results of our study will aid in understanding the response of terrestrial ecosystems to climate change and provide the regional case for global ecosystem models.     

Greater Abundance of <Emphasis Type="Italic">Betula nana</Emphasis> and Early Onset of the Growing Season Increase Ecosystem CO<Subscript>2</Subscript> Uptake in West Greenland

Expansion of deciduous shrubs is a common observation throughout the Arctic, with implications for carbon (C) cycling. Shrubs may increase net ecosystem C uptake through greater leaf area and gross ecosystem photosynthesis (GEP), and/or through cooler summer soils and reduced ecosystem respiration (ER). We used a space-for-time substitution combined with experimental warming at a Low Arctic site in West Greenland to examine the biophysical effects of increased temperature and Betula nana abundance on ecosystem CO₂ exchange. Communities dominated by Betula were much stronger C sinks than graminoid communities due to greater GEP and lower ER. The warming treatment had little effect on GEP, ER, or net ecosystem CO₂ exchange (NEE). The start of the growing season has been advancing at our study site, as indicated by long-term observations of plant phenology. In a retrospective analysis, we estimate that earlier onset of the growing season has increased the strength of the ecosystem C sink at rates of 1.3 and 2.1 g C m^?2 y^?1 in Betula and graminoid tundra, respectively, since 2002. However, earlier, and presumably longer, growing seasons may be associated with greater potential for drought stress. Our data suggest that mid-summer drought-induced GEP declines may partially offset C gains associated with an earlier start to the growing season. Our results suggest greater deciduous shrub abundance and longer growing seasons will likely lead to greater net C uptake in our study area, while highlighting important complexities associated with drought and plant community composition.     

Differential ecophysiological response of deciduous shrubs and a graminoid to long-term experimental snow reductions and additions in moist acidic tundra, Northern Alaska

Changes in winter precipitation that include both decreases and increases in winter snow are underway across the Arctic. In this study, we used a 14-year experiment that has increased and decreased winter snow in the moist acidic tussock tundra of northern Alaska to understand impacts of variation in winter snow depth on summer leaf-level ecophysiology of two deciduous shrubs and a graminoid species, including: instantaneous rates of leaf gas exchange, and δ¹³C, δ¹⁵N, and nitrogen (N) concentrations of Betula nana, Salix pulchra, and Eriophorum vaginatum. Leaf-level measurements were complemented by measurements of canopy leaf area index (LAI) and depth of thaw. Reductions in snow lowered summer leaf photosynthesis, conductance, and transpiration rates by up to 40 % compared to ambient and deep snow conditions for Eriophorum vaginatum, and reduced Salix pulchra conductance and transpiration by up to 49 %. In contrast, Betula nana exhibited no changes in leaf gas exchange in response to lower or deeper snow. Canopy LAI increased with added snow, while reduced winter snow resulted in lower growing season soil temperatures and reduced thaw depths. Our findings indicate that the spatial and temporal variability of future snow depth will have individualistic consequences for leaf-level C fixation and water flux by tundra species, and that these responses will be manifested over the longer term by changes in canopy traits, depth of thaw, soil C and N processes, and trace gas (CO₂ and H₂O) exchanges between the tundra and the atmosphere.     

Vegetation shift from deciduous to evergreen dwarf shrubs in response to selective herbivory offsets carbon losses: evidence from 19 years of warming and simulated herbivory in the subarctic tundra

Selective herbivory of palatable plant species provides a competitive advantage for unpalatable plant species, which often have slow growth rates and produce slowly decomposable litter. We hypothesized that through a shift in the vegetation community from palatable, deciduous dwarf shrubs to unpalatable, evergreen dwarf shrubs, selective herbivory may counteract the increased shrub abundance that is otherwise found in tundra ecosystems, in turn interacting with the responses of ecosystem carbon (C) stocks and CO₂ balance to climatic warming. We tested this hypothesis in a 19‐year field experiment with factorial treatments of warming and simulated herbivory on the dominant deciduous dwarf shrub Vaccinium myrtillus. Warming was associated with a significantly increased vegetation abundance, with the strongest effect on deciduous dwarf shrubs, resulting in greater rates of both gross ecosystem production (GEP) and ecosystem respiration (ER) as well as increased C stocks. Simulated herbivory increased the abundance of evergreen dwarf shrubs, most importantly Empetrum nigrum ssp. hermaphroditum, which led to a recent shift in the dominant vegetation from deciduous to evergreen dwarf shrubs. Simulated herbivory caused no effect on GEP and ER or the total ecosystem C stocks, indicating that the vegetation shift counteracted the herbivore‐induced C loss from the system. A larger proportion of the total ecosystem C stock was found aboveground, rather than belowground, in plots treated with simulated herbivory. We conclude that by providing a competitive advantage to unpalatable plant species with slow growth rates and long life spans, selective herbivory may promote aboveground C stocks in a warming tundra ecosystem and, through this mechanism, counteract C losses that result from plant biomass consumption.     

Phenology and species determine growing‐season albedo increase at the altitudinal limit of shrub growth in the sub‐Arctic

Arctic warming is resulting in reduced snow cover and increased shrub growth, both of which have been associated with altered land surface–atmospheric feedback processes involving sensible heat flux, ground heat flux and biogeochemical cycling. Using field measurements, we show that two common Arctic shrub species (Betula glandulosa and Salix pulchra), which are largely responsible for shrub encroachment in tundra, differed markedly in albedo and that albedo of both species increased as growing season progressed when measured at their altitudinal limit. A moveable apparatus was used to repeatedly measure albedo at six precise spots during the summer of 2012, and resampled in 2013. Contrary to the generally accepted view of shrub‐covered areas having low albedo in tundra, full‐canopy prostrate B. glandulosa had almost the highest albedo of all surfaces measured during the peak of the growing season. The higher midsummer albedo is also evident in localized MODIS albedo aggregated from 2000 to 2013, which displays a similar increase in growing‐season albedo. Using our field measurements, we show the ensemble summer increase in tundra albedo counteracts the generalized effect of earlier spring snow melt on surface energy balance by approximately 40%. This summer increase in albedo, when viewed in absolute values, is as large as the difference between the forest and tundra transition. These results indicate that near future (<50 years) changes in growing‐season albedo related to Arctic vegetation change are unlikely to be particularly large and might constitute a negative feedback to climate warming in certain circumstances. Future efforts to calculate energy budgets and a sensible heating feedback in the Arctic will require more detailed information about the relative abundance of different ground cover types, particularly shrub species and their respective growth forms and phenology.     

Ecotypic differences in the phenology of the tundra species Eriophorum vaginatum reflect sites of origin

Eriophorum vaginatum is a tussock‐forming sedge that contributes significantly to the structure and primary productivity of moist acidic tussock tundra. Locally adapted populations (ecotypes) have been identified across the geographical distribution of E. vaginatum; however, little is known about how their growth and phenology differ over the course of a growing season. The growing season is short in the Arctic and therefore exerts a strong selection pressure on tundra species. This raises the hypothesis that the phenology of arctic species may be poorly adapted if the timing and length of the growing season change. Mature E. vaginatum tussocks from across a latitudinal gradient (65–70°N) were transplanted into a common garden at a central location (Toolik Lake, 68°38′N, 149°36′W) where half were warmed using open‐top chambers. Over two growing seasons (2015 and 2016), leaf length was measured weekly to track growth rates, timing of senescence, and biomass accumulation. Growth rates were similar across ecotypes and between years and were not affected by warming. However, southern populations accumulated significantly more biomass, largely because they started to senesce later. In 2016, peak biomass and senescence of most populations occurred later than in 2015, probably induced by colder weather at the beginning of the growing season in 2016, which caused a delayed start to growth. The finish was delayed as well. Differences in phenology between populations were largely retained between years, suggesting that the amount of time that these ecotypes grow has been selected by the length of the growing seasons at their respective home sites. As potential growing seasons lengthen, E. vaginatum may be unable to respond appropriately as a result of genetic control and may have reduced fitness in the rapidly warming Arctic tundra.