Leaf temperatures mediate alpine plant communities’ response to a simulated extended summer

We use a quantitative model of photosynthesis to explore leaf‐level limitations to plant growth in an alpine tundra ecosystem that is expected to have longer, warmer, and drier growing seasons. The model is parameterized with abiotic and leaf trait data that is characteristic of two dominant plant communities in the alpine tundra and specifically at the Niwot Ridge Long Term Ecological Research Site: the dry and wet meadows. Model results produce realistic estimates of photosynthesis, nitrogen‐use efficiency, water‐use efficiency, and other gas exchange processes in the alpine tundra. Model simulations suggest that dry and wet meadow plant species do not significantly respond to changes in the volumetric soil moisture content but are sensitive to variation in foliar nitrogen content. In addition, model simulations indicate that dry and wet meadow species have different maximum rates of assimilation (normalized for leaf nitrogen content) because of differences in leaf temperature. These differences arise from the interaction of plant height and the abiotic environment characteristic of each plant community. The leaf temperature of dry meadow species is higher than wet meadow species and close to the optimal temperature for photosynthesis under current conditions. As a result, 2°C higher air temperatures in the future will likely lead to declines in dry meadow species’ carbon assimilation. On the other hand, a longer and warmer growing season could increase nitrogen availability and assimilation rates in both plant communities. Nonetheless, a temperature increase of 4°C may lower rates of assimilation in both dry and wet meadow plant communities because of higher, and suboptimal, leaf temperatures.     

Greater deciduous shrub abundance extends tundra peak season and increases modeled net CO2 uptake

Satellite studies of the terrestrial Arctic report increased summer greening and longer overall growing and peak seasons since the 1980s, which increases productivity and the period of carbon uptake. These trends are attributed to increasing air temperatures and reduced snow cover duration in spring and fall. Concurrently, deciduous shrubs are becoming increasingly abundant in tundra landscapes, which may also impact canopy phenology and productivity. Our aim was to determine the influence of greater deciduous shrub abundance on tundra canopy phenology and subsequent impacts on net ecosystem carbon exchange (NEE) during the growing and peak seasons in the arctic foothills region of Alaska. We compared deciduous shrub‐dominated and evergreen/graminoid‐dominated community‐level canopy phenology throughout the growing season using the normalized difference vegetation index (NDVI). We used a tundra plant‐community‐specific leaf area index (LAI) model to estimate LAI throughout the green season and a tundra‐specific NEE model to estimate the impact of greater deciduous shrub abundance and associated shifts in both leaf area and canopy phenology on tundra carbon flux. We found that deciduous shrub canopies reached the onset of peak greenness 13 days earlier and the onset of senescence 3 days earlier compared to evergreen/graminoid canopies, resulting in a 10‐day extension of the peak season. The combined effect of the longer peak season and greater leaf area of deciduous shrub canopies almost tripled the modeled net carbon uptake of deciduous shrub communities compared to evergreen/graminoid communities, while the longer peak season alone resulted in 84% greater carbon uptake in deciduous shrub communities. These results suggest that greater deciduous shrub abundance increases carbon uptake not only due to greater leaf area, but also due to an extension of the period of peak greenness, which extends the period of maximum carbon uptake.     

Sensitivity analysis of ecosystem CO<Subscript>2</Subscript> 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.     

Effects of lengthened growing season and soil warming on the phenology and physiology of Polygonum bistorta

The phenological and physiological responses of arctic tundra plant species are key to predicting their survival in a warmer climate. One of the consequences of a warmer climate in the Arctic will be a longer growing season. We examined the effects of lengthened growing season and soil warming on the widely distributed forb, Polygonum bistorta L. Three treatments were established near Toolik Lake, Alaska in 1995 and 1996: extended season, extended season with soil warming, and an unmanipulated control. The season was extended by removing the snow load in the spring and keeping the treatments free of snow in the autumn. The spring snow removal extended the snow‐free period over that of controls by 8 d in 1995 and 24 d in 1996. As a result, the number of accumulated soil thaw days and consequently the depth of soil thaw increased on the treatment plots. Polygonum bistorta responded to the treatments by becoming active earlier and senescing earlier, resulting in a growth period of similar duration to that of the controls. Leaf size and leaf number were unaffected by the treatments, as were leaf photosynthetic assimilation rates and nutrient concentrations. The results indicate that internal constraints limit the response of this species to lengthened growing season, suggesting that it is a determinant or periodic species. With climate warming, this periodic growth will put P. bistorta at a competitive disadvantage relative to plants that can respond to lengthened growing season.     

Fast and furious: Early differences in growth rate drive short‐term plant dominance and exclusion under eutrophication

1. The reduction of plant diversity following eutrophication threatens many ecosystems worldwide. Yet, the mechanisms by which species are lost following nutrient enrichment are still not completely understood, nor are the details of when such mechanisms act during the growing season, which hampers understanding and the development of mitigation strategies.
2. Using a common garden competition experiment, we found that early‐season differences in growth rates among five perennial grass species measured in monoculture predicted short‐term competitive dominance in pairwise combinations and that the proportion of variance explained was particularly greater under a fertilization treatment.
3. We also examined the role of early‐season growth rate in determining the outcome of competition along an experimental nutrient gradient in an alpine meadow. Early differences in growth rate between species predicted short‐term competitive dominance under both ambient and fertilized conditions and competitive exclusion under fertilized conditions.
4. The results of these two studies suggest that plant species growing faster during the early stage of the growing season gain a competitive advantage over species that initially grow more slowly, and that this advantage is magnified under fertilization. This finding is consistent with the theory of asymmetric competition for light in which fast‐growing species can intercept incident light and hence outcompete and exclude slower‐growing (and hence shorter) species. We predict that the current chronic nutrient inputs into many terrestrial ecosystems worldwide will reduce plant diversity and maintain a low biodiversity state by continuously favoring fast‐growing species. Biodiversity management strategies should focus on controlling nutrient inputs and reducing the growth of fast‐growing species early in the season.

     

Diurnal patterns of CO2 and H2O exchange of the Arctic sedges Eriophorum angustifolium and E. vaginatum (Cyperaceae)

Eriophorum vaginatum and E. angustifolium are dominant arctic sedges of the well-drained tussock tundra and the permanently flooded wet-sedge tundra, respectively. We determined diurnal courses of gas exchange and water relations of the two species in their natural habitat and compared their responses to changes in light, air temperature, and humidity. Mean photosynthetic response to light was similar between E. angustifolium and E. vaginatum and carbon gain in both species was light limited during most of the growing season. On sunny and dry days, both species closed stomata in response to high leaf-to-air vapor pressure deficits. Even though E. angustifolium was growing in standing water, it exhibited a tighter control of transpirational water loss and had lower hydraulic conductivity in the soil-root-shoot pathway than E. vaginatum. The different response pattern between the two species is discussed in the context of differences in habitat conditions.     

Changes in high arctic tundra plant reproduction in response to long‐term experimental warming

We provide new information on changes in tundra plant sexual reproduction in response to long‐term (12 years) experimental warming in the High Arctic. Open‐top chambers (OTCs) were used to increase growing season temperatures by 1–2 °C across a range of vascular plant communities. The warming enhanced reproductive effort and success in most species; shrubs and graminoids appeared to be more responsive than forbs. We found that the measured effects of warming on sexual reproduction were more consistently positive and to a greater degree in polar oasis compared with polar semidesert vascular plant communities. Our findings support predictions that long‐term warming in the High Arctic will likely enhance sexual reproduction in tundra plants, which could lead to an increase in plant cover. Greater abundance of vegetation has implications for primary consumers – via increased forage availability, and the global carbon budget – as a function of changes in permafrost and vegetation acting as a carbon sink. Enhanced sexual reproduction in Arctic vascular plants may lead to increased genetic variability of offspring, and consequently improved chances of survival in a changing environment. Our findings also indicate that with future warming, polar oases may play an important role as a seed source to the surrounding polar desert landscape.     

Plant phenological responses to a long‐term experimental extension of growing season and soil warming in the tussock tundra of Alaska

Climate warming is strongly altering the timing of season initiation and season length in the Arctic. Phenological activities are among the most sensitive plant responses to climate change and have important effects at all levels within the ecosystem. We tested the effects of two experimental treatments, extended growing season via snow removal and extended growing season combined with soil warming, on plant phenology in tussock tundra in Alaska from 1995 through 2003. We specifically monitored the responses of eight species, representing four growth forms: (i) graminoids (Carex bigellowii and Eriophorum vaginatum); (ii) evergreen shrubs (Ledum palustre, Cassiope tetragona, and Vaccinium vitis‐idaea); (iii) deciduous shrubs (Betula nana and Salix pulchra); and (iv) forbs (Polygonum bistorta). Our study answered three questions: (i) Do experimental treatments affect the timing of leaf bud break, flowering, and leaf senescence? (ii) Are responses to treatments species‐specific and growth form‐specific? and (iii) Which environmental factors best predict timing of phenophases? Treatment significantly affected the timing of all three phenophases, although the two experimental treatments did not differ from each other. While phenological events began earlier in the experimental plots relative to the controls, duration of phenophases did not increase. The evergreen shrub, Cassiope tetragona, did not respond to either experimental treatment. While the other species did respond to experimental treatments, the total active period for these species did not increase relative to the control. Air temperature was consistently the best predictor of phenology. Our results imply that some evergreen shrubs (i.e., C. tetragona) will not capitalize on earlier favorable growing conditions, putting them at a competitive disadvantage relative to phenotypically plastic deciduous shrubs. Our findings also suggest that an early onset of the growing season as a result of decreased snow cover will not necessarily result in greater tundra productivity.     

Changes in the structure and function of northern Alaskan ecosystems when considering variable leaf‐out times across groupings of species in a dynamic vegetation model

The phenology of arctic ecosystems is driven primarily by abiotic forces, with temperature acting as the main determinant of growing season onset and leaf budburst in the spring. However, while the plant species in arctic ecosystems require differing amounts of accumulated heat for leaf‐out, dynamic vegetation models simulated over regional to global scales typically assume some average leaf‐out for all of the species within an ecosystem. Here, we make use of air temperature records and observations of spring leaf phenology collected across dominant groupings of species (dwarf birch shrubs, willow shrubs, other deciduous shrubs, grasses, sedges, and forbs) in arctic and boreal ecosystems in Alaska. We then parameterize a dynamic vegetation model based on these data for four types of tundra ecosystems (heath tundra, shrub tundra, wet sedge tundra, and tussock tundra), as well as ecotonal boreal white spruce forest, and perform model simulations for the years 1970–2100. Over the course of the model simulations, we found changes in ecosystem composition under this new phenology algorithm compared with simulations with the previous phenology algorithm. These changes were the result of the differential timing of leaf‐out, as well as the ability for the groupings of species to compete for nitrogen and light availability. Regionally, there were differences in the trends of the carbon pools and fluxes between the new phenology algorithm and the previous phenology algorithm, although these differences depended on the future climate scenario. These findings indicate the importance of leaf phenology data collection by species and across the various ecosystem types within the highly heterogeneous Arctic landscape, and that dynamic vegetation models should consider variation in leaf‐out by groupings of species within these ecosystems to make more accurate projections of future plant distributions and carbon cycling in Arctic regions.     

Nitrogen availability increases in a tundra ecosystem during five years of experimental permafrost thaw

Perennially frozen soil in high latitude ecosystems (permafrost) currently stores 1330–1580 Pg of carbon (C). As these ecosystems warm, the thaw and decomposition of permafrost is expected to release large amounts of C to the atmosphere. Fortunately, losses from the permafrost C pool will be partially offset by increased plant productivity. The degree to which plants are able to sequester C, however, will be determined by changing nitrogen (N) availability in these thawing soil profiles. N availability currently limits plant productivity in tundra ecosystems but plant access to N is expected improve as decomposition increases in speed and extends to deeper soil horizons. To evaluate the relationship between permafrost thaw and N availability, we monitored N cycling during 5 years of experimentally induced permafrost thaw at the Carbon in Permafrost Experimental Heating Research (CiPEHR) project. Inorganic N availability increased significantly in response to deeper thaw and greater soil moisture induced by Soil warming. This treatment also prompted a 23% increase in aboveground biomass and a 49% increase in foliar N pools. The sedge Eriophorum vaginatum responded most strongly to warming: this species explained 91% of the change in aboveground biomass during the 5 year period. Air warming had little impact when applied alone, but when applied in combination with Soil warming, growing season soil inorganic N availability was significantly reduced. These results demonstrate that there is a strong positive relationship between the depth of permafrost thaw and N availability in tundra ecosystems but that this relationship can be diminished by interactions between increased thaw, warmer air temperatures, and higher levels of soil moisture. Within 5 years of permafrost thaw, plants actively incorporate newly available N into biomass but C storage in live vascular plant biomass is unlikely to be greater than losses from deep soil C pools.     

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.     

Decadal warming causes a consistent and persistent shift from heterotrophic to autotrophic respiration in contrasting permafrost ecosystems

Soil carbon in permafrost ecosystems has the potential to become a major positive feedback to climate change if permafrost thaw increases heterotrophic decomposition. However, warming can also stimulate autotrophic production leading to increased ecosystem carbon storage—a negative climate change feedback. Few studies partitioning ecosystem respiration examine decadal warming effects or compare responses among ecosystems. Here, we first examined how 11 years of warming during different seasons affected autotrophic and heterotrophic respiration in a bryophyte‐dominated peatland in Abisko, Sweden. We used natural abundance radiocarbon to partition ecosystem respiration into autotrophic respiration, associated with production, and heterotrophic decomposition. Summertime warming decreased the age of carbon respired by the ecosystem due to increased proportional contributions from autotrophic and young soil respiration and decreased proportional contributions from old soil. Summertime warming's large effect was due to not only warmer air temperatures during the growing season, but also to warmer deep soils year‐round. Second, we compared ecosystem respiration responses between two contrasting ecosystems, the Abisko peatland and a tussock‐dominated tundra in Healy, Alaska. Each ecosystem had two different timescales of warming (<5 years and over a decade). Despite the Abisko peatland having greater ecosystem respiration and larger contributions from heterotrophic respiration than the Healy tundra, both systems responded consistently to short‐ and long‐term warming with increased respiration, increased autotrophic contributions to ecosystem respiration, and increased ratios of autotrophic to heterotrophic respiration. We did not detect an increase in old soil carbon losses with warming at either site. If increased autotrophic respiration is balanced by increased primary production, as is the case in the Healy tundra, warming will not cause these ecosystems to become growing season carbon sources. Warming instead causes a persistent shift from heterotrophic to more autotrophic control of the growing season carbon cycle in these carbon‐rich permafrost ecosystems.     

The incidence and implications of clouds for cloud forest plant water relations

Although clouds are the most recognisable and defining feature of tropical montane cloud forests, little research has focussed on how clouds affect plant functioning. We used satellite and ground‐based observations to study cloud and leaf wetting patterns in contrasting tropical montane and pre‐montane cloud forests. We then studied the consequences of leaf wetting for the direct uptake of water accumulated on leaf surfaces into the leaves themselves. During the dry season, the montane forest experienced higher precipitation, cloud cover and leaf wetting events of longer duration than the pre‐montane forest. Leaf wetting events resulted in foliar water uptake in all species studied. The capacity for foliar water uptake differed significantly between the montane and pre‐montane forest plant communities, as well as among species within a forest. Our results indicate that foliar water uptake is common in these forest plants and improves plant water status during the dry season.     

No increase in alpine snowbed productivity in response to experimental lengthening of the growing season

Climate change effects on snow cover and thermic regime in alpine tundra might lead to a longer growing season, but could also increase risks to plants from spring frost events. Alpine snowbeds, i.e. alpine tundra from late snowmelt sites, might be particularly susceptible to such climatic changes. Snowbed communities were grown in large monoliths for two consecutive years, under different manipulated snow cover treatments, to test for effects of early (E) and late (L) snowmelt on dominant species growth, plant functional traits, leaf area index (LAI) and aboveground productivity. Spring snow cover was reduced to assess the sensitivity of snowbed alpine species to severe early frost events, and dominant species freezing temperatures were measured. Aboveground biomass, productivity, LAI and dominant species growth did not increase significantly in E compared to L treatments, indicating inability to respond to an extended growing season. Edapho‐climatic conditions could not account for these results, suggesting that developmental constraints are important in controlling snowbed plant growth. Impaired productivity was only detected when harsher and more frequent frost events were experimentally induced by early snowmelt. These conditions exposed plants to spring frosts, reaching temperatures consistent with the estimated freezing points of the dominant species (～?10 °C). We conclude that weak plasticity in phenological response and potential detrimental effects of early frosts explain why alpine tundra from snowbeds is not expected to benefit from increased growing season length.     

Predicted responses of arctic and alpine ecosystems to altered seasonality under climate change

Global climate change is already having significant impacts on arctic and alpine ecosystems, and ongoing increases in temperature and altered precipitation patterns will affect the strong seasonal patterns that characterize these temperature‐limited systems. The length of the potential growing season in these tundra environments is increasing due to warmer temperatures and earlier spring snow melt. Here, we compare current and projected climate and ecological data from 20 Northern Hemisphere sites to identify how seasonal changes in the physical environment due to climate change will alter the seasonality of arctic and alpine ecosystems. We find that although arctic and alpine ecosystems appear similar under historical climate conditions, climate change will lead to divergent responses, particularly in the spring and fall shoulder seasons. As seasonality changes in the Arctic, plants will advance the timing of spring phenological events, which could increase plant nutrient uptake, production, and ecosystem carbon (C) gain. In alpine regions, photoperiod will constrain spring plant phenology, limiting the extent to which the growing season can lengthen, especially if decreased water availability from earlier snow melt and warmer summer temperatures lead to earlier senescence. The result could be a shorter growing season with decreased production and increased nutrient loss. These contrasting alpine and arctic ecosystem responses will have cascading effects on ecosystems, affecting community structure, biotic interactions, and biogeochemistry.     

Analysis of developmental preformation in the alpine herb Caltha leptosepala (Ranunculaceae)

Developmental preformation is ubiquitous among alpine and arctic tundra plant species and may cause a delay in plant morphological responses to environmental variation. The duration of preformation and seasonal pattern of development were examined in Caltha leptosepala to identify characteristics of architecture and development that may influence the timing of plant responses to environmental cues, both within a single growing season and between years. All structures in C. leptosepala are preformed: leaves are initiated one or two growing seasons before they mature and flowers are initiated one growing season before maturation. Features of development and architecture in C. leptosepala, however, appear to differ from the determinate growth patterns of other exclusively preforming species, and may allow within-season variability in the seasonal development and maturation of structures. Cohorts of leaves initiated are asynchronous with maturation cohorts, and each year the number of leaf primordia per plant at snowmelt exceeds the number to mature aboveground. Therefore, some flexibility in whether leaves complete a 2-yr or 3-yr developmental trajectory might occur. Plasticity in reproductive phenotype might also occur via the process of floral abortion. Despite developmental characteristics that might facilitate the expression of phenotypic plasticity, only slight variability was observed in the duration of preformation or in the seasonal pattern of initiation and emergence of structures. Growth patterns of C. leptosepala thus appear to be fundamentally constrained, and limitations to annual growth may assure that sufficient preformed primordia remain belowground at the end of each growing season for maturation of a full cohort during the subsequent season.     

Deepened winter snow cover enhances net ecosystem exchange and stabilizes plant community composition and productivity in a temperate grassland

Global warming has greatly altered winter snowfall patterns, and there is a trend towards increasing winter snow in semi‐arid regions in China. Winter snowfall is an important source of water during early spring in these water‐limited ecosystems, and it can also affect nutrient supply. However, we know little about how changes in winter snowfall will affect ecosystem productivity and plant community structure during the growing season. Here, we conducted a 5‐year winter snow manipulation experiment in a temperate grassland in Inner Mongolia. We measured ecosystem carbon flux from 2014 to 2018 and plant biomass and species composition from 2015 to 2018. We found that soil moisture increased under deepened winter snow in early growing season, particularly in deeper soil layers. Deepened snow increased the net ecosystem exchange of CO₂ (NEE) and reduced intra‐ and inter‐annual variation in NEE. Deepened snow did not affect aboveground plant biomass (AGB) but significantly increased root biomass. This suggested that the enhanced NEE was allocated to the belowground, which improved water acquisition and thus contributed to greater stability in NEE in deep‐snow plots. Interestingly, the AGB of grasses in the control plots declined over time, resulting in a shift towards a forb‐dominated system. Similar declines in grass AGB were also observed at three other locations in the region over the same time frame and are attributed to 4 years of below‐average precipitation during the growing season. By contrast, grass AGB was stabilized under deepened winter snow and plant community composition remained unchanged. Hence, our study demonstrates that increased winter snowfall may stabilize arid grassland systems by reducing resource competition, promoting coexistence between plant functional groups, which ultimately mitigates the impacts of chronic drought during the growing season.     

Productivity–diversity patterns in arctic tundra vegetation

Productivity has long been argued to be a major driver of species richness patterns. In the present study we test alternative productivity–diversity hypotheses using vegetation data from the vast Eurasian tundra. The productivity–species pool hypothesis predicts positive relationships at both fine and coarse grain sizes, whereas the productivity–interaction hypothesis predicts unimodal patterns at fine grain size, and monotonic positive patterns at coarse grain size. We furthermore expect to find flatter positive (productivity–species pool hypothesis) or more strongly negative (productivity–interaction hypothesis) relationships for lichens and bryophytes than for vascular plants, because as a group, lichens and bryophytes are better adapted to extreme arctic conditions and more vulnerable to competition for light than the taller‐growing vascular plants. The normalised difference vegetation index (NDVI) was used as a proxy of productivity. The generally unimodal productivity–diversity patterns were most consistent with the productivity–interaction hypothesis. There was a general trend of decreasing species richness from moderately to maximally productive tundra, in agreement with an increasing importance of competitive interactions. High richness of vascular plants and lichens occurred in moderately low productive tundra areas, whereas that of bryophytes occurred in the least productive tundra habitats covered by this study. The fine and coarse grain richness trends were surprisingly uniform and no variation in beta diversity along the productivity gradient was seen for vascular plants or bryophytes. However, lichen beta diversity varied along the productivity gradient, probably reflecting their sensitivity to habitat conditions and biotic interactions. Overall, the results show evidence that productivity–diversity gradients exist in tundra and that these appear to be largely driven by competitive interactions. Our results also imply that climate warming‐driven increases in productivity will strongly affect arctic plant diversity patterns.     

Distinct impacts of different mammalian herbivore assemblages on arctic tundra CO2 exchange during the peak of the growing season

Herbivores play a key role in the carbon (C) cycle of arctic ecosystems, but these effects are currently poorly represented within models predicting land–atmosphere interactions under future climate change. Although some studies have examined the influence of various individual species of herbivores on tundra C sequestration, few studies have directly compared the effects of different herbivore assemblages. We measured peak growing season instantaneous ecosystem carbon dioxide (CO₂) exchange (photosynthesis, respiration and net ecosystem exchange) on replicated plots in arctic tundra which, for 14 years, have excluded different portions of the herbivore population (grazed controls, large mammals excluded, both small and large mammals excluded). Herbivory suppressed photosynthetic CO₂ uptake, but caused little change in ecosystem respiration. Despite evidence that small mammals consume a greater portion of plant biomass in these ecosystems, the effect of excluding only large herbivores was indistinguishable from that of excluding both large and small mammals. The herbivory‐induced decline in photosynthesis was not entirely attributable to a decline in leaf area but also likely reflects shifts in plant community composition and/or species physiology. One shrub species – Betula nana – accounted for only around 13% of total aboveground vascular plant biomass but played a central role in controlling ecosystem CO₂ uptake and release, and was suppressed by herbivory. We conclude that herbivores can have large effects on ecosystem C cycling due to shifts in plant aboveground biomass and community composition. An improved understanding of the mechanisms underlying the distinct ecosystem impacts of different herbivore groups will help to more accurately predict the net impacts of diverse herbivore communities on arctic C fluxes.     

How ecological neighbourhoods influence the structure of the scavenger guild in low arctic tundra

Aim How the ecological neighbourhoods of coast and forest affect arctic tundra ecosystems is a pressing question as the circumpolar tundra belt is shrinking under global warming. Mobile facultative scavengers are likely to negatively impact tundra biodiversity as dominant competitors or predators, if they spill over into tundra. Here, we provide the first quantitative assessments of the structure of a scavenger guild in low arctic tundra with emphasis on how it changes along spatial gradients from neighbouring ecosystems (i.e. forest and coast) and with altitude (i.e. productivity gradients). We also assess the likelihood of interactions between guild members that may negatively impact vulnerable tundra species. Location North‐eastern part of Norway. Methods Extensive records of scavenger prevalence were obtained by deploying automatic digital cameras at experimental carcasses in tundra regions covering several thousand square kilometres and three winters in northern Norway. Main conclusions We found short‐range neighbourhood effects of forest and coast within the tundra scavenger guild. Species richness declined steeply with decreasing distance from the neighbouring ecosystems, in particular subarctic forest, and with increasing altitude. Bird species with strongholds in forest (golden eagle Aquila chrysaetos and hooded crow Corvus cornix) or along the coast (white‐tailed eagle Haliaeetus albicilla) were mostly responsible for short‐range neighbourhood effects on guild structure. However, the two most abundant guild members, the common raven Corvus corax and the red fox Vulpes vulpes, exhibited no spatial patterns within the range of neighbourhoods and altitudes examined. There was a clear diurnal segregation in the use of carcasses between birds and mammals reducing the likelihood of direct interactions between these two taxa. Presence of red fox appeared to exclude the arctic fox Vulpes lagopus, the only endemic tundra species within the guild, from carcasses.