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.     

Interactions of herbivore exclusion with warming and N addition in a grass-dominated temperate old field

Field experiments used to explore the effects of global change drivers, such as warming and nitrogen deposition on plant productivity and species composition, have typically focused on bottom-up processes. However, both direct and indirect responses of herbivores to the treatments could result in important interactions between top-down and bottom-up effects. These interactions may be complicated by the simultaneous effects of multiple herbivore taxa. We used rodent and mollusc exclosures in the plots of a warming and N addition field experiment to examine how herbivore removal would influence plant biomass responses to the treatments. The effect of rodent exclusion on grass biomass more than doubled in response to nitrogen addition, but did not respond to warming, whereas the effect of mollusc exclusion on grass biomass increased in response to warming, but not nitrogen. In contrast, the effect of rodent exclusion on total biomass (grasses and forbs combined) increased in response to both nitrogen and warming, while the effect of mollusc exclusion on total biomass was insensitive to nitrogen and warming. In no cases were there interactions between nitrogen and warming with respect to their influence on exclosure effects. Overall, our results demonstrated substantial and variable effects of multiple herbivore taxa on plant biomass responses to warming and N addition, despite the absence of conspicuous damage to the plant canopy. These results therefore highlight the potential importance of interactions between top-down and bottom-up factors in global change field experiments.     

Regulation of boreal forest understory vegetation: the roles of resources and herbivores

To understand inter-trophic linkages between components of the boreal forest understory vegetation, three hypotheses were tested: survival, growth and abundance of grasses and legumes were controlled by (i) resource availability alone, (ii) by herbivores alone, and (iii) by both resource availability and herbivores. The hypotheses were tested using three experimental treatments – fertilization, herbivore exclusion, and fertilization plus herbivore exclusion – in three areas having different densities of resident herbivores, mostly snowshoe hares and ground squirrels. The highest density of snowshoe hares is comparable to natural levels during peaks in the snowshoe hare cycle. As the density of herbivores increased so too did the level of response by the measured variables – survival, growth of transplants and leaf area index of established vegetation. In general, fertilization resulted in a decrease in survival and growth of transplants, and fences increased survival and growth; both responses were more noticeable at higher herbivore densities. Fertilizer and herbivore exclosure fences had only negligible effects on established grass and legume abundance at all hare densities. We have shown that some hypotheses of vegetation regulation are over-simplified because different species groups (i.e., grasses and legumes) are regulated by different factors, at different life history stages, and sometimes these factors act in opposing directions. We argue that during the increase phase and peak of the snowshoe hare cycle (high herbivore density), growth and survival of establishing plants is regulated by herbivores. During the decline and low phases of the snowshoe hare cycle herbivores will have little impact on early life stages, whereas the established, mature, vegetation will be resource-regulated. Because of the variability in responses to the same manipulations we may begin to understand which plant life history stages are most vulnerable to consumer and resource regulation, the magnitudes of these sources of regulation at each of these stages, and how these vary among species groups and types of environments.     

Herbivory and nutrient limitation protect warming tundra from lowland species’ invasion and diversity loss

Herbivory and nutrient limitation can increase the resistance of temperature‐limited systems to invasions under climate warming. We imported seeds of lowland species to tundra under factorial treatments of warming, fertilization, herbivore exclusion and biomass removal. We show that warming alone had little impact on lowland species, while exclusion of native herbivores and relaxation of nutrient limitation greatly benefitted them. In contrast, warming alone benefitted resident tundra species and increased species richness; however, these were canceled by negative effects of herbivore exclusion and fertilization. Dominance of lowland species was associated with low cover of tundra species and resulted in decreased species richness. Our results highlight the critical role of biotic and abiotic filters unrelated to temperature in protecting tundra under warmer climate. While scarcity of soil nutrients and native herbivores act as important agents of resistance to invasions by lowland species, they concurrently promote overall species coexistence. However, when these biotic and abiotic resistances are relaxed, invasion of lowland species can lead to decreased abundance of resident tundra species and diminished diversity.     

The Response of Soil CO2 Fluxes to Progressively Excluding Vertebrate and Invertebrate Herbivores Depends on Ecosystem Type

Grasslands support large populations of herbivores and store up to 30% of the world’s soil carbon (C). Thus, herbivores likely play an important role in the global C cycle. However, most studies on how herbivory impacts the largest source of C released from grassland soils—soil carbon dioxide (CO₂) emissions—only considered the role of large ungulates. This ignores all other vertebrate and invertebrate herbivores and their collective effects on ecosystem properties. We progressively excluded large, medium, and small vertebrates and invertebrates from two subalpine grasslands (productive, heavily grazed short-grass; less productive, lightly grazed tall-grass) using size-selective fences, assessed the impact on soil CO₂ emissions and related biotic and abiotic variables. Exclusion resulted in significant changes in soil CO₂ emissions in both vegetation types. Short-grass soil CO₂ emissions progressively increased when large and medium mammals were excluded. However, no difference was detected among plots were all or no herbivores grazed. In contrast, tall-grass soil CO₂ emissions were not affected by mammal exclusion, but excluding all herbivores lead to reduced emissions. Soil micro-climatic parameters best predicted the patterns of soil CO₂ emissions in short-grass vegetation, whereas root biomass was the best predictor of CO₂ release in tall-grass vegetation. Our results showed that diverse herbivore communities affect soil respiration differently than assumed from previous studies that only excluded large ungulates. Such information is important if we are to understand how changes in herbivore species composition—as could happen through altered management practices, extinction or invasion—impact grassland C storage and release.     

Warming Reduces Carbon Losses from Grassland Exposed to Elevated Atmospheric Carbon Dioxide

The flux of carbon dioxide (CO₂) between terrestrial ecosystems and the atmosphere may ameliorate or exacerbate climate change, depending on the relative responses of ecosystem photosynthesis and respiration to warming temperatures, rising atmospheric CO₂, and altered precipitation. The combined effect of these global change factors is especially uncertain because of their potential for interactions and indirectly mediated conditions such as soil moisture. Here, we present observations of CO₂ fluxes from a multi-factor experiment in semi-arid grassland that suggests a potentially strong climate – carbon cycle feedback under combined elevated [CO₂] and warming. Elevated [CO₂] alone, and in combination with warming, enhanced ecosystem respiration to a greater extent than photosynthesis, resulting in net C loss over four years. The effect of warming was to reduce respiration especially during years of below-average precipitation, by partially offsetting the effect of elevated [CO₂] on soil moisture and C cycling. Carbon losses were explained partly by stimulated decomposition of soil organic matter with elevated [CO₂]. The climate – carbon cycle feedback observed in this semiarid grassland was mediated by soil water content, which was reduced by warming and increased by elevated [CO₂]. Ecosystem models should incorporate direct and indirect effects of climate change on soil water content in order to accurately predict terrestrial feedbacks and long-term storage of C in soil.     

The interaction of plant genotype and herbivory decelerate leaf litter decomposition and alter nutrient dynamics

We examined how plant genetic variation and a common herbivore (the leaf-galling aphid, Pemphigus betae ) influenced leaf litter quality, decomposition, and nutrient dynamics in a dominant riparian tree ( Populus spp .). Based on both observational studies and a herbivore exclusion experiment using trees of known genotype, we found four major patterns: 1) the quality of galled vs non-galled or gall-excluded litter significantly differed in the concentration of condensed tannins, lignin, nitrogen and phosphorus; 2) the difference in litter quality resulted in galled litter decomposing at rates 34 to 40% slower than non-galled litter; 3) plant genotype and herbivory had similar effects on the magnitude of decomposition rate constants; and 4) plant genotype mediated the herbivore effects on leaf litter quality and decomposition, as there were genotype-specific responses to herbivory independent of herbivore density. In contrast to other studies that have demonstrated accelerated ecosystem properties in response to arthropod herbivory, our findings argue that herbivore-induced secondary compounds decelerated ecosystem properties though their "after-life" effects on litter quality. Furthermore, these data are among the first to suggest that genotype-specific responses to herbivores can have a major impact on decomposition and nutrient flux, which likely has important consequences for the spatial distribution of nutrients at the landscape level. Due to the magnitude of these effects, we contend that it is important to incorporate a genetic perspective into ecosystem studies.     

Multi‐factor climate change effects on insect herbivore performance

The impact of climate change on herbivorous insects can have far‐reaching consequences for ecosystem processes. However, experiments investigating the combined effects of multiple climate change drivers on herbivorous insects are scarce. We independently manipulated three climate change drivers (CO₂, warming, drought) in a Danish heathland ecosystem. The experiment was established in 2005 as a full factorial split‐plot with 6 blocks × 2 levels of CO₂ × 2 levels of warming × 2 levels of drought = 48 plots. In 2008, we exposed 432 larvae (n = 9 per plot) of the heather beetle (Lochmaea suturalis Thomson ), an important herbivore on heather, to ambient versus elevated drought, temperature, and CO₂ (plus all combinations) for 5 weeks. Larval weight and survival were highest under ambient conditions and decreased significantly with the number of climate change drivers. Weight was lowest under the drought treatment, and there was a three‐way interaction between time, CO₂, and drought. Survival was lowest when drought, warming, and elevated CO₂ were combined. Effects of climate change drivers depended on other co‐acting factors and were mediated by changes in plant secondary compounds, nitrogen, and water content. Overall, drought was the most important factor for this insect herbivore. Our study shows that weight and survival of insect herbivores may decline under future climate. The complexity of insect herbivore responses increases with the number of combined climate change drivers.     

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.     

Nutrient availability controls the impact of mammalian herbivores on soil carbon and nitrogen pools in grasslands

Grasslands are subject to considerable alteration due to human activities globally, including widespread changes in populations and composition of large mammalian herbivores and elevated supply of nutrients. Grassland soils remain important reservoirs of carbon (C) and nitrogen (N). Herbivores may affect both C and N pools and these changes likely interact with increases in soil nutrient availability. Given the scale of grassland soil fluxes, such changes can have striking consequences for atmospheric C concentrations and the climate. Here, we use the Nutrient Network experiment to examine the responses of soil C and N pools to mammalian herbivore exclusion across 22 grasslands, under ambient and elevated nutrient availabilities (fertilized with NPK + micronutrients). We show that the impact of herbivore exclusion on soil C and N pools depends on fertilization. Under ambient nutrient conditions, we observed no effect of herbivore exclusion, but under elevated nutrient supply, pools are smaller upon herbivore exclusion. The highest mean soil C and N pools were found in grazed and fertilized plots. The decrease in soil C and N upon herbivore exclusion in combination with fertilization correlated with a decrease in aboveground plant biomass and microbial activity, indicating a reduced storage of organic matter and microbial residues as soil C and N. The response of soil C and N pools to herbivore exclusion was contingent on temperature – herbivores likely cause losses of C and N in colder sites and increases in warmer sites. Additionally, grasslands that contain mammalian herbivores have the potential to sequester more N under increased temperature variability and nutrient enrichment than ungrazed grasslands. Our study highlights the importance of conserving mammalian herbivore populations in grasslands worldwide. We need to incorporate local‐scale herbivory, and its interaction with nutrient enrichment and climate, within global‐scale models to better predict land–atmosphere interactions under future climate change.     

In situ warming strengthens trophic cascades in a coastal food web

Global warming may affect most organisms and their interactions. Theory and simple mesocosm experiments suggest that consumer top–down control over primary producer biomass should strengthen with warming, since consumer respiration increases faster with warming than plant photosynthesis. However, these predictions have so far not been tested on natural communities that have experienced warming over many generations. Natural systems display a higher diversity, heterogeneity and complexity than mesocosms, which could alter predicted effects of warming. Here we used an artificially heated part of the northern Baltic Sea (the Forsmark Biotest basin) to test how warming influences trophic interactions in a shallow coastal food web with four trophic levels: omnivorous fish, invertivorous fish, herbivorous invertebrates, and filamentous macroalgae. Monitoring of fish assemblages over six years showed that small invertivorous fish (gobiids, sticklebacks and minnows) were much less abundant in the heated basin than in unheated references areas. Stomach content analyses of the dominating omnivorous fish – Eurasian perch Perca fluviatilis – revealed a strikingly different diet within and outside the Biotest basin; gammarid crustaceans were the dominating prey at heated sites, whereas invertivorous fish (e.g. gobiids) dominated at unheated sites. A 45‐day cage experiment showed that fish exclusion did not affect the biomass of algal herbivores (gastropods and gammarids), but reduced algal biomass in heated sites (but not unheated). This suggests that warming induced a trophic cascade from fish to algae, and that this effect was mediated by predator‐induced changes in herbivore behavior, rather than number. Overall, our study suggests that warming has effectively compressed the food chain from four to three trophic levels (algae, gammarids and perch), which have benefitted the primary producers by reducing grazing pressure. Consequently, warming appears to have restructured this coastal food web through a combination of direct (physiological) and indirect (species interactions) effects.     

Warming the tundra: reciprocal responses of invertebrate herbivores and plants

Rapid warming in northern ecosystems is simultaneously influencing plants, herbivores and the interactions among them. Recent studies suggest that herbivory could buffer plant responses to environmental change, but this has only been shown for vertebrate herbivores so far. The role of invertebrate herbivory in tundra ecosystems is often overlooked, but can be relevant in determining the structure and dynamics of tundra plant communities and may also affect how plants respond to warming. Invertebrate herbivores are also likely to respond more rapidly to warming than vertebrates because their behaviour and life cycles strongly depend on temperature. We investigated the effects of current season warming on Arctic moth caterpillars, their herbivory rates, and the subsequent responses of two common tundra plants, Salix arctica and Dryas octopetala. We manipulated both herbivore presence and temperature in a full‐factorial field experiment at two elevations, using enclosures and passive warming chambers. Changes in temperature achieved through elevation and/or experimental warming directly affected caterpillars, herbivory and the responses of plants. Caterpillars performed worse (higher respiration rates and lower growth rates) in warmer, lower elevation plots and shifted their diets towards more nutritious foods, such that the relative intensity of herbivory changed for the two studied plants. Within‐season responses of both forage plant species were weak, but invertebrate herbivores affected the responses of plants to elevation or experimental warming. Our results suggest that increased temperatures can reduce the performance of cold‐adapted invertebrate herbivores, with potential consequences to the longer term responses of tundra plants to warming due to changes in herbivory rates and selective foraging.     

Evolutionary responses to environmental change: trophic interactions affect adaptation and persistence

According to recent reviews, the question of how trophic interactions may affect evolutionary responses to climate change remains unanswered. In this modelling study, we explore the evolutionary dynamics of thermal and plant–herbivore interaction traits in a warming environment. We find the herbivore usually reduces adaptation speed and persistence time of the plant by reducing biomass. However, if the plant interaction trait and thermal trait are correlated, herbivores can create different coevolutionary attractors. One attractor has a warmer plant thermal optimum, and the other a colder one compared with the environment. A warmer plant thermal strategy is given a head start under warming, the only case where herbivores can increase plant persistence under warming. Persistence time of the plant under warming is maximal at small or large thermal niche width. This study shows that considering trophic interactions is necessary and feasible for understanding how ecosystems respond to climate change.     

Top predator control of plant biodiversity and productivity in an old-field ecosystem

Abstract Predators can have strong indirect effects on plants by altering the way herbivores impact plants. Yet, many current evaluations of plant species diversity and ecosystem function ignore the effects of predators and focus directly on the plant trophic level. This report presents results of a 3‐year field experiment in a temperate old‐field ecosystem that excluded either predators, or predators and herbivores and evaluated the consequence of those manipulations on plant species diversity (richness and evenness) and plant productivity. Sustained predator and predator and herbivore exclusion resulted in lower plant species evenness and higher plant biomass production than control field plots representing the intact natural ecosystem. Predators had this diversity‐enhancing effect on plants by causing herbivores to suppress the abundance of a competitively dominant plant species that offered herbivores a refuge from predation risk.     

Carbon storage in terrestrial ecosystems: do browsing and grazing herbivores matter?

Large mammalian herbivores manifest a strong top‐down control on ecosystems that can transform entire landscapes, but their impacts have not been reviewed in the context of terrestrial carbon storage. Here, we evaluate the effects of plant biomass consumption by large mammalian herbivores (>10 kg adult biomass), and the responses of ecosystems to these herbivores, on carbon stocks in temperate and tropical regions, and the Arctic. We calculate the difference in carbon stocks resulting from herbivore exclusion using the results of 108 studies from 52 vegetation types. Our estimates suggest that herbivores can reduce terrestrial above‐ and below‐ground carbon stocks across vegetation types but reductions in carbon stocks may approach zero given sufficient periods of time for systems to respond to herbivory (i.e. decades). We estimate that if all large herbivores were removed from the vegetation types sampled in our review, increases in terrestrial carbon stocks would be up to three orders of magnitude less than many of the natural and human‐influenced sources of carbon emissions. However, we lack estimates for the effects of herbivores on below‐ground biomass and soil carbon levels in many regions, including those with high herbivore densities, and upwards revisions of our estimates may be necessary. Our results provide a starting point for a discussion on the magnitude of the effects of herbivory on the global carbon cycle, particularly given that large herbivores are common in many ecosystems. We suggest that herbivore removal might represent an important strategy towards increasing terrestrial carbon stocks at local and regional scales within specific vegetation types, since humans influence populations of most large mammals.     

Fire frequency drives habitat selection by a diverse herbivore guild impacting top–down control of plant communities in an African savanna

In areas with diverse herbivore communities such as African savannas, the frequency of disturbance by fire may alter the top–down role of different herbivore species on plant community dynamics. In a seven year experiment in the Kruger National Park, South Africa, we examined the habitat use of nine common herbivore species across annually burned, triennially burned and unburned areas. We also used two types of exclosures (plus open access controls) to examine the impacts of different herbivores on plant community dynamics across fire disturbance regimes. Full exclosures excluded all herbivores > 0.5 kg (e.g. elephant, zebra, impala) while partial exclosures allowed access only to animals with shoulder heights ≤ 0.85 m (e.g. impala, steenbok). Annual burns attracted a diverse suite of herbivores, and exclusion of larger herbivores (e.g. elephant, zebra, wildebeest) increased plant abundance. When smaller species, mainly impala, were also excluded there were declines in plant diversity, likely mediated by a decline in open space available for colonization of uncommon plant species. Unburned areas attracted the least diverse suite of herbivores, dominated by impala. Here, herbivore exclusion, especially of impala, led to strong declines in plant richness and diversity. With no fire disturbance, herbivore exclusion led to competitive exclusion via increases in plant dominance and light limitation. In contrast, on triennial burns, herbivore exclusion had no effect on plant richness or diversity, potentially due to relatively little open space for colonization across exclosure treatments but also little competitive exclusion due to the intermediate fire disturbance. Further, the diverse suite of grazers and browsers on triennial burns may have had a compensating effect of on the diversity of grasses and forbs. Ultimately, our work shows that differential disturbance regimes can result in differential consumer pressure across a landscape and result in heterogeneous patterns in top–down control of community dynamics.     

Responses of belowground communities to large aboveground herbivores: Meta‐analysis reveals biome‐dependent patterns and critical research gaps

The importance of herbivore–plant and soil biota–plant interactions in terrestrial ecosystems is amply recognized, but the effects of aboveground herbivores on soil biota remain challenging to predict. To find global patterns in belowground responses to vertebrate herbivores, we performed a meta‐analysis of studies that had measured abundance or activity of soil organisms inside and outside field exclosures (areas that excluded herbivores). Responses were often controlled by climate, ecosystem type, and dominant herbivore identity. Soil microfauna and especially root‐feeding nematodes were negatively affected by herbivores in subarctic sites. In arid ecosystems, herbivore presence tended to reduce microbial biomass and nitrogen mineralization. Herbivores decreased soil respiration in subarctic ecosystems and increased it in temperate ecosystems, but had no net effect on microbial biomass or nitrogen mineralization in those ecosystems. Responses of soil fauna, microbial biomass, and nitrogen mineralization shifted from neutral to negative with increasing herbivore body size. Responses of animal decomposers tended to switch from negative to positive with increasing precipitation, but also differed among taxa, for instance Oribatida responded negatively to herbivores, whereas Collembola did not. Our findings imply that losses and gains of aboveground herbivores will interact with climate and land use changes, inducing functional shifts in soil communities. To conceptualize the mechanisms behind our findings and link them with previous theoretical frameworks, we propose two complementary approaches to predict soil biological responses to vertebrate herbivores, one focused on an herbivore body size gradient, and the other on a climate severity gradient. Major research gaps were revealed, with tropical biomes, protists, and soil macrofauna being especially overlooked.     

The direct and indirect effects of fire on the assembly of insect herbivore communities: examples from the Florida scrub habitat

Disturbance is a major source of spatial and temporal heterogeneity. In fire-maintained systems, disturbance by fire is often used as a management tool to increase biological diversity, restore degraded habitats, and reduce pest outbreaks. Much attention has been given to how plant communities recover from fire, but relatively few studies have examined post-fire responses of higher order species, such as insect herbivores. Because dynamic feedbacks occur between plants and their consumers, which can in turn influence the response of the entire ecosystem, incorporating higher trophic level responses into our understanding of the effects of fire is essential. In this study, we used structural equation modeling (SEM) to tease apart the direct and indirect effects of fire on insect herbivore assemblages found on three common oak species in the Florida scrub (Quercus inopina, Q. chapmanii, and Q. geminata). We investigated how fire affected herbivore abundance, richness, and community composition both directly and indirectly through environmental heterogeneity at different spatial scales (e.g., leaf quality, plant architecture, and habitat structure). We also investigated how seasonality and landscape heterogeneity influenced post-fire responses of insect herbivores and whether fire effects on herbivore assemblages varied among different host plants. Our general findings were that fire effects were (1) largely indirect, mediated through habitat structure (although direct fire effects were observed on Q. inopina herbivores), (2) non-linear through time due to self-thinning processes occurring in the scrub habitat, and (3) varied according to herbivore assemblage as a result of differences in the composition of species in each herbivore community. To the best of our knowledge, this is the first comprehensive study to examine how fire influences the assembly of insect herbivore communities through both direct and indirect pathways and at multiple spatial scales.     

Complex carbon cycle responses to multi‐level warming and supplemental summer rain in the high Arctic

The Arctic has experienced rapid warming and, although there are uncertainties, increases in precipitation are projected to accompany future warming. Climate changes are expected to affect magnitudes of gross ecosystem photosynthesis (GEP), ecosystem respiration (ER) and the net ecosystem exchange of CO₂ (NEE). Furthermore, ecosystem responses to climate change are likely to be characterized by nonlinearities, thresholds and interactions among system components and the driving variables. These complex interactions increase the difficulty of predicting responses to climate change and necessitate the use of manipulative experiments. In 2003, we established a long‐term, multi‐level and multi‐factor climate change experiment in a polar semidesert in northwest Greenland. Two levels of heating (30 and 60 W m^?2) were applied and the higher level was combined with supplemental summer rain. We made plot‐level measurements of CO₂ exchange, plant community composition, foliar nitrogen concentrations, leaf δ¹³C and NDVI to examine responses to our treatments at ecosystem‐ and leaf‐levels. We confronted simple models of GEP and ER with our data to test hypotheses regarding key drivers of CO₂ exchange and to estimate growing season CO₂‐C budgets. Low‐level warming increased the magnitude of the ecosystem C sink. Meanwhile, high‐level warming made the ecosystem a source of C to the atmosphere. When high‐level warming was combined with increased summer rain, the ecosystem became a C sink of magnitude similar to that observed under low‐level warming. Competition among our ER models revealed the importance of soil moisture as a driving variable, likely through its effects on microbial activity and nutrient cycling. Measurements of community composition and proxies for leaf‐level physiology suggest GEP responses largely reflect changes in leaf area of Salix arctica, rather than changes in leaf‐level physiology. Our findings indicate that the sign and magnitude of the future High Arctic C budget may depend upon changes in summer rain.     

Introduced ungulate herbivore alters soil processes after fire

Ungulate herbivory can have profound effects on ecosystem processes by altering organic inputs of leaves and roots as well as changing soil physical and chemical properties. These effects may be especially important when the herbivore is an introduced species. Utilizing large mammal exclosures to prevent access by introduced elk at multiple sites along a fire chronosequence, we examined the effects of elk herbivory and fire on soil microbial activity and nutrient availability. Using time since fire as a co-variate and herbivore exclosures, paired with areas outside of the exclosures, we hypothesized that reductions in plant biomass due to herbivory would reduce organic inputs to soils and impact soil microbial activities and nutrient storage. We found three major patterns: (1) when elk were excluded, surface mineral soils had higher soil organic carbon (C), total nitrogen (N), microbial N pools, and increased extra-cellular enzyme activity of a C-acquiring enzyme across a gradient of time since fire. (2) When introduced elk are present, the activity of some extracellular enzymes as well as NO₃ ⁻ availability are enhanced in the soil but the post-fire patterns described above with respect to nutrient accrual over time are delayed. (3) Herbivory by an introduced ungulate upsets the trajectory of ecosystem “recovery” after wildfire and delays soil C and N dynamics by an estimated 14.5–21 years, respectively. These results suggest that introduced, browsing herbivores significantly decelerate ecosystem processes but herbivory by exotics may also result in unpredictability in specific soil responses.