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Alpine tundra ecosystems are highly vulnerable to climate warming but are governed by local‐scale abiotic heterogeneity, which makes it difficult to predict tundra responses to environmental change. Although land models are typically implemented at global scales, they can be applied at local scales to address process‐based ecological questions. In this study, we ran ecosystem‐scale Community Land Model (CLM) simulations with a novel hillslope hydrology configuration to represent topographically heterogeneous alpine tundra vegetation across a moisture gradient at Niwot Ridge, Colorado, USA. We used local observations to evaluate our simulations and investigated the role of topography and aspect in mediating patterns of snow, productivity, soil moisture, and soil temperature, as well as the potential exposure to climate change across an alpine tundra hillslope. Overall, our simulations captured observed gradients in abiotic conditions and productivity among heterogeneous, hydrologically connected vegetation communities (moist, wet, and dry). We found that south facing aspects were characterized by reduced snowpack and drier and warmer soils in all communities. When we extended our simulations to the year 2100, we found that earlier snowmelt altered the timing of runoff, with cascading effects on soil moisture, productivity, and growing season length. However, these effects were not distributed equally across the tundra, highlighting potential vulnerabilities of alpine vegetation in dry, wind‐scoured, and south facing areas. Overall, our results demonstrate how land model outputs can be applied to advance process‐based understanding of climate change impacts on ecosystem function.

Fine‐scale microclimate variation due to complex topography can shape both current vegetation distributional patterns and how vegetation responds to changing climate. Topographic heterogeneity in mountains is hypothesized to mediate responses to regional climate change at the scale of metres. For alpine vegetation especially, the interplay between changing temperatures and topographically mediated variation in snow accumulation will determine the overall impact of climate change on vegetation dynamics.

Hybridization between taxa generates new pools of genetic variation that can lead to different environmental responses and demographic trajectories over time than seen in parental lineages. The potential for hybrids to have novel environmental tolerances may be increasingly important in mountainous regions, which are rapidly warming and drying due to climate change. Demographic analysis makes it possible to quantify within‐ and among‐species responses to variation in climate and to predict population growth rates as those conditions change. We estimated vital rates and population growth in 13 natural populations of two cinquefoil taxa (Potentilla hippianaandP. pulcherrima) and their hybrid across elevation gradients in the Southern Rockies. Using three consecutive years of environmental and demographic data, we compared the demographic responses of hybrid and parental taxa to environmental variation across space and time. All three taxa had lower predicted population growth rates under warm, dry conditions. However, the magnitude of these responses varied among taxa and populations. Hybrids had consistently lower predicted population growth rates thanP. hippiana. In contrast, hybrid performance relative toP. pulcherrimavaried with population and climate, with the hybrid maintaining relatively stable growth rates while populations ofP. pulcherrimashrank under warm, dry conditions. Our findings demonstrate that hybrids in this system are neither intrinsically unfit nor universally more vigorous than parents, suggesting that the demographic consequences of hybridization are context‐dependent. Our results also imply that shifts to warmer and drier conditions could have particularly negative repercussions forP. pulcherrima, which is currently the most abundant taxon in the study area, possibly as a legacy of more favorable historical climates. More broadly, the distributions of these long‐lived taxa are lagging behind their demographic trajectories, such that the currently less commonP. hippianacould become the most abundant of thePotentillataxa as this region continues to warm and dry.

Bacterial and fungal root endophytes can impact the fitness of their host plants, but the relative importance of drivers for root endophyte communities is not well known. Host plant species, the composition and density of the surrounding plants, space, and abiotic drivers could significantly affect bacterial and fungal root endophyte communities. We investigated their influence in endophyte communities of alpine plants across a harsh high mountain landscape using high-throughput sequencing. There was less compositional overlap between fungal than bacterial root endophyte communities, with four ‘cosmopolitan’ bacterial OTUs found in every root sampled, but no fungal OTUs found across all samples. We found that host plant species, which included nine species from three families, explained the greatest variation in root endophyte composition for both bacterial and fungal communities. We detected similar levels of variation explained by plant neighborhood, space, and abiotic drivers on both communities, but the plant neighborhood explained less variation in fungal endophytes than expected. Overall, these findings suggest a more cosmopolitan distribution of bacterial OTUs compared to fungal OTUs, a structuring role of the plant host species for both communities, and largely similar effects of the plant neighborhood, abiotic drivers, and space on both communities.

Climate change is increasing temperature, decreasing precipitation, and increasing atmospheric CO₂concentrations in many ecosystems. As atmospheric carbon rises, plants may increase carbon‐based defenses, such as phenolics, thereby potentially affecting food quality, foraging habits, and habitat suitability for mammalian herbivores. In alpine habitats, the American pika (Ochotona princeps) is a model species for studying effects of changing plant chemistry on mammals. To survive between growing seasons, pikas cache “haypiles” of plants rich in phenolics. Although they are toxic to pikas, phenolic compounds facilitate retention of plant biomass and nutrition during storage, and they degrade over time. Alpine avens (Geum rossii, Rosales: Rosaceae) is a high‐phenolic plant species that comprises up to 75% of pika haypiles in Colorado. Here, we tested the hypothesis that contemporary climate change has affected the nutritional value of alpine avens to pikas in the last 30 years. Specifically, we compared phenolic activity, nutritional quality, and overwinter preservation of plants collected at Niwot Ridge, Colorado (USA), in 1992 to those collected between 2010 and 2018, spanning nearly three decades of climate change. Phenolic activity increased in alpine avens since 1992, while fiber and nitrogen content decreased. Importantly, overwinter preservation of plant biomass also increased, particularly on windblown slopes without long‐lasting snow cover. Previous studies indicate that pikas at this site still depend on alpine avens for their winter food caches. Higher phenolic content in alpine avens could therefore enhance the preservation of haypiles over winter; however, if pikas must further delay consuming these plants to avoid toxicity or invest extra energy in detoxification, then the nutritional gains from enhanced preservation may not be beneficial. This study provides important insights into how climate‐driven changes in plant chemistry will affect mammalian herbivores in the future.

Global change is altering patterns of community assembly, with net outcomes dependent on species' responses to the abiotic environment, both directly and mediated through biotic interactions. Here, we assess alpine plant community responses in a 15‐year factorial nitrogen addition, warming and snow manipulation experiment. We used a dynamic competition model to estimate the density‐dependent and ‐independent processes underlying changes in species‐group abundances over time. Density‐dependent shifts in competitive interactions drove long‐term changes in abundance of species‐groups under global change while counteracting environmental drivers limited the growth response of the dominant species through density‐independent mechanisms. Furthermore, competitive interactions shifted with the environment, primarily with nitrogen and drove non‐linear abundance responses across environmental gradients. Our results highlight that global change can either reshuffle species hierarchies or further favour already‐dominant species; predicting which outcome will occur requires incorporating both density‐dependent and ‐independent mechanisms and how they interact across multiple global change factors.

Global change alters ecosystems and their functioning, and biotic interactions can either buffer or amplify such changes. We utilized a long‐term nitrogen (N) addition and species removal experiment in the Front Range of Colorado, USA to determine whether a codominant forb and a codominant grass, with different effects on nutrient cycling and plant community structure, would buffer or amplify the effects of simulated N deposition on soil bacterial and fungal communities. While the plant community was strongly shaped by both the presence of dominant species and N addition, we did not find a mediating effect of the plant community on soil microbial response to N. In contrast to our hypothesis, we found a decoupling of the plant and microbial communities such that the soil microbial community shifted under N independently of directional shifts in the plant community. These findings suggest there are not strong cascading effects of N deposition across the plant–soil interface in our system.