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A key challenge to understanding the effects of climate change and nutrient deposition on ecosystem functioning is our lack of knowledge about nutrient limitations of heterotrophic and phototrophic microbial communities. This is especially true in high elevation ecosystems where it has been shown that earlier melt-out of snow beds and glacial retreat is allowing photosynthetic microbes and plants to move into previously unvegetated areas. We used landscape-level analyses of microbial enzyme stoichiometries combined with soil microcosm fertilization studies to determine which nutrients are limiting to microbes in plant-free or sparsely vegetated, snow bed areas of the Colorado Front Range. Both of these independent approaches indicated that the ultimate limiting nutrient in unvegetated and sparsely vegetated soils is phosphorus (P) for phototrophic microbes, with co-limitation by carbon (C) for the entire microbial community. In contrast, vegetated soils in the same watersheds showed more balanced nitrogen (N), P and C co-limitation similar to patterns seen in other plant-dominated ecosystems. In microcosm experiments, P additions resulted in increased growth rates and percent cover by phototrophs, whereas N additions decreased the relative abundances of phototrophs. Taken together, our findings indicate that the colonization of high elevation ecosystems being impacted by N deposition and climate warming will likely be constrained by P limitation of both heterotrophic and phototrophic microbes and by negative impacts of N on microbial phototrophs. These effects may in turn limit the ability of these fragile ecosystems to immobilize inputs of atmospheric N causing increased runoff of excess N to downstream ecosystems. [ABSTRACT FROM AUTHOR]

Climate warming is a key factor driving species range shifts. While previous work has focused on shifts of aboveground plant communities, changes in climate and vegetation should affect soil communities and hence ecosystem-level nutrient cycling and ecosystem functioning. High alpine ecosystems are particularly sensitive to climate warming because snow is among the main drivers of ecosystem structure and function. Climate-warming snow cover changes at Niwot Ridge in the Colorado Rocky Mountains have resulted in a consistent plant colonization of previously unvegetated soils generating a natural gradient of soil habitats ranging from unvegetated to increasingly vegetated. We used this gradient of plant communities at different successional stages to determine if nematodes respond to climate-driven changes in this high-alpine landscape and if they play a role in changes in soil C and N. We hypothesized that: 1) there would be clear shifts in nematode communities along the gradient as a function of snow cover, plant richness and density, and water holding capacity but that these shifts would be dependent on nematode feeding habits and their positioning in the soil foodweb and 2) the shifts would be associated with accumulating soil C and N. To test these hypotheses, we measured nematodes, plants, and soil microbes, snow cover, pH, soil water holding capacity, and different forms of soil C and N in 98 plots across the plant successional gradient. As predicted, nematode communities exhibited extensive shifts from a few individuals of a single species in unvegetated soils to hundreds of individuals and tens of species within every feeding group under complex plant communities. Representatives of omnivorous and bacterivorous K-strategists preceded plants and plant parasites and root associates depended on plants most. Linear regression models indicated that plants, microbial communities and soil water holding capacity, but not snow cover, were the most predictive factors of nematode diversity and density across all trophic levels and that all nematode groups were positively related to all measures of soil C and N. Structural equation models confirmed these results, but also indicated that effects of climate warming on nematodes were indirect primarily through shifts in plant and microbial communities and changes of soil water holding capacity. Moreover, nematode trophic group densities, but not their diversity, played a potential role in the accumulation of soil N, and to a lesser degree of soil C. Because nematode communities at Niwot Ridge are largely at their early phases of assembly, with continuing climate warming, we predict their increasing abundance and diversity will likely continue, as will their impact on soil C and N processes. • Climate warming is a key factor driving species range shifts. • High alpine ecosystems are particularly sensitive to climate warming. • Soil nematodes expand their range of distribution akin to plants. • As nematodes become a part of the high alpine they contribute to C and N sequestration. [ABSTRACT FROM AUTHOR]

Two sources of complexity make predicting plant community response to global change particularly challenging. First, realistic global change scenarios involve multiple drivers of environmental change that can interact with one another to produce non-additive effects. Second, in addition to these direct effects, global change drivers can indirectly affect plants by modifying species interactions. In order to tackle both of these challenges, we propose a novel population modeling approach, requiring only measurements of abundance and climate over time. To demonstrate the applicability of this approach, we model population dynamics of eight abundant plant species in a multifactorial global change experiment in alpine tundra where we manipulated nitrogen, precipitation, and temperature over 7 years. We test whether indirect and interactive effects are important to population dynamics and whether explicitly incorporating species interactions can change predictions when models are forecast under future climate change scenarios. For three of the eight species, population dynamics were best explained by direct effect models, for one species neither direct nor indirect effects were important, and for the other four species indirect effects mattered. Overall, global change had negative effects on species population growth, although species responded to different global change drivers, and single-factor effects were slightly more common than interactive direct effects. When the fitted population dynamic models were extrapolated under changing climatic conditions to the end of the century, forecasts of community dynamics and diversity loss were largely similar using direct effect models that do not explicitly incorporate species interactions or best-fit models; however, inclusion of species interactions was important in refining the predictions for two of the species. The modeling approach proposed here is a powerful way of analyzing readily available datasets which should be added to our toolbox to tease apart complex drivers of global change., (© 2013 John Wiley & Sons Ltd.)