Emily C. Farrer

Sensitivity of grassland plant community composition to spatial vs. temporal variation in precipitation

Climate gradients shape spatial variation in the richness and composition of plant communities. Given future predicted changes in climate means and variability, and likely regional variation in the magnitudes of these changes, it is important to determine how temporal variation in climate influences temporal variation in plant community structure. Here, we evaluated how species richness, turnover, and composition of grassland plant communities responded to interannual variation in precipitation by synthesizing long-term data from grasslands across the United States. We found that mean annual precipitation,(MAP) was a positive predictor of species richness across sites, but a positive temporal relationship between annual precipitation and richness was only evident within two sites with low MAP. We also found higher average rates of species turnover in dry sites that in turn had a high proportion of annual species, although interannual rates of species turnover were surprisingly high across all locations. Annual species were less abundant than perennial species at nearly all sites, and our analysis showed that the probability of a species being lost or gained from one year to the next increased with decreasing species abundance. Bray-Curtis dissimilarity from one year to the next, a measure of species composition change that is influenced mainly by abundant species, was insensitive to precipitation at all sites. These results suggest that the richness and turnover patterns we observed were driven primarily by rare species, which comprise the majority of the local species pools at these grassland sites. These findings are consistent with the idea that short-lived and less abundant species are more sensitive to interannual climate variability than longer-lived and more abundant species. We conclude that, among grassland ecosystems, xeric grasslands are likely to exhibit the greatest responsiveness of community composition (richness and turnover) to predicted future increases in interannual precipitation variability. Over the long-term, species composition may shift to reflect spatial patterns of mean precipitation; however, perennial-dominated systems will be buffered against rising interannual variation, while systems that have a large number of rare, annual species will show the greatest temporal variability in species composition in response to rising interannual variability in precipitation.

Biotic mechanisms of community stability shift along a precipitation gradient

Understanding how biotic mechanisms confer stability in variable environments is a fundamental quest in ecology, and one that is becoming increasingly urgent with global change. Several mechanisms, notably a portfolio effect associated with species richness, compensatory dynamics generated by negative species covariance and selection for stable dominant species populations can increase the stability of the overall community. While the importance of these mechanisms is debated, few studies have contrasted their importance in an environmental context. We analyzed nine long-term data sets of grassland species composition to investigate how two key environmental factors, precipitation amount and variability, may directly influence community stability and how they may indirectly influence stability via biotic mechanisms. We found that the importance of stability mechanisms varied along the environmental gradient: strong negative species covariance occurred in sites characterized by high precipitation variability, whereas portfolio effects increased in sites with high mean annual precipitation. Instead of questioning whether compensatory dynamics are important in nature, our findings suggest that debate should widen to include several stability mechanisms and how these mechanisms vary in importance across environmental gradients.

Nitrogen deposition alters plant–fungal relationships: linking belowground dynamics to aboveground vegetation change

Nitrogen (N) deposition rates are increasing globally due to anthropogenic activities. Plant community responses to N are often attributed to altered competitive interactions between plants, but may also be a result of microbial responses to N, particularly root‐associated fungi (RAF), which are known to affect plant fitness. In response to N, Deschampsia cespitosa, a codominant plant in the alpine tundra at Niwot Ridge (CO), increases in abundance, while Geum rossii, its principal competitor, declines. Importantly, G. rossii declines with N even in the absence of its competitor. We examined whether contrasting host responses to N are associated with altered plant–fungal symbioses, and whether the effects of N are distinct from effects of altered plant competition on RAF, using 454 pyrosequencing. Host RAF communities were distinct (only 9.4% of OTUs overlapped). N increased RAF diversity in G. rossii, but decreased it in D. cespitosa. D. cespitosa RAF communities were more responsive to N than G. rossii RAF communities, perhaps indicating a flexible microbial community aids host adaptation to nutrient enrichment. Effects of removing D. cespitosa were distinct from effects of N on G. rossii RAF, and D. cespitosa presence reversed RAF diversity response to N. The most dominant G. rossii RAF order, Helotiales, was the most affected by N, declining from 83% to 60% of sequences, perhaps indicating a loss of mutualists under N enrichment. These results highlight the potential importance of belowground microbial dynamics in plant responses to N deposition.

Nitrogen deposition, plant carbon allocation, and soil microbes: Changing interactions due to enrichment

PREMISE OF THE STUDY Nitrogen (N) inputs to the terrestrial environment have doubled worldwide during the past century. N negatively impacts plant diversity, but it is unknown why some species are more susceptible than others. While it is often assumed that competition drives species decline, N enrichment also strongly affects soil microbial communities. Can these changes affect plant-microbe interactions in ways that differentially influence success of plant species? Furthermore, can altered plant-microbe interactions lead to carbon (C) limitation in plants? METHODS We focused on a species that increases (Deschampsia cespitosa) and one that decreases (Geum rossii) in abundance in N-fertilized plots in alpine tundra. We measured soil microbes using phospholipid fatty acids, and C limitation and transfer using a (13)C tracer experiment, C:N ratios, nonstructural carbohydrates, and leaf preformation. KEY RESULTS While N profoundly influenced microbial communities, this change occurred similarly in association with both plant species. N addition did not alter total C allocation to microbes in either species, but it changed patterns of microbial C acquisition more in Geum, specifically in gram-negative bacteria. Geum showed evidence of C limitation: it allocated less C to storage organs, had lower C:N and carbohydrate stores, and fewer preformed leaves in N plots. CONCLUSIONS Carbon limitation may explain why some species decline with N enrichment, and the decline may be due to physiological responses of plants to N rather than to altered plant-microbe interactions. Global change will alter many processes important in structuring plant communities; noncompetitive mechanisms of species decline may be more widespread than previously thought.

Shrub Expansion Over the Past 62 Years in Rocky Mountain Alpine Tundra: Possible Causes and Consequences

Abstract Woody plants are encroaching into many herbaceous-dominated communities across the globe, including arctic and alpine tundra. Quantifying the encroachment rate, testing which factors contribute to encroachment, and determining how encroachment is taking place and in which community types encroachment is occurring are essential for predicting shifts in tundra vegetation and carbon (C) storage. We examined willow cover changes from 1946 to 2008 in 18 ha of alpine tundra in Colorado using aerial photographs. We linked this pattern of change with experimental assessment of the effects of increasing summer temperatures, winter precipitation, and nitrogen (N) deposition—factors that this region has experienced over this period—on willow growth and survival. Shrub cover expanded by 441% over 62 years and is increasing at an exponential rate, corresponding to increases in C storage of 137 kg ha-1. Nitrogen and temperature facilitate willow growth and snow increases survival, although N and the combination of N plus snow decrease survival. We find clonal growth (78%) accounts for more expansion than seed dispersal (22%), and that shrubs have expanded into wet, moist, and dry meadow. In addition to a release from grazing, we suggest that global change could be driving shrub expansion.

Vegetation change at high elevation: scale dependence and interactive effects on Niwot Ridge

Background: High-elevation mountain systems may be particularly responsive to climate change. Aims: Here we investigate how changes along elevation gradients in mountain systems can aid in predicting vegetation distributional changes in time, focusing on how changing climatic controls affect meso-scale transitions at the lower and upper boundaries of alpine vegetation (with forest and subnival zones, respectively) as well as micro-scale transitions among plant communities within the alpine belt. We focus on climate-related drivers, particularly in relation to climate change, but also consider how species interactions, dispersal and responses to disturbance may influence plant responses to these abiotic drivers. Results: Empirical observations and experimental studies indicate that changing climatic controls influence both meso-scale transitions at the upper and lower boundaries of alpine vegetation and micro-scale transitions among plant communities within tundra. Micro-scale heterogeneity appears to buffer response in many cases, while interactions between climate and other changes may often accelerate change. Conclusions: Interactions with microtopography and larger edaphic gradients have the capacity to both facilitate rapid changes and reinforce stability, and that these interactions will affect the responsiveness of vegetation to climate change at different spatial scales.

Indirect effects of global change accumulate to alter plant diversity but not ecosystem function in alpine tundra

© 2014 British Ecological Society. Summary: Environmental change can affect species directly by altering their physical environment and indirectly by altering the abundance of interacting species. A key challenge at the interface of community ecology and conservation biology is to predict how direct and indirect effects combine to influence response in a changing environment. In particular, little is known about how direct and indirect effects on biodiversity develop over time or their potential to influence ecosystem function. We studied how nitrogen (N), winter precipitation (snow) and warming influenced diversity and ecosystem function over 6 years in alpine tundra. We used path analyses to partition direct effects of environmental manipulations from indirect effects due to changes in the abundance of two dominant plants. We hypothesize that (i) indirect effects will develop more slowly but will become stronger than direct effects over time and (ii) after 6 years, indirect effects will more strongly influence diversity while direct effects will influence ecosystem function. Indirect effects of N on diversity were consistently stronger than direct effects and actually developed quickly, prior to direct effects. Direct effects of snow on diversity were detected in year 2 but then subsequently were reversed, while indirect effects were detected in year 4 and grew stronger over time. Overall in year 6, indirect effects were much stronger than direct effects on diversity. Direct effects predominated for three of four ecosystem functions we measured (productivity, N mineralization, winter N availability). The only indirect effects we found were that N and snow indirectly affected microbial biomass N by influencing Geum abundance. Across all four ecosystem measures, indirect effects were infrequent and weaker than direct effects. Synthesis. Increasing indirect effects on diversity over time indicate that short-term experiments or monitoring of natural systems may underestimate the full magnitude of global change effects on plant communities. Moreover, explicitly accounting for changes in dominant plant abundance may be necessary for forecasting plant community response to environmental change. Conversely, weak indirect effects for ecosystem processes suggest that predicting ecosystem function without knowledge of plant responses to global change may be possible. Increasing indirect effects on diversity over time indicate that short-term experiments or monitoring of natural systems may underestimate the full magnitude of global change effects on plant communities. Explicitly accounting for changes in dominant plant abundance may be necessary for forecasting plant community response to environmental change. Conversely, weak indirect effects for ecosystem processes suggest that predicting ecosystem function without knowledge of plant responses to global change may be possible.

Teasing apart plant community responses to N enrichment: the roles of resource limitation, competition and soil microbes.

Although ecologists have documented the effects of nitrogen enrichment on productivity, diversity and species composition, we know little about the relative importance of the mechanisms driving these effects. We propose that distinct aspects of environmental change associated with N enrichment (resource limitation, asymmetric competition, and interactions with soil microbes) drive different aspects of plant response. We test this in greenhouse mesocosms, experimentally manipulating each factor across three ecosystems: tallgrass prairie, alpine tundra and desert grassland. We found that resource limitation controlled productivity responses to N enrichment in all systems. Asymmetric competition was responsible for diversity declines in two systems. Plant community composition was impacted by both asymmetric competition and altered soil microbes, with some contributions from resource limitation. Results suggest there may be generality in the mechanisms of plant community change with N enrichment. Understanding these links can help us better predict N response across a wide range of ecosystems.

Plant-microbe interactions at multiple scales across a high-elevation landscape

Here we review the numerous studies of plant–microbe interactions conducted at the Niwot Ridge LTER site in Colorado, USA. By synthesising work at scales ranging from the rhizosphere to the landscape, we offer a mechanistic view of how these interactions are essential to understanding the spatial and temporal structuring of plant and microbial communities across this diverse and changing landscape. These new insights are also important for making predictions about how both plant and microbial communities and populations will respond to future changes in this environment, especially with regard to the potential uphill movement of plants and microbes in response to climate change and nitrogen deposition. We predict that the uphill movement of plants and microbes will be especially apparent, and have the most impact, in areas of the alpine that are now mostly plant free. These areas are currently undergoing a shift from a microbe-dominated ecosystem to one where microbe–plant interactions will play a critical role in reducing nutrient losses to downstream ecosystems.

Assembly of root-associated bacteria communities: interactions between abiotic and biotic factors

Nitrogen (N) deposition in many areas of the world is over an order of magnitude greater than it would be in absence of human activity. We ask how abiotic (N)and biotic (plant host and neighborhood) effects interact to influence root-associated bacterial (RAB)community assembly. Using 454 pyrosequencing, we examined RAB communities from two dominantal pine tundra plants, Geum rossii and Deschampsia cespitosa, under control, N addition and D. cespitosa removal treatments, implemented in a factorial design. We hypothesized that host would have the strongest effect on RAB assembly, followed by N,then neighbor effects.The most dominant phyla were Proteobacteria (mostly Gammaproteobacteria), Actinobacteria,Bacteroidetes and Acidobacteria. We found RAB communities were host specific, with only 17% overlap in operational taxonomic units. Host effects on composition were over twice as strong as Neffects. D. cespitosa RAB diversity declined with N, while G. rossii RAB did not. D. cespitosa removal did not influence G. rossii RAB community composition, but G. rossii RAB diversity declined with N only when D. cespitosa was absent. We conclude that RAB of both hosts are sensitive to N enrichment, and RAB response to N is influenced by host identity and plant neighborhood.