Community recovery from disturbance and nutrient deposition

Melissa H. DeSiervo, Botany, University of Wyoming, Lauren L. Sullivan, Division of Biological Sciences, University of Missouri, Columbia, MO, Eric W. Seabloom, Ecology, Evolution, and Behavior, University of Minnesota, Saint Paul, MN and Lauren Shoemaker, Department of Botany, University of Wyoming, Laramie, WY

Background/Question/Methods
Both theory and empirical evidence suggest that increasing nutrient supply rate will decrease species diversity, for example causing local extinction of nitrogen fixing species. This suggests that local communities with high resource supply rates will show convergence in their community composition. However long-term data to address this question is limited, and furthermore, the combined effect of resource addition and environmental disturbance may disrupt this prediction. For example, processes such as the order and timing of species arrival after disturbance, and the history of the landscape may cause communities to diverge in composition, even if they are subject to the same abiotic conditions. We examined the long-term community composition of experimental grasslands at the Cedar Creek ecosystem reserve, comparing trajectories of community assembly in three old fields with nutrient additions crossed with disturbance. Half of each field was plowed in 1982 and the entire field was subjected to a gradient of nutrient supply rates for 4 decades. We ask, do communities with higher resource supply rates show more convergence in community assembly over successional time? Does disturbance alter the likelihood of convergence? And, finally, does land-use history have an impact on the degree to which communities diverge or converge?
Results/Conclusions
In accordance with our predictions, communities in undisturbed plots became increasingly similar over 30+ years, with more convergence in fertilized plots. However, with both soil disturbance and nutrient addition, community trajectories became context dependent, with some communities becoming increasingly dissimilar from one another in some fields but increasingly similar in other locations. Nonetheless, some species, like the C3 grass Agropyron repens were highly predictable, becoming more dominant in communities with the highest resource supply rates across all fields. The context dependent effects of disturbance on community assembly suggests the need for globally replicated tests of community assemble theory in the face of global change; the central question being addressed in a new global experiment, Disturbance Response Across Grasslands Network (DRAGNet). The DRAGNet experiment will also provide insight on the spatial and temporal dynamics of seed dispersal, improving our mechanistic understanding of how processes such as dispersal and colonization influence community recovery across a wide array of abiotic conditions.