Effects of Fire and Drought on Ecological Processes Via Plant-Soil Interactions

Understanding how biotic organisms are affected by abiotic conditions and, in turn, affect the functioning of their environment is one of the most unifying goals of ecological research. Particularly in the context of a changing environment, a generalizable understanding of the factors which underpin plant functioning has crucial implications for predicting how and why ecosystems may function differently under future climate scenarios. In my dissertation, I assess the implications of two fundamental abiotic drivers - soil resource supply and disturbance regime - on community and ecosystem dynamics. I use a combination of greenhouse and field experiments to iteratively examine how responses to these drivers at the levels of individual and species scale up to influence competitive interactions, biomass regeneration, and productivity.

In chapter two, I quantify a short-lived increase in nitrogen availability following prescribed fire. In chapter three, I then test the extent to which a similar nitrogen pulse is utilized by co-occurring plant species, and relate their nitrogen uptake to regrowth when planted with a stronger or weaker competitor. This chapter demonstrates that species differ significantly in their ability to capture a pulse of nitrogen but that this has no effect on their competitive ability during resprouting. Instead, both functions are correlated in opposite directions with the same root trait, suggesting that they may exemplify opposing life history strategies. In chapter four I examine the role of nitrogen availability on biomass regrowth in complex field communities to assess how nitrogen supply and disturbance history affect community responses to subsequent disturbances. I found that historical disturbance frequency had a much stronger impact on disturbance response than nitrogen availability, although nitrogen availability mediated the disturbance response of some species. Together these findings suggest that root traits can be used to predict competitive strategies for plants in this frequently-disturbed and pulse-driven ecosystem. If competitive strategies are optimized either following nitrogen pulses or disturbance, species abundances may fluctuate over time and under future conditions depending on the frequency of each abiotic driver. Finally, in chapter five I extend these questions to water-limited conditions: in mesocosms planted with tree saplings that form associations with either arbuscular or ectomycorrhizal fungi, I compare the resistance and recovery of belowground respiration to drought and rewetting. I find that mycorrhizal association affects respiration resistance to drought, but both mycorrhizal types fail to recover following rewetting. The association of trees with mycorrhizal fungi may, then, have implications for the drought tolerance of host tree species.