UC Irvine

The overall aim of my dissertation was to determine how microbial decomposers may respond to future climate change in the American Southwest. In my first chapter, I investigated how attenuation of ambient ultraviolet radiation (UV) affected microbial litter decomposition during a one-year field study. Using flow cytometry to quantify bacteria, microscopy to quantify fungi, and assays to quantify potential extracellular enzyme activity, I determined that attenuation of ambient UV reduced decomposition rates and the effectiveness of extracellular enzymes produced by microbial decomposers in plant litter.

In my second chapter, I observed how microbial decomposer communities varied with climate and litter chemistry across a regional climate gradient that extends from cold, wet conditions to hot, dry conditions. Changes along this gradient emulate the potential impacts of future climate change in the American Southwest. I determined that differences in microbial activity were not driven directly by precipitation, but rather by differences in protein degradation rates, enzyme turnover, and the availability of carbon substrates.

In my third chapter, I transplanted grassland microbial communities into five sites along the same regional climate gradient to simulate climate change effects on microbial properties and decomposition rates in the field. The transplant design allowed me to determine if grassland microbial communities were constrained in their ability to respond to climate forcing. I found that temperature and precipitation interact to limit decomposition rates in the coldest and driest sites. I also found that grassland microbial communities were not constrained in their ability to degrade litter in any site along the gradient.

Together, the results of my dissertation research indicate that transitions from forests to both grasslands and scrublands in southern California marks a significant shift in litter chemistry and enzyme dynamics, while the transitions from grasslands to drier scrublands and deserts and from cold subalpine sites to milder montane forests may be the most significant for rates of C-cycling. In a more arid future, shifts in climate and resulting shifts in plant communities in these transition zones are therefore likely have the most significant effects on future decomposer activity and C-cycling rates.