PNNL

Abstract

The intensity and frequency of ecosystem disturbances are shifting in the wake of climate change, and multiple disturbances in close succession have the potential to compound their independent effects and strongly alter ecosystem structure and function. In this work, we examine the effects of an extreme precipitation event on a montane forest landscape that was previously decimated by wildfire (37 months prior) relative to an unburned reference site in the same ecosystem. We assessed responses in ecosystem structure and function in terms of soil edaphic properties (soil chemistry), bacterial community composition and assembly (16S rDNA sequencing and statistical null models), and soil enzyme activities involved in organic C and N acquistion (extracellular enzyme assays). Our research reveals that previously burned landscapes are uniquely succeptible to a subsequent extreme precipitation event via significant increases in soil pH in contrast to unburned soils that are more resistant to flooding. Flooding also increased the abundance of specific microbial clades within burned soils, specifically Beta- and Delta-proteobacteria known to be highly dynamic in early successional ecosystems, and microbial investment in N- vs. C-acquiring extracellular enzymes. Finally, we connected variation in ecological selective pressures on bacterial communities associated with pH change to these differences in microbial mediated soil enzyme activity. Thus, this research demonstrates how multiple, compounding disturbances drive distinct changes in soil edaphic properties, bacterial community structure, and ultimately ecosystem function relative to systems experiencing a single disturbance.