Precipitation drives interannual variation in summer soil respiration in a Mediterranean-climate, mixed-conifer forest

Predictions of future climate change rely on models of how both environmental conditions and disturbance impact carbon cycling at various temporal and spatial scales. Few multi-year studies, however, have examined how carbon efflux is affected by the interaction of disturbance and interannual climate variation. We measured daytime soil respiration (R (s)) over five summers (June-September) in a Sierra Nevada mixed-conifer forest on undisturbed plots and plots manipulated with thinning, burning and their combination. We compared mean summer R (s) by year with seasonal precipitation. On undisturbed plots we found that winter precipitation (PPTw) explained between 77-96% of interannual variability in summer R (s). In contrast, spring and summer precipitation had no significant effect on summer R (s). PPTw is an important influence on summer R (s) in the Sierra Nevada because over 80% of annual precipitation falls as snow between October and April, thus greatly influencing the soil water conditions during the following growing season. Thinning and burning disrupted the relationship between PPTw and R-s, possibly because of significant increases in soil moisture and temperature as tree density and canopy cover decreased. Our findings suggest that R (s) in some moisture-limited ecosystems may be significantly influenced by annual snowpack and that management practices which reduce tree densities and soil moisture stress may offset, at least temporarily, the effect of predicted decreases in Sierran snowpack on R (s).