Combined effects of drought and forest thinning on soil microbial community composition and functioning

Aim: Summer droughts are increasing in frequency and severity in Europe with detrimental effects on forests. Reducing the number of trees per area through thinning has been found to improve tree performance during and after drought, but little is known about what happens to the soil. Here, we studied the effects of intense summer drought on soil microbial communities and their functioning, and whether thinning can alter their responses to drought. Methods: We simulated a summer drought using rain-out shelters within a replicated block design of different thinning intensities in Dutch Pinus sylvestris stands. Soil samples were collected before, during and after drought to track changes in fungal community composition, total and ectomycorrhizal fungal biomass, and extracellular enzymatic activities. Throughout the growing season, soil respiration was regularly measured and litter bags were sequentially harvested to monitor decomposition rates. Results: Generally, effects of thinning were larger than those of drought, and drought responses did not differ between harvest intensities. Fungal biomass was not affected by drought, but was lower for heavy thinning and clearcutting compared to unharvested control and light thinning. Respiration and decomposition rates were slower for heavy thinning and clearcutting, and drought also lowered process rates overall. Moreover, soil respiration was still affected by drought after a few months of recovery, as was decomposition of litter incubated over the entire growing season. We will further explore how the fungal community composition responded to drought, honing in on differences between ectomycorrhizal and saprotrophic fungi. Conclusions: We found no evidence that light thinning mitigates the short-term impact of summer drought on soil microbial communities in Pinus sylvestris forests.