Do biological legacies moderate the effects of forest harvesting on soil microbial community composition and soil respiration

Ecological forestry is a management approach that uses natural disturbance processes as models for designing silvicultural prescriptions that restore or sustain ecosystem biodiversity and function in actively managed forests. We evaluated how a novel ecologically-based multi-cohort silvicultural treatment affects the soil microbial community (SMC) and tested whether supplemental dead wood in the form of girdled trees alters these effects. We also tested SMC function by measuring soil CO2 flux over multiple growing seasons, and examined if these patterns were related to soil microbial groups. Our experimental harvests were conducted in second-growth northern hardwood forests in northern Wisconsin, USA. Treatments included a modified shelterwood harvest (SH), a shelterwood harvest plus dead wood supplementation (SH + CWD), and an unharvested control; here we report responses three to five years post-treatment. The SMC composition (determined using PLFA) in both harvests was significantly different from the control, a difference driven by greater bacterial abundance in the harvested areas, and particularly by gram negative bacteria in SH. Microbial community composition was not significantly different between the two harvests (SH and SH + CWD). Total soil respiration was significantly lower in SH than in the control and SH + CWD treatments, a difference most likely driven by a reduction of the autotrophic respiration component in SH treatments due to harvesting, while in the SH + CWD treatment roots from living girdled trees contributed to autotrophic soil respiration. The relationship between the SMC and soil respiration varied with treatment and season. In general, soil respiration in the unharvested controls was most significantly correlated with microbes that relate to autotrophic respiration sources, while respiration in SH + CWD was most significantly correlated with heterotrophic microbes. These results indicate that, although the SMC composition was affected by forest harvesting practices incorporating live and dead biological legacies, supplementing the number of standing dead trees through girdling and felling maintained SMC function, as measured through total soil respiration, an indicator of some important aspects of ecosystem function.