Assessing the contribution of dead wood to ecosystem respiration in the UMBS forest: field observations.

Abstract

As CO2 continues to accumulate in the atmosphere, it has become necessary to better understand the role of terrestrial sinks in storing C. Succession is one factor that can influence a forest's ability to act as a C sink or source. As a forest ages and approaches a successional transition, it will likely increase its pool of dead wood, or coarse woody debris (CWD), and the efflux from this component can have a huge influence on overall forest C storage. Few studies have quantified this contribution to total ecosystem respiration, however. To better understand this contribution, we modeled the flux rate of CWD at the University of Michigan Biological Station in northern Lower Michigan, USA. The field study in this collaborative project aimed to assess bigtooth aspen (Populus grandidentata) CWD respiration response under ambient temperature and moisture conditions. The field study mirrored a laboratory component that measured respiration under a range of temperature and moisture manipulations, and the field study served as a means of validating the laboratory results. In the field, we also modeled the relationship between CWD moisture and soil moisture. We generated predictive models of CWD respiration from moisture and temperature and then scaled to the ecosystem level using the model of soil moisture against CWD moisture along with past annual soil moisture and temperature data. We predicted that respiration would be strongly correlated to moisture and temperature and that the models would reflect a potential increase in CWD efflux for the UMBS forests. In both the laboratory and field results, respiration rates appeared to increase with temperature, especially in highly decayed samples, though there was no statistically significant relationship. However, respiration rates also increased with moisture, and statistical significance was found in both the laboratory and field data. Scaling to the ecosystem, we were able to illustrate the flux of CWD over the course of a year, and we observed seasonal responses in CWD respiration and a strong respiration response to temperature. We estimated the yearly flux of CWD in the UMBS forests to be 0.2 Mg C ha-1 yr-1. With approaching succession and the subsequent increase in CWD mass, this flux can potentially play a significant role in upsetting the balance between C sequestration and respiration, possibly turning the region's forests into a C source, a change from the C sink thi it is presently. The results from this study supplement previous, current, and future studies on the C flux of the UMBS area (gathered by the UMBS Forest Carbon Cycle Research Program) and, more generally, the C flux of temperate forests.