Coarse woody decay rates vary by physical position in tropical seasonal rainforests of SW China

Decomposition of woody detritus is an important but often ignored process in forest ecosystems. Moisture and temperature regimes are dominant controls over woody decay, contributing to significant variability at local, regional, and global scales. Our focus was on local variability in woody decay rates depending on their physical position. Woody detritus may decay on the forest floor, aboveground, or combination of both, depending on the mortality agent. In this study, we measured decay rates of logs, large branches on the forest floor, and snags over a three-year period. We also collected monthly respiration estimates, and analyzed woody detritus N and P content throughout the study. Logs exhibited the greatest mass loss with a decay-rate constant of k = 0.606 ± 0.020, followed by large branches (k = 0.316 ± 0.012) and snags (k = 0.268 ± 0.008). Heterotrophic respiration was greatest prior to the peak of rainy season, and was greatest for snag material during the first two years of sampling, probably a result of water saturation in ground material. Both N and P were released in all materials, but the rate of P release was much slower in snags. There were large differences of P concentration and C:P among the materials, but their value became similar after three years, indicating P limitation on microbial activities. Our results presented robust evidence for the physical-position-dependence of coarse woody detritus decomposition in the forests.