Decomposition of Douglas-fir coarse woody debris in response to differing moisture content and initial heterotrophic colonization

Abstract

This experiment was designed to test the effects of moisture content and hetrotrophic community composition on the decomposition of coarse woody debris (CWD). In 1995, 360 freshly cut Douglas-fir logs were randomly assigned to 5 replicate sites, with 12 heterotrophic colonization × moisture treatment combinations at each site. Half of the logs in each heterotrophic treatment were placed under elevated clear plastic shelters to reduce precipitation inputs while the other half received unrestricted inputs. The heterotrophic colonization treatments (HCT) were designed to test the effect on decomposition of the initial timing of physical penetration of the bark by drilling holes into the phloem and sapwood, and insect vectoring of fungi by injecting basidiomycete and ascomycete inoculum into the sapwood. In summer of 2001, respiration was measured from one set of experimental logs and density change, the amount of area affected by brown rot and wood borer galleries (br/wbg) from another set of logs. The sheltered logs had significantly higher respiration rates than the unsheltered logs (0.70 g CO 2 m −2 d −1 versus 0.57 g CO 2 m −2 d −1 respectively). Sheltered logs also had significantly higher percentage of the log's total volume affected by br/wbg (6.3%) than in the non-sheltered logs (1.1%). Mean moisture content in the sheltered logs was 36%, significantly less than in the unsheltered logs (45%). There was also evidence that earlier insect entry into CWD can facilitate faster decomposition as indicated by higher respiration rates in the logs that had been experimentally penetrated. In sum, the results suggest that even small decreases in moisture content can significantly increase respiration rates through increased fungal and insect activity in the early stages of decomposition. However, after 6 years the differences in hetrotrophic activity did not result in significantly higher density loss in the drier logs.