Coarse woody debris is affected by the frequency and intensity of historical harvesting and fire in an open eucalypt forest

Abstract

ABSTRACT We examined the effects of timber harvesting and fire history on coarse woody debris (CWD) at 48 sites dispersed across the dry sclerophyll jarrah (Eucalyptus marginata) forest of south-western Western Australia. These sites represent a range of fire and harvesting histories. The mean total volume of CWD (119 ± 10 m3 ha-1), which varied greatly across the sites, was greatest on the recent harvest category (140 m3 ha−1) and least on the never harvested category (77 m3 ha−1). The ‘old harvest’ category harvested more than 40 years prior had an intermediate value (89 m3 ha−1). The increased volume of CWD on the recent harvest category consisted largely of small diameter (10–50 cm) and less decayed CWD which had been on the ground for approximately 10–60 years. Although the total volume of large CWD (diameter >50 cm) was similar across the recent, old and never harvested categories, harvesting skewed the distribution of large CWD toward the less decayed classes. Multiple linear regression was used to examine the effects of harvest intensity (amount of basal area removed), harvest history, fire history and site attributes on CWD volume. The strongest single predictors of CWD volume were the total reduction in basal area from recent and historical harvesting, and the number of prescribed fires since 1937; both positively correlated with CWD volume (r = 0.47 for both). The next strongest single predictor of the CWD volume was the number of wildfires (negatively correlated, r = −0.40). Unlike prescribed fires, wildfires reduced the accumulated volume of CWD. Over time, timber harvesting and prescribed burning increased CWD loads and reduced the volume of large diameter highly decayed CWD. Increased CWD loads resulting from past harvesting and prescribed burns are a substantial carbon store that may also benefit CWD dependent species. However, forest managers need to balance the potential benefits of maintaining large volumes of CWD against the risk that fires burning under dry summer conditions will consume a high proportion of CWD resulting in severe heating of soil and vegetation, and substantial emissions of carbon to the atmosphere.