Estimating canopy bulk density and canopy base height for interior western US conifer stands

Abstract

Crown fire hazard is often quantified using effective canopy bulk density (CBD) and canopy base height (CBH). When CBD and CBH are estimated using nonlocal crown fuel biomass allometries and uniform crown fuel distribution assumptions, as is common practice, values may differ from estimates made using local allometries and nonuniform distributions. We estimated CBD and CBH for mostly pure, even-aged stands of seven conifer species by modifying the Fire and Fuels Extension to the Forest Vegetation Simulator to use nonuniform crown fuel distributions, which allowed us to determine whether distribution effects on CBD and CBH estimates were species specific or general. For two species, we also compared estimates from local and nonlocal allometries to ascertain whether there was a consistent bias in CBD and CBH estimates associated with application of allometries outside their geographic area of origin. Using nonuniform distributions caused consistent increases in average CBD estimates of ~10 –30% for all species compared to estimates obtained using uniform distributions. Effects on CBH varied. Using local allometries did not result in CBD or CBH estimates that were consistently larger or smaller than those obtained using nonlocal allometries. We conclude that using nonuniform distributions invariably increases estimates of CBD for even-aged conifer stands, whereas effects on CBH estimates are difficult to predict and that allometric relationships vary widely among stands in the southern Rocky Mountains.