Influence of crown biomass estimators and distribution on canopy fuel characteristics in ponderosa pine stands of the Black Hills

Abstract

Two determinants of crown fire hazard are canopy bulk density (CBD) and canopy base height (CBH). The Fire and Fuels Extension to the Forest Vegetation Simulator (FFE-FVS) is a model that predicts CBD and CBH. Currently, FFE-FVS accounts for neither geographic variation in tree allometries nor the nonuniform distribution of crown mass when one is estimating CBH and CBD. We develop allometric equations specific to ponderosa pine (Pinus ponderosa Dougl. ex Laws.) in the Black Hills to predict crown mass and use the Weibull distribution to model the vertical distribution of crown mass within individual trees. We present parameter prediction models that, in turn, predict the vertical distribution of crown mass based on stand- and tree-level attributes. With use of an FFE-FVS executable incorporating local crown mass equations and the parameter prediction models, new estimates of CBD and CBH were produced. Locally derived biomass equations predicted substantially greater estimates of foliage mass than currently predicted by FFE-FVS. The increase in CBD resulting from the local biomass and vertical distribution models averaged 78% over original estimates. Our results suggest that locally derived crown mass equations in addition to nonuniform estimates of crown mass distribution be used to calculate CBH and CBD as used in fire prediction models.