Estimating canopy fuels and their impact on potential fire behavior for ponderosa pine in the Black Hills, South Dakota.

We evaluate whether current procedures used in fire behavior prediction models such as FVS-FFE provide predictions of CBD and CBH suitable for evaluating fire behavior in response to fuel treatments. Currently, FFE-FVS uses a geographic non-specific set of tree allometries and assumes a uniform distribution of crown mass when estimating CBH and CBD. We develop allometric equations to predict crown mass specific to ponderosa pine in the Black Hills (Pinus ponderosa Dougl. ex Laws.) from a sample of 80 felled trees in 16 forest stands spanning a wide range in tree size and stand. We develop a non-uniform description of vertical crown mass within individual trees using the Weibul distribution. We relate the parameters of the distribution to stand structure, so that a vertical canopy biomass profile can be estimated for any stand from standard inventory information. We modify the existing FVS-FFE program to include our results. Estimates of CBD increased by an average of 78% when using our local biomass and non-uniform vertical distribution models compared to current procedures in FVS-FFE. On average, 47% of the underprediction of the current procedure compared to our new models resulted from site specific allometries and additional 31% of the under prediction resulted from a non-uniform distribution of crown mass. Our results suggest locally-derived crown mass equations in addition to non-uniform estimates of crown mass distribution should be used to calculate CBH and CBD as used in fire prediction models. Current management efforts to create stand structures more resistant to the initiation and spread of crown fire include increasing CBH and reducing CBD below the threshold where crown fire can be initiated and carried through the tree canopy. Of the 16 stands sampled in this study, only two had CBD estimates >0.100 kg m-3 (i.e. the CBD where active crown fire would be expected) as currently implemented in FFE-FVS. When local crown mass equations and distribution models were applied to the data, 12 out of the 16 stands had CBD estimates >0.100 kg m-3 threshold. Consequently, FFE-FVS, as presently formulated, would misdiagnose fire hazard in a substantial number of Black Hills ponderosa pine stands. Further, where FFE-FVS is used to design and evaluate fuels treatments, it is probable that either the amount of density reduction necessary to achieve a desired effect will be underestimated, or the longevity of effectiveness of a given treatment will be overestimated.