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ABSTRACT: Equations to predict uncompacted crown ratio as a function of compacted crown ratio, tree diameter, and tree height are developed for the main tree species in Oregon, Washington, and California using data from the Forest Health Monitoring Program, USDA Forest Service. The uncompacted crown ratio was modeled with a logistic function and fitted using weighted, nonlinear regression. The models were evaluated using cross-validation. Mean squared error of predicted uncompacted crown ratio was between 0.1 and 0.15, overall bias was negligible, and correlation between the predicted and observed uncompacled crown ratio was high for most species. The sensitivity of crown fire risk to crown ratio estimation method was evaluated using the Fire and Fuels Extension of the Forest Vegetation Simulator. Torching index, an estimate of the wind speed needed for a crown fire to develop, was significantly greater when compacted crown ratio was used instead of uncompacted crown ratio. The close agreement in torching indices simulated using predicted and observed uncompacted crown ratio provides further evidence of the utility of the models developed in this xtudv. West. J. Appl. For. 19(4):260-267.
Key Words: Crown fire risk, crown base height, cross-validation.
The crown ratio of a tree is defined as the portion of the tree height supporting live foliage. Thus, a crown ratio of 1 would indicate that the foliage reaches the ground, and a crown ratio close to 0 would indicate a very small crown. Although crown ratio is frequently estimated in forest inventories, what is actually measured depends on the protocol used and the objective of the inventory. Trees often have asymmetric crowns, so determining the base of the crown tends to be subjective, and crown ratio depends on the criteria used to define the base of the crown (Maguire and Harm 1987).
Many forest inventories, including the Forest Inventory and Analysis (FIA) and Forest Health Monitoring (FHM), conducted by the USDA Forest Service, measure the "compacted crown ratio" (CCR). Crown base height is determined by mentally transferring lower live branches to fill in large holes in the upper portion of the crown, until a full crown is generated. Then, the ratio of the tree height covered hy this compacted crown to its total height is estimated visually and recorded as CCR....