Forest Fuels and Wildfire Hazard in Two Fire-Excluded Old-Growth Ponderosa Pine Stands: Contrasting Stand-Average Calculations with Measures of Spatial Heterogeneity

Forest managers in the northern Rocky Mountains are charged with conducting restoration treatments that will enhance the resilience of fire-dependent old-growth stands, and reduce their susceptibility to stand-replacing fire. Yet, stand-average metrics that are routinely used for prescription development poorly characterize the typically heterogeneous stand structure and forest fuels that are associated with old-growth forests. We conducted a proof-of-concept study to compare stand-average calculations of forest fuels and associated wildfire hazard to the within-stand spatial heterogeneity of those properties. We analyzed two fire-excluded old-growth ponderosa pine stands in western Montana, encompassing the moisture gradient across which the forest type occurs in this region. At one site, we also analyzed the effect of a restoration prescription to reduce fuels and abate wildfire hazards. Fixed-area plot sample data were analyzed to describe within-stand heterogeneity by analyzing the distributions of current and anticipated post-treatment plot conditions, and contrasting these with conventional stand-average calculations. Distributions of overstory structures, fuel loads, and modeled fire behaviors were typically non-normal, skewed, and varied widely. Generally, standaverage calculations poorly represented the range of within-stand conditions. The study’s findings highlight the heterogeneity of stand structure, forest fuels, and wildfire fire hazard in old-growth ponderosa pine stands, and reveal the shortcomings of analytical methods that simplify spatially heterogeneous stand data.