Many historic and reconstruction datasets suggest that on average frequent-fire forests were relatively open, low-density stands with canopy cover between 20-40%. Yet some sensitive species, such as the spotted owl and fisher, which persisted under active fire regimes, are associated with high-density forests with > 70% canopy cover. Such stands today are often associated with heavy fuel loading and increased risk of drought-related mortality. Forest managers need a better understanding of where such dense forest conditions were present before logging and fire suppression altered modern forest conditions. We used three historical datasets of forest structure with general spatial information (plot locations up to 120 m of error) in the northern, central and southern Sierra Nevada to examine the relationship between stand conditions (i.e., basal area, composition and stem density) and climatic and topographic variables.
Results/Conclusions
For all three datasets, factors associated with moisture availability such as precipitation, elevation and slope aspect were correlated with historical basal area. In contrast, conditions that might influence fuels drying and fire intensity such as slope position and steepness, and climatic water deficit were most strongly associated with stem density. These factors explained a significant, but relatively low proportion of variance in historical structure, probably due to location errors, soil effects on water holding capacity and the stochastic nature of fire. We did identify one variable, topographic convergence index, a GIS measure of potential water accumulation from upslope collection, which was significantly correlated at a local scale with both basal area and density. Our results suggest managers might avoid thinning in some areas with high stem densities, fuel loads and canopy cover in locations where climatic and water accumulation variables suggest these habitat conditions may have been present in historical forests that were resilient to fire and drought.