Influence of fire disturbance and biophysical heterogeneity on pre-settlement ponderosa pine and mixed conifer forests

Fire frequency is assumed to have exerted a strong influence on historical forest communities in the inland Pacific Northwest. This study reconstructs forest structure and composition in the year 1890 and fire frequency from 1760 to 1890 at 10 sites spanning a broad productivity gradient in the southern Blue Mountains of eastern Oregon. We tested for the relative influence of fire frequency, climate, soils, and topography by fitting variables to ordinations of forest structural and compositional configurations. We also built formal statistical models using non-parametric permutational multivariate analysis of variance. Because fire disturbance and biophysical influences on forest structure and composition may vary depending on the scale at which relationships are examined, we tested the influence of variables at the scale of 4- to 12-ha sites and at the scale of three equal-sized areas within each site. The proportion of fire-intolerant species basal area reconstructed within sites in the year 1890 ranged from 0% to 43%. The proportion of fire-intolerant species basal area reconstructed within equal-sized areas within sites ranged from 0% to 75%. Despite significant differences in historical species composition between and within sites, fire frequencies were relatively similar. Mean fire return intervals (MFRIs) calculated for sites ranged from 10.6 to 21.2 yr. MFRIs calculated for equal-sized areas within sites ranged from 10.6 to 28.8 yr. Fitting fire frequency and biophysical variables to ordinations and model building with perMANOVA showed that topographic position index and vapor pressure deficit had stronger influences on site-scale forest structure and composition than fire frequency metrics. Available soil water was the most important influence on forest structure and composition within equal-sized areas within sites. Relatively frequent fire across a broad range of forest types in the southern Blues appears to have been a relatively uniform influence on forest dynamics modulated by fine-scale biophysical heterogeneity. If return to historical conditions is a goal of management, treatments to reduce fuel and restore frequent fire are appropriate across a broad productivity gradient in the southern Blues.