Historical forest conditions in frequent-fire forests on the eastern slopes of the Oregon Cascade Range

Abstract

Records from a 1914-25 timber inventory of the forested areas on two large Indian reservations reveal historical conditions at the landscape-level in fire-prone forests in the eastern Cascade Range in Oregon. Live conifers >15 cm dbh (diameter at breast height) were tallied by species and size class in a 20% sample of over 180,000 hectares (ha). Forests were predominantly low density relative to current conditions (roughly a third to a quarter of current mean density). Total stand density, large tree (>53 cm dbh) density, and ponderosa pine density were relatively stable across a wide moisture gradient (40-180 cm annual precipitation). Large fire- and drought-tolerant trees dominated basal area (>70% of total mean basal area) and were widely distributed across the landscape (present on 97% of transects). Currently ponderosa pine and large trees no longer dominate total basal area, and large trees are not as uniformly distributed across the landscape as they were historically. Higher-density values (>120 tph, 95th percentile), although rare, were widely distributed across the mixed-conifer habitat while treeless areas (transects on which no conifers >15 cm dbh were recorded) were almost entirely restricted to documented burned areas at higher elevations in colder, wetter habitat types. Historical forest conditions in frequent-fire forests may be increasingly useful in guiding contemporary forest management given 1) projections for increased drought; 2) increases in vertical and horizontal connectivity of forest canopies related to changes in land use; and 3) documented resilience and resistance of historical forest conditions to fire and drought-related stressors in fire-prone forests. This systematic sample of a large landscape provides information about variability in species composition, densities, and structures at multiple spatial scales, which are highly relevant to management activities to restore and conserve desired ecosystem functions.