Robust historical timber inventories describe patterns of forest structure, composition, and competition across a wide latitudinal gradient

Frequent-fire forests had fewer, larger trees and experienced less competition for resources prior to widespread fire suppression and logging in the early 20th century, yet landscape-scale assessments of historical forest structure, composition, and competition remain limited. We compiled five systematic timber inventories sampled from 1911 to 1936 to assess the historical structure and composition of ponderosa pine and mixed-conifer forests of the Sierra Nevada of California and the Cascades of Oregon and California. We summarized measures of tree size, density, species, and basal area for each dataset. We calculated the relative Stand Density Index (SDI) to quantify degrees of stand competition and utilized Principal Component Analysis and K-means clustering to assess patterns of forest structure across a wide latitudinal gradient. On average, historical frequent-fire forests had low tree densities, larger trees, high representation of fire-tolerant pines, and minimal competition, with variability within and among sites. Clustering analysis identified two distinct groups of stand conditions, but each site had membership within both groups, revealing landscape-level similarities despite our extensive study area. Our synthesis of historical timber inventories illustrates a spectrum of frequent-fire forest characteristics across broad spatial scales, providing managers with reference conditions that can inform contemporary forest restoration and support aspirations of operational resilience.