Age Structure and Species Composition Changes Resulting from Altered Disturbance Regimes on the Eastern Slopes of the Cascades Range, Washington

Abstract

Age and diameter distributions are powerful tools for assessing changes in forest structure and composition over time. This study analyzed the age distribution of 2,345 trees from 487 plots in the eastern Washington Cascades to document increases in per-hectare tree densities and shifts in species composition since Euro-American settlement. Diameter distributions of these trees plus an additional 814 snags, stumps, and logs were analyzed to determine the extent and pattern of structural and compositional change resulting from post-settlement fire exclusion and preferential harvest of large ponderosa pine (Pinus pon-derosaDougl. Ex Laws.) and western larch (Larix occidentalisNutt.). Per-hectare densities of most species increased following settlement, with shade-tolerant/fire-intolerant species showing the biggest gain. A comparison of diameter distributions for live and dead trees indicates existing stands may not provide snags and logs of adequate dimensions for future habitat needs. Changes in forest structure and composition over the past century increase risk for insect outbreaks, diseases, and catastrophic wildfires.