Compositional changes in selected forest ecosystems of the western United States

Investigation of forest changes over large areas may support and inform stand scale studies. I used the oldest and most recent available USDA Forest Service Forest Inventory and Analysis surveys in nine states of the western United States to identify increasing and decreasing composition (i.e., percent of total trees) of species and groups over large spatial extents. Nineteen species decreased and nine species increased at least 5% in total composition over a minimum of 5000 km2 between the 1980-1994 surveys and the 2000-2013 surveys. Common pinyon, quaking aspen, ponderosa pine, lodgepole pine, Douglas-fir, and five oak species decreased relative to other species across the greatest extents, while Rocky Mountain juniper and Utah juniper were the most abundant species that increased. Decreasing species generally either were fire-tolerant species or post-fire-disturbance species that initially benefitted from less frequent fire intervals but now are declining with extended fire exclusion, whereas increasing species predominantly were fire-sensitive species. Widespread compositional changes to fire-sensitive species indicates increased tree density in forests. In the western United States, historically extensive open, low density ponderosa pine ecosystems are shifting in state to closed, high density forests composed of a suite of fire-sensitive species, which has critical implications for forest management and fire policy. Increased tree densities of smaller understory trees that burn relatively easily will spread fire to large diameter trees in the overstory, resulting in high severity fires.