Vegetation Response to Thinning and Burning in a Ponderosa Pine Forest, Washington

We evaluated, over nine years, the effects of thinning and burning on vegetation response in a ponderosa pine/pinegrass forest. Ponderosa pine forests in the eastern Cascade Mountains of Washington support dense, stands in which crown fires are probable, owing to post-settlement sheep grazing (increased tree seedling establishment), logging, and fire exclusion. In 1991, the Okanogan-Wenatchee National Forest began to apply long-term management techniques to mitigate post-settlement changes in ponderosa pine forests. We used a randomized complete block design with a linear regression component to analyze post-fire responses of the vegetation. Thinning and burning had little effect on the canopy layer of ponderosa pine, western larch, and Douglas-fir, but small grand fir trees, and nearly half of the saplings of all species, were killed. Shrubs in the middle forest layer were top-killed, but resprouted during the first post-fire growing season, and increased dramatically after 3 yr and 9 yr. Frequency and cover maintained or increased for species in the lower forest layer in post-fire years 1, 3, and 9. The thinning and fire treatments reduced the middle layer of small trees and shrubs in the first post-fire year, but by the third and ninth post-fire years tree seedlings, especially grand fir and ponderosa pine, and small shrubs were abundant in the understory. Thinning trees and removing excess fuels, coupled with low intensity fall prescribed burns, offers a promising management strategy for restoring the pre-settlement structure of the ponderosa pine/pinegrass community in Beehive Forest.