Understory response to fuel reduction treatments in the blue mountains of northeastern Oregon

Understory response to fuel reduction treatments was evaluated in fire-adapted ponderosa pine-Douglas-fir forests in northeastem Oregon. Treatments included: no management (control), prescribed fall burning (burn), low thinning (thin), and low thinning followed by prescribed fall burning (thin/burn), replicated four times in a completely randomized design. Treatment effects were observed in the first growing season after burning and three seasons after thinning. Species richness of the understory vegetation was significantly lower in the thin than in the control, but Shannon-Weaver's index of diversity was not affected by fuel reduction treatments. Graminoid cover was not influenced by treatment, forb cover was reduced in treatments that included thinning, and shrub and total cover were reduced in treatments that included burning. Individual species responded to treatment in a manner consistent with their life history characteristics. Prairie Junegrass cover increased in those treatments that included burning, while cover of other graminoid species was not significantly influenced. The thin treatment significantly lowered elk sedge and total cover, but did not strongly influence the cover of other species. Prairie Junegrass frequency increased significantly in the burned treatments, while western needlegrass frequency was significantly reduced. Frequency of other species tended to remain the same in all treatments. Resilience of community diversity to fire and the consistent effect of burning on individual species demonstrate their adaptation to frequent low-intensity fire, and the subsequent moderate impact of low thinning and fall prescribed burning on understory vegetation.