Effects of thinning and prescribed fire on understory vegetation in interior pine forests of California

An array of treatments were applied to ponderosa pine (Pinus ponderosa) stands in northeastern California in a large-scale study designed to evaluate the effects of management on accelerating late-successional attributes. Treatments include both mechanical manipulation (thinning) and prescribed fire. The three factors associated with late-successional stands considered in this study are the diameter distribution, species composition, and fire frequency. A potential treatment response is the change in understory composition following treatment. Understory vegetation was sampled in the second growing season after the bum treatment, and 3 to 5 growing seasons after mechanical treatments. A total of 79 distinct species were identified within the treatment units. Species richness appeared to be unaffected by treatments. The distribution of species within treatment units was affected, but response was limited to only a few species. In general, understory species appeared to be slightly more evenly distributed in treated units. Shrub species, primarily snowbrush (Ceanothus velutinus), were more common in all treated areas, and white fir (Abies concolor) was less common, particularly in burned treatments. Bitterbrush (Purshia tridentata) was not found through much of the study area, so conclusions for this key browse species are difficult to make. There was no short-term effect of treatments on pine regeneration. The infrequency of pine regeneration, however, suggests the need for long-term evaluation.