The effects of forest management on plant species diversity in a Sierran conifer forest

We used a large data set collected as part of a wildlife habitat study at the Blodgett Forest Research Station in the Sierra Nevada of California to assess the influence of management practices on vascular plant diversity in the mixed conifer forest. In addition to the existing data, we conducted botanical inventories in 32 plots (0.28 ha in size) from five different silvicultural regimes in 1997. Based on these inventories, understory species richness normalized to a total area sampled of 1.13 ha was significantly greater in plots under plantation (80 species) and shelterwood (77 species) management compared to plots in reserve sites (i.e., approximately 80 year with no active management, 48 species). This pattern in species richness was consistent in the larger data set. Based on 372 plots sampled between 1977 and 1996, plantations and shelterwoods routinely had the highest species richness and the reserve units routinely were the least rich. Stands under single-tree selection had species richness values closer to those of the reserve stands. The relative diversity observed in group selection plots varied from year to year. Based on the 1997 data, canopy closure and seedbed were significantly correlated to understory plant diversity. Plots that were more open and had more bare ground tended to have higher species richness. This result supports the suggestion that in addition to the total amount of biomass removed, post-harvest practices also affect plant diversity. There were considerable differences in the composition of the understory communities among silvicultural treatments. Plots under a group selection, single tree selection or reserve management regime had a greater proportion of late-seral vs. early-seral species and a lower proportion of introduced exotic species compared to plantations and shelterwoods. In this forest, more intensive management maximized species richness, but the lower intensity practices better conserved understory plants typical of late-seral stands.