|Abstract or Summary
- Disturbance and microclimate interact to play a central role influencing the composition and structure of plant communities. In this thesis, I examined plant community composition and structure twenty years after high severity wildfires with and without post-fire management (salvage logging, fuel treatment, tree planting, and shrub release) under contrasting microclimatic conditions in the Klamath region of northern California. The general distribution of cover among the main life form groups: shrubs, hardwoods, and conifers, was similar between unmanaged and managed sites. The abundance of key species within each group, however, changed as a result of post-fire management. Ceanothus integerrimus and Pinus ponderosa responded positively to post-fire management, while Arctostaphylos viscida responded negatively. At the community level, a general pattern was a reduction of heterogeneity in managed sites in terms of both vertical structure and species composition. Based on regeneration strategies after fire, species were assigned to three regeneration behavior groups or regenerative traits, reflecting, in part, responses to disturbance. Strong positive correlations were found between species that regenerate from the seed bank and increasing levels of heat load. Positive interactions were also found between post-fire management and aspect with the abundance of N-fixing Ceanothus spp., which increased in abundance on managed sites on south aspects.
In a second study, I evaluated tree composition in the same areas. Aspect and elevation were important factors controlling conifer and hardwood composition. No effect of post-fire management was detected on composition. Frequency and density of conifers were higher on north aspects relative to south aspects. Post-fire management had no significant effect on density of conifer species; however, results suggested a positive effect of post-fire management on Pinus ponderosa’s density on south aspects. Conifer frequency was higher on managed sites relative to unmanaged sites. These results suggest that post-fire management did not change the number of conifers but increased their spatial evenness on the sites.
Results from these studies suggest that post-fire management changed composition and structure of the early seral communities under study. Furthermore, these results suggest that post-fire management effects on plant communities were strongest on warmer, drier aspects.