Masters Thesis

Long-term, post-fire dynamics of a sagebrush steppe community in northeastern California

This study examined the long-term, post-fire dynamics of sagebrush steppe communities at two burn areas located in Modoc County in northeastern California. Pre and post-fire vegetative cover and herbaceous productivity data were collected at two burn areas across a nearly thirty-year period. The cover and productivity data from the study area were also analyzed with respect to their relationship to the number of post-fire growing seasons and annual weather conditions. As expected, fire removed the woody, shrub component of the plant community and shifted the plant community to an herbaceous, grass-dominated community. Vegetative cover and productivity of herbaceous species increased significantly in the years following the burns. While the invasive annual grasses (primarily cheatgrass, Bromus tectorum) dominated in the plant community in the years immediately following the fires, native perennial grasses overtook the annual grasses after 10 post-fire growing seasons. A substantial amount of the variability in annual grass cover and productivity could be explained by the number of post-fire growing seasons. Annual grass cover and productivity were also significantly influenced by annual variations in weather, particularly fall and winter precipitation. Perennial grass productivity peaked 10 years following the burns, while perennial grass cover reached maximum levels 20 years post-fire. Perennial grass cover and productivity began to decline towards the end of the monitoring period, but were still greater than the pre-fire levels. Perennial grasses appeared to be minimally influenced by annual variations in temperature and precipitation. Both annual and perennial forbs generally increased following the fire, but varied both spatially and temporally. Annual fluctuations in temperature and precipitation explained more of the variation in annual forb cover and productivity than post-fire time. Shrub recovery was slow following the prescribed burns, with minimal shrub cover noted until 10 to 20 growing seasons. Following nearly 30 post-fire growing seasons, sagebrush cover returned to pre-fire levels and was comparable to cover levels to unburned areas. Shrub recovery rates were relatively stable across the study area, and were not influenced by annual variations in temperature or precipitation. By 2009, there was evidence of western juniper returning to both of the burn areas. This indicates that a 30-to-40 year fire return interval might be recommended to reduce western juniper encroachment and to maintain a productive and diverse ecosystem. Even with the relatively small spatial scale of study area, there were significant differences in the post-fire vegetation between the sampling sites. The research confirms that sagebrush steppe plant communities are highly heterogeneous and can vary in their post-fire succession.