PS 7-68 - Long-term monitoring before and after a fire in southern California elucidates changes in vegetative cover and composition.

Background/Question/Methods

Vegetation succession after fire has been examined in many different ecosystems, but most studies have relied on post-fire comparisons of burned and unburned sites due to the unpredictability of wildfires. While these comparisons are useful, it is difficult to determine whether ecosystems have truly recovered or shifted away from pre-fire compositions as the burned sites may have already differed from the unburned sites prior to fire. We used long-term vegetation monitoring data (2007 to 2021) with point-intercept and quadrat census methods to examine post-fire recovery of coastal sage scrub and grassland ecosystems after a 2017 fire in southern California. We addressed two questions. (1) How does fire change the vegetation cover of native vs. non-native species, as well as different functional groups in these ecosystems, (2) Did diversity and community composition in burned sites return to their pre-fire state four years after fire – and is this recovery different in timescale for different functional groups or species? Cover and diversity were analyzed with linear mixed models, and community composition was analyzed with constrained ordination methods. For all types of analysis, we compared burned and unburned sites post-fire and also compared burned sites post-fire to the same sites pre-fire.

Results/Conclusions

Burned coastal sage scrub showed lingering changes four growing seasons after fire in both cover and species composition when compared either with unburned sites or with their own pre-burn data. Furthermore, burned shrubland sites changed differently in species composition between pre-fire and the first year after fire than did unburned sites. Driving these results was an increase in native shrubs such as Salvia mellifera and Artemisia californica in unburned sites after 2017, but an increase in non-native grass and forbs, followed by an increase in the native fire-follower, Acmispon glaber, in burned sites. By contrast, community composition was less perturbed by fire in grassland sites. In the grasslands, burned and unburned sites differed in community composition prior to the fire, and both burned and unburned sites showed decreased native Stipa lepida and increased invasion from non-native Brassica nigra in recent years. Our analysis elucidated the impacts of fire on specific forb and grass species that are difficult to study without pre-fire data and demonstrated stronger and longer lasting effects of the fire in coastal sage scrub than grassland habitat.