Vegetation Change in Response to Extreme Events: The Effect of a Short Interval between Fires in California Chaparral and Coastal Scrub

TLDR

The study examines how shrub species abundance changed after consecutive fires in 1979 and 1980 on Otay Mountain, California. The second fire in 1980 reburned part of the first fire’s area that had been seeded with annual ryegrass for erosion control. The reburn caused severe vegetation shifts, nearly eliminating Ceanothus, drastically reducing Adenostoma density by up to 97%, killing Xylococcus, and these changes are expected to persist for decades, indicating that human introduction of aggressive annual grasses has amplified abrupt post‑fire vegetation change.

Abstract

This study describes changes in the abundance of shrub species after two fires in 1979 and 1980 on Otay Mountain in San Diego County, California. The 1979 fire burned a large area of dense chaparral and coastal sage scrub. The 1980 fire burned a portion of the 1979 fire area that had been seeded with annual ryegrass (Lolium multiflorum) as an erosion protection measure. Changes in the vegetation caused by the 1979 fire alone were similar to those commonly seen in chaparral wildfire, but the reburning of the vegetation in 1980 caused drastic changes in some areas. Ceanothus oliganthus was almost completely eliminated from the area of the 1980 burn. Adenostoma fasciculatum, the most abundant shrub at the study site, was reduced in density by up to 97%. Even Xylococcus bicolor, which normally resprouts with complete success after fire, suffered substantial mortality with reburning. It is concluded that the changes brought about by the 1980 fire will certainly persist for many decades. While sudden shifts in vegetation composition probably occurred without human intervention, we believe that human activity, especially after the introduction of aggressive annual grasses 200 yr ago, has caused an increase in the instances of abrupt change.