Change over 70 years in a southern California chaparral community related to fire history

Question: What changes in species composition and cover have occurred in chaparral as a function of fire history across an ecoregion? Location: San Diego County, California, USA. Methods: Stands in which 40 mid-elevation chaparral vegetation plots (each 400 m² in area) were located in the 1930s were resurveyed in 2001. We stratified the stands into Infrequently versus Frequently burned (0-1 versus 2 or more fires recorded in the 91-yr period), and Immature versus Mature (≤31 yr versus >31 yr since last fire), resulting in four groups. Ten stands were randomly selected from each of these groups for survey. Results: There were no major shifts in life form composition, e.g., live oak trees were not invading chaparral that had experienced little or no fire, nor were subshrubs or herbaceous species replacing shrubs in areas that had experienced more frequent fires. However, there was a notable increase in the frequency of the subshrub Eriogonum fasciculatum across all fire history groups. In the mature stands with infrequent fire, average cover of resprouting shrubs increased (from 72 to 91%) and cover of obligate seeding shrubs (species with fire-cued germination) decreased (from 21 to 6%) significantly. Mature stands with frequent fire showed a significant decrease in resprouter cover (from 87 to 80%) and increase in obligate seeders (from 10 to 16%). Conclusions: While the tremendous changes in land use in southern California have been predicted to cause shifts in chaparral composition, these shifts are difficult to detect because species longevity and fire cycles are on the order of decades to a century. In this study, the expected trends could only be detected in groups that were mature at the time of the second survey.