UC San Diego

Coastal sage scrub (CSS) is an important vegetation type in Southern California, which is under threat from multiple environmental changes such as nitrogen deposition, invasion by exotic species, and accelerating fire regimes. While previous studies have found that CSS can recover pre -fire cover through a successional process, there is concern that high exotic annual grass abundance after fire could prevent native shrub recovery, creating an alternate stable state which requires an active management for restoration of native shrub cover. This study evaluated the performance of focal native and exotic species at early demographic stages (germination, first-year growth and survival) in one recently burned site, and then placed these short-term results in context by examining long-term vegetation dynamics across four CSS sites in San Diego County. Exotic species out-performed native species during all early demographic stages, especially following fire. In the long-term (20 years) cover by native CSS shrubs returned to pre-fire levels at three of the four sites monitored. These results indicate that invasion slows the rate of CSS successional recovery post-fire, and that native CSS vegetation can recover given a long-enough fire return interval. Accelerating fire frequencies, however, may be creating invaded "apparent stable states," because full native shrub recovery does not effectively occur between closely spaced fires