Long-term prospects for restoration of coastal sage scrub: invasive species, nitrogen deposition, and novel ecosystems.
Coastal sage scrub (CSS) is one of the most endangered ecosystem-types in California and is undergoing extensive restoration efforts. Major threats to CSS include agriculture and urban development, fragmentation, invasive species, frequent fire, and high levels of anthropogenic nitrogen (N) deposition that increases exotic species productivity, further increasing fuel for fire. In this review we compare a range of techniques that have been used with varying success to restore CSS, using examples from published and unpublished sources. Techniques that treat large scales and reduce the exotic seedbank are the most effective, such as herbicides or solarization, but each may also have drawbacks. Other methods include mulch, seeding or planting species with competitive functional traits, grazing, and fire. Regardless of method, invasive species recolonize to varying extents following restoration, and periodic treatment is needed. CSS in sites receiving more than the critical load of 11 kg ha-1 yr-1 of N deposition may become type-converted to exotic annual grassland in the absence of other disturbance such as fire. Such sites are not good candidates for restoration. Inability to control exotic species reinvasion and restore diversity of native forbs results in novel ecosystems with reduced conservation value and ecosystem services.