Soil Ecology Restoration Group

last update October 8, 2000

What are the Limits to Restoration of Coastal Sage Scrub in Southern California?

 


Abstract

The coastal sage scrub vegetation of southern California is becoming one of the most intractable vegetation types to restore. It is subject to weed invasion, fragmentation, frequent fire, nitrogen deposition, and other disturbances that have reduced the shrub density and increased the frequency of Mediterranean weeds. Nation recolonization of native shrubs and forbs back into disturbed coastal sage scrub is often slow, especially where exotic weed abundance and disturbance frequency are high.

Where natural regeneration is slow, restoration will be necessary to return the native shrub and forb cover. Restoration trials with six species of shrubs showed that they would not establish from seed unless the Mediterranean annual grasses were removed. However, establishment of seedlings of Artemisia californica was successful if they were initiated as three month old greenhouse transplants. Transplant survival and growth rate were increased by weeding, and those transplants that survived the first growing season with grass competition survived into the second season. Thus, weed management, including weed seedbank control, is the most critical factor for shrub establishment.

An additional problem related to weed invasion may be nitrogen deposition from automobile exhaust, which is up to 45 kg/ha/yr in the Los Angeles air basin. Anthropogenic N deposition causes increased weed productivity in other areas around the globe, and may in part explain the vegetation type conversion of coastal sage shrubland to annual grassland in southern California. Thus, even our best efforts at restoration may fail in the most polluted areas, where we have measured extractable soil N up to 87 µg/g.

Reestablishing biodiversity is one of the most important goals of many restoration projects, but even the best stands of restored coastal sage shrub have low biodiversity. This vegetation type contains contains 600 sensitive species, but restoration of many of these is limited by insufficient availability of seed, lack of knowledge of propagation, and of course the high cost associated with handling so many species. However, the greatest threats to biodiversity for coastal sage shrub restoration are competition from weeds and possibly also elevated soil N in regions with N deposition. A restoration strategy for the region will include an understanding of what sites are most likely to respond to restoration, given problems inherent in areas that suffer from the highest air pollution and greatest weed invasion.