Influence of the Summer Marine Layer on Maritime Chaparral
and Implications for Conservation Policy in the California Coastal Zone
Michael C. Vasey
The California coast is renowned for its exceptional diversity of local endemic species, particularly in evergreen, sclerophyllous shrubland known as maritime chaparral. This species rich, fire-adapted shrubland is typical of other regions in the world with Mediterranean-type climates characterized by cool, wet winters and long, dry summers. The dry season in California, however, is moderated by a persistent layer of fog and low cloud cover that hugs the coast during much of the summer (summer marine layer). I investigated the potential influence of the summer marine layer on shrub water relations along a coast-to-interior climate gradient in the central California region. I also tested the possible impact the summer marine layer might have on chaparral species diversity patterns in this region. Finally, since maritime chaparral is a legally protected natural community under Environmentally Sensitive Habitat Area (ESHA) policy, I investigated the origin and implementation of ESHA policy to determine if natural science insights might help to inform the conservation of this globally rare vegetation. Using water potentials, stable carbon isotope ratios, and xylem vulnerability analysis of Arctostaphylos shrubs, we found evidence that plant water relations in coastal chaparral are more favorable than in interior chaparral. Dry season climate variables associated with evaporative demand were found to dominate the environmental variance of the 87 plots investigated; however, for upland coastal (transition) plots, there was significantly more winter rain than coastal lowlands or interior uplands. I found high levels of beta diversity in both coastal uplands and coastal lowlands compared to low beta diversity in interior chaparral. Since "maritime chaparral" has legal status under ESHA policy, I propose that the designation of maritime chaparral in most cases should include both coastal lowland and coastal upland chaparral. I found that the mandatory language of ESHA policy, its focus on habitat rather than species, and its grounding in local land use planning is a powerful combination for biodiversity conservation that provides an interesting model for landscape scale conservation strategies.