Patterns of chaparral diversity along a moisture gradient in the Mediterranean-type climate region of the central coast of California

Background/Question/Methods

The central coast of California is renowned as a hotspot of local endemism, especially in woody species that require fire-dependent recruitment.  In this study, we investigate the hypothesis that climatic factors associated with the summer marine layer best explain chaparral composition patterns along the west central coast of California. We randomly sampled chaparral species composition in 0.1-hectare plots along a coast-to- interior gradient. For each plot, climatic variables were estimated (PRISM and BIOCLIM) and soil samples were analyzed.  We used Cluster Analysis and Principle Components Analysis to categorize sites into climate zone groups. Climate variables, vegetation composition and various diversity measures were compared across climate zone groups using ANOVA and non-metric multidimensional scaling. 

Results/Conclusions

Differences in climatic variables related to summer moisture stress explained the majority of variance in measured conditions and coincided with three distinctive chaparral communities: maritime (lowland coast where summer marine cloud cover was strongest), transition (upland coastal elevations with some fog influence and greater winter precipitation), and interior sites with greater summer drought stress.  Beta-diversity was higher in maritime and transition sites than in interior sites. Coastal chaparral (both maritime and transition) differs from interior chaparral in having a higher obligate seeder-to-facultative seeder (sprouter) ratio and by being dominated by various Arctostaphylos species as opposed to the interior dominant, Adenostoma fasciculatum. In summary, the California central coast is characterized by a moisture gradient that correlates with patterns of woody plant composition and β-diversity.  Summer fog in coastal lowlands and higher winter precipitation in coastal uplands combine to lower late dry season water stress in coastal chaparral and contribute to longer fire return intervals that favor obligate seeders and higher endemism. Soil nutrients are comparatively less important in explaining plant community composition, but the heterogeneous edaphic template contributes to local endemism and promotes chaparral patches within the dominant forest vegetation along the coast.