Does the age of exposure of serpentine explain variation in endemic plant diversity in California?

We analyzed botanical, geologic, and climatic data in an effort to understand why there are many more serpentine endemics - i.e., plant species restricted to ultramafic substrates-in some regions of California than in others. We found that, in addition to elevational range (which subsumed the effect of serpentine area) and annual rainfall, the age of exposure of serpentine explained significant geographic variation in the diversity of endemic plant species. Estimated ages of subaerial exposure are generally higher in the Klamath and northern Coast Range serpentines than in Sierra Nevadan and southern Coast Range analogues, paralleling the trends in endemic plant diversity. However, the fit of the relationship is weakened by the southern Sierran serpentines, which are apparently long-exposed yet poor in endemic plant species. These results have significance for understanding local ecological processes, because we have also found that the diversity of endemic species within 50 x 10 m field plots is best predicted by the diversity of endemics in the surrounding region, which in turn is a function of the age of subaerial exposure.