Climatic nuclei, warm/dry microrefugia and the potential for leading edge migration: Mini-deserts in the Bay Area

Background/Question/Methods

Xeric microclimates at the mesic edges of species distributions may have provided climatic microrefugia for relatively warm-adapted species in the context of historical regional cooling trends.  Currently, these isolated climatically suitable pockets may support relict populations from the former northern extent of species main distributions during the Altithermal period in California.  We used herbarium records, field surveys, and fine-scale climate and species distribution modeling to analyze the importance of these isolated leading edge populations to distributional expansion with projected regional warming/drying trends.   

Results/Conclusions

We have identified such areas in species with predominantly southeastern California and desert distributions, but which also occur in isolated pockets of unusually hot and dry microclimates in the East San Francisco Bay Area, beyond the northern/western limits of their main distributions. Under multiple climate futures derived from the CMIP5 ensemble model, large parts of the Bay Area shift vegetation types, becoming more suitable for semi-desert-adapted vegetation.  Thus these current warm/dry ‘microrefugia’ may create important nuclei for future native species’ dispersal and colonization of a climatically shifting Bay Area.  Considering warming/drying trends and model-projected vegetation shifts, these areas may have a disproportionate influence, compared to southeastern contiguous areas of the distribution, on leading edge migration.