Correlates Of Plant Biodiversity In Mediterranean Baja California Mexico

Northwestern Baja California is an area of exceptionally high local endemism, nestled at the southernmost end of the California Floristic Province (CFP), one of the world's mediterranean-climate regions. The ecotonal position between the CFP and the central deserts of the Baja California Peninsula reflects the transition of the major climatic regimes. This area appears to have been more climatically stable than adjacent areas, and the local endemism along the coast is largely the result of the persistent marine layer, which buffers temperature extremes and maintains moisture during the drier months of the year. The driving hypothesis of this dissertation is that long-term climatic stability provided by the cold coastal upwelling of the California Current (driven in turn by the large-scale circulation of the seas) has buffered climatic conditions along the coast, reducing species extinctions and allowing adaptive processes leading to high levels of endemism in the local flora. The high productivity of the coastal upwelling has also had an impact on local plant diversity through the impact of prehistoric shell middens on soil heterogeneity: Ocean deposits (aragonite shells) extracted by indigenous people have added to landscape complexity and increased biological diversity along the coast. Shell middens are scattered along the coast, and idiosyncratically change soil conditions and the resultant plant communities. Shell middens consistently increase native plant diversity, but with very different species compositions, overlapping conservation priorities for archeology and biodiversity. The climate-buffering effect of the coastal currents can be evaluated in the flowering dynamics of the local plant communities: Plant phenology is predominantly controlled by season and available moisture. Fogs dampen seasonality in flowering times and plants flower for longer time periods at the coast, allowing probable mechanisms for speciation. This stability is consistent with the high levels of endemism that can be seen along the coast, and in microclimate effects on phenology along the inland-to-coast gradient. Conservation of coastal habitats in this southern edge of the California Floristic Province is of elevated importance due to the refugium effect of the marine layer, and the presence of archeological sites that increase biodiversity.