ABSTRACT
Southern California’s plant communities and the animals that inhabit them evolved with mountain building, a cooling and drying trend over the past 30 million years, the onset of glacial and interglacial cycles 2 million years ago, and the arrival of western Europeans 500 years ago. Floristic history is inferred from fossils that consist mostly of the durable parts of plants such as pollen, spores, seeds, cones, hard fruits, and leaves.
In the early Cenozoic 65 million years ago, most of western North America was steeped in a mild wet climate, and the diversity of plant species exceeded anything known elsewhere on the continent. At its southern extent, the dense coastal rainforest of the early Cenozoic merged with a tropical savannah. The nearest counterparts of that flora today are found in southern Mexico and in the middle of the United States, the West Indies, and parts of southern Asia. The same cooling and drying that affected the northern forests at the end of the Eocene forced the retreat of this tropical element ever southward. Most of the California constituents disappeared by the end of the Miocene 5 million years ago.
A third vegetation element of the early Tertiary developed in southwestern North America as the Madro-Tertiary Geoflora. Its expansion and evolutionary fate had the greatest impact on Southern California as a floristic unit today.
Today, California is home to about 5,800 species of vascular plants, a number that represents 32% of all plant species in the United States. Of these, 2,150 species and subspecies are endemic.
