Influences of Latitude and Climate on the Distribution of Sierra Nevada Vegetation: A Historical Perspective

The Sierra Nevada has been identified as a biodiversity hotspot as part of the California Floristic Province due to a high level of endemic species, many of which are vulnerable to habitat loss within the Province. Throughout the Tertiary period, species have been responding to a changing climate. As a result, the Tertiary period is one basis from which we can try to understand current plant distributions. Raven and Axelrod presented a set of biogeographical theories dealing with origins of flora in North America. They defined Arcto-Tertiary flora as having originated in the northern and polar latitudes then migrating south as the climate changed to cooler-drier conditions in the middle Oligocene (28.4 Ma). They also defined Madro-Tertiary flora as having moved from its southern origin in the area south of the Rocky Mountains and the Sierra Madre Occidental in Mexico to northern areas in California as the dry climate continued to expand. I attempt to explore a generalized approach using the Arcto-Tertiary and Madro-Tertiary flora classifications as the basis for understanding how species are currently distributed and what that may say about future responses to climate. AT species are distributed all throughout the Sierra Nevada (40 degrees N to 35 degrees N latitude) but their main distribution lies between 37 and 39 degrees N latitude. Latitude tended to be a poor predictor overall, especially for MT species. The two species groups separated out well based on climatic variables and environmental plot characteristics. This shows that the groups have relations to their climatic origins but not to latitude.