The latitudinal diversity gradient of plant functional groups in California vernal pools

Vincent Chen, Nina Tortosa and Jamie M. Kneitel, Department of Biological Sciences, California State University, Sacramento, Sacramento, CA, Jennifer Buck-Diaz, Vegetation Program, California Native Plant Society, Sacramento, CA

Background/Question/Methods
The latitudinal diversity gradient is characterized by increasing species richness with decreasing latitude. It has been a well-established pattern found in a wide range of taxonomic groups and ecosystems. Temporary aquatic ecosystems, however, have been largely understudied in this regard. In California, precipitation increases and temperature decreases with increasing latitude. Consequently, temporary aquatic ecosystems may be larger and persist longer with increasing latitude, resulting in a reverse gradient documented with aquatic fauna. Further, species traits are also know to influence patterns of diversity at different spatial scales. We used data from approximately 800 California vernal pools from across 10 degrees of latitude. Over 400 plant species were identified during sampling, and species were categorized into four trait categories, all combinations of native or exotic and graminoid or forb. Pool and site characteristics were also measured, including pool surface area, disturbances, and climate. Pools were grouped into the USFWS vernal pool regions, and alpha, beta, and gamma diversity were calculated. Multiple regression, partial correlations, and multivariate approaches were used to evaluate what variables were associated with diversity patterns.
Results/Conclusions
Total alpha diversity increased with latitude (reverse latitudinal gradient), pool size, and the climate variables (temperature and precipitation). In contrast, beta diversity decreased with latitude and pool size. The individual plant groups (native graminoids, native forbs, exotic graminoids, and exotic forbs) responded differently to variables, but there were some commonalities. Alpha diversity in most groups was positively associated with latitude and beta diversity was negatively associated with pool size; both were reduced by thatch cover. Many differences among functional groups were notable. For forbs and grasses, native alpha diversity increased with pool size, but exotic alpha diversity decreased with increased precipitation. Beta diversity among groups also responded differently to precipitation and temperature averages and variations. Similar to previous studies on invertebrates in this and comparable systems, a reverse latitudinal diversity gradient was exhibited. Pool size, climate, and location all influenced diversity at alpha, beta, and gamma scales. Plant functional groups responded similarly to some variables, but there were major differences. Management and restoration of vernal pools will need to consider how these factors affect plant diversity of functional groups. Further ecological research into vernal pools should consider the effects of species interactions (e.g., competition), land use, and future climate change.