OOS 22-5
Long term trajectories of plant communities in restored California vernal pools
Long term dynamics of wetland plant communities are tied to multiple physical and ecological factors, including hydrological conditions, interspecific interactions, and species dispersal mediated by landscape connectivity. We have studied naturally occurring and restored vernal pools in California to trace the importance of these factors in determining temporal fluctuations in species composition. We initiated a field experiment in 1999 with the construction of 256 vernal pools at a site on Travis Air Force Base, California. Pool size and connectivity were varied to study spatial effects on community development, and seeding treatments were applied to support analyses of dispersal limitation and priority effects in five native species. Through annual sampling, we have followed community development in these constructed pools and in a set of naturally occurring pools within the study site. Because many of the pools declined dramatically in native species abundance during the past decade, we initiated a new field manipulation in December 2013 to examine factors that may influence the re-emergence of native species dominance in these pools. In sampling plots adjacent to our long-term study plots, we manipulated water depth and presence of litter to determine their separate and combined effects on native plant performance.
Results/Conclusions
Our long-term measurements show high variability among pools in native species richness and abundance, but a general and sharp decline in abundance of native species from 2000-2013. Declines typically occurred in pools that had maximum water depths of 5 cm or less during the wet season and in pools that received few seeds in our initial seeding treatments. In our newly manipulated sampling plots, we observed effects of both pool depth and presence of litter on re-emergence of native species from the soil seed bank. Our data suggest that plant community composition in restored vernal pools may shift in response to interventions that modify both hydrological conditions and interactions of native and exotic species via litter manipulation. We conclude that long-term community trajectories are malleable and responsive to changes in local conditions due to the persistence of species in the soil seed bank.