Linking ecological theory with restoration: A ten-year study of plant community dynamics in California vernal pools

Background/Question/Methods   Ecological restoration presents unique opportunities to examine factors that affect the formation, structure and stability of ecological communities. Restoration of natural communities can furnish insights into whether community composition is deterministic, following predictable trajectories; or if it depends significantly on variation in species arrivals that can drive community trajectories in different directions. Our long-term studies of the ecological dynamics of vernal pool plant communities in California focus on: 1) effects of dispersal limitation, order of colonization, and frequency of colonization on plant community composition; 2) invasion of vernal pools by non-native species, and 3) comparison of restored vernal pools to that of naturally-occurring pools. We initiated a field experiment in 1999 by constructing 250 vernal pool basins that received different seeding treatments to assess environmental constraints on plant community formation.

Results/Conclusions   We observed dispersal limitation by all five seeded species; three of four species exerted strong priority effects, with much higher abundance in pools where they were seeded first compared to pools where they were seeded in the second year of the study; and one species had much higher abundance where it was seeded more frequently. Several features of historical contingence affected the early formation of vernal pool plant communities, but we also observed reversals in community trajectories, suggesting that these effects may be lost within a decade. Native species were initially more abundant than non-native species, but non-natives eventually dominated the experimental vernal pools. Pools that received more seeds of native species were less invaded, providing evidence for biotic resistance, but this resistance was eventually overcome. Our results suggest that biotic resistance was overcome more easily in more benign environments, and the harshest environments were only invaded after biotic resistance was removed. Finally, we observed that constructed vernal pool basins exhibited similar hydrology as reference pools, and vernal pool species established populations in many of the constructed pools. Constructed pools with appropriate hydrologic conditions were more likely to support populations of vernal pool plant species, but seeding increased the abundance of these species in constructed pools. In summary, our research has revealed that colonization events, pool hydrologic conditions, and biotic interactions all influence the formation and persistence of vernal pool plant communities.