Plastic response of vernal pool vegetation to yearly differences in amount and timing of precipitation and temperature causes remarkable temporal fluctuation of species abundance in vernal pool plant communities, which are dominated by annual species. It creates difficulties in interpretation of monitoring results especially for understanding and forecasting impacts of climate change. Understanding plant community diversity and natural range of variation is necessary to distinguish between annual fluctuations and long-term vegetation changes related to climate change. The objective of this study is to establishing a network of vernal pools representative of vegetation and habitat diversity and evaluate the range of variation of their floristic composition and environmental parameters. During 2001-1009 we collected data from >2500 plots in >900 pools from 110 locations representing all 17 Californian vernal pool regions, each of which has a unique set of climate, vegetation, landform, soil, and ecosystem traits. At each sample plot (10 sq. m) both floristic and environmental data were collected including a complete species list with cover estimates for each species. Data were stored in Turboveg database and classified using Juice and PC-ORD software. For each classification unit the range of variation of selected parameters was documented.
Results/Conclusions In each vernal pool region we recognized 3-4 associations, many of which did not occur in other regions. Associations were characterized by diagnostic species, floristic composition, environmental settings, and the range of these parameters observed under current climate condition. Evaluating sites for their potential inclusion in the network was based on diversity of plant associations, range of habitat traits, floristic richness, presence of endemic species and associations, and geographic location. This approach allowed us to include all vegetation variation in Californian vernal pools in a relatively few number of reference sites. These sites are recommended by managing agencies for long-term monitoring of wetland conditions and trends. Such a design is necessary to separate long-term trends from the inter-annual plastic responses.