Characterization and Global Distribution of Vernal Pools

We define vernal pools as precipitation-filled seasonal wetlands inundated during periods when temperature is sufficient for plant growth, followed by a brief waterlogged-terrestrial stage and culminating in extreme desiccating soil conditions of extended duration. These factors have played a significant selective role in shaping the vernal pool flora and fauna. Inundation during the growing season largely eliminates establishment of upland species in the pool basins and the terrestrial period is sufficiently desiccating to prevent establishment of many typical wetland taxa. Pool filling is predominantly from precipitation and drainage is from a relatively small watershed, consequently nutrient input is largely autogenous. Vernal pools tend towards oligotrophic and are poorly buffered with dramatic diel changes in CO2, O2, and pH. These factors have played a role in selection of CAM photosynthesis in Isoëtes and Crassula, and may further contribute to elimination of many other typical wetland plants. The flora comprises two elements: cosmopolitan aquatic taxa and vernal pool specialists. The former are species in genera found worldwide in aquatic habitats, whereas specialists are species from terrestrial genera in western North America. While the fauna includes vernal pool endemics, most all species are in globally widespread aquatic genera. California vernal pools comprise a diverse array of seasonally aquatic habitats. Although variable in their origin, size, shape, type of duripan, depth, duration of inundation and species composition, three factors (duration, timing, and source of inundation) are common across all systems generally classified as Californian vernal pools. Outside of California, vernal pools are present in other Mediterranean climate regions, being best developed in Chile and Western Australia. Vernal pools are also found in non-Mediterranean climates, for example on the well-studied “granite outcrops” of the southeastern U.S., where shallow basins fill during the winter and, despite summer-rains, high temperatures and shallow substrates result in desiccating summer conditions. Other seasonal wetlands such as desert playas, Great Plains buffalo wallows and prairie playas or potholes have sufficient differences to be excluded by our definition of a vernal pool. CITATION. Pages 1-14 in: C.W. Witham, E.T. Bauder, D. Belk, W.R. Ferren Jr., and R. Ornduff (Editors). Ecology, Conservation, and Management of Vernal Pool Ecosystems – Proceedings from a 1996 Conference. California Native Plant Society, Sacramento, CA. 1998.