Responses of vernal pool plant species to annual fluctuations in weather in California’s Central Valley
Hardpan vernal pools in California’s Central Valley are seasonal wetlands which support a high diversity of native plant species but face many challenges including continuing habitat loss and effects of climate change. Individual vernal pools have unique hydrology due to variation in such factors as catchment area, depth to water-restricting soil layers and total annual precipitation. Over the last 7 years, hardpan vernal pools at Mather Field (Sacramento County), CA were studied to determine how plant species respond to changes in pool hydrology through time. Pool inundation periods were calculated based on hourly measurements of water depths obtained with water level dataloggers. Detailed measurements of plant locations within pools were obtained using a real-time kinematic global positioning system so that hydroperiods could be calculated for individual species within pools.
Inundation periods for individual pools varied substantially from year to year (e.g., 50-142 days of inundation in one pool). Timing of seasonal precipitation had a strong effect on pool inundation period length, with late precipitation resulting in shorter inundation periods even if total rainfall was greater. Hydroperiods for individual plant species were typically narrow within individual pools in any year (e.g., Eleocharis macrostachya had a mean hydroperiod of 42.8 +/- 2.1 days in 2013), but across years or across pools within years, hydroperiods for species were quite variable (e.g., in just one pool E. macrostachya’s mean hydroperiod ranged from 49.2 to 136.2 days across all years). Despite increased abundance of non-native species in years with late and/or low rainfall, native species again dominated pools when pools inundated earlier and/or longer. These annual dynamics highlight the effects that climate change will have on vernal pool systems. Estimates of rainfall and temperature change for the next century predict that vernal pools in the Sacramento Valley will have longer inundation periods, favoring perennial species like E. macrostachya, while those in San Joaquin Valley will have shorter inundation periods, likely resulting in increased invasion by upland or non-native species.