A five year study of hardpan vernal pools at Mather Field, CA: Responses of vernal pool plant species to variability in hydroperiod

Background/Question/Methods

Vernal pools are ephemeral wetlands which support a high diversity of native plant species. Individual pools have unique hydrology due to variation in factors such as soil depth, presence and integrity of a subsurface water restricting layer, and catchment area.  In addition, annual fluctuations in precipitation and evapotranspiration can differentially affect individual pool hydrology and vegetation. To understand how plant species respond to annual variability in pool hydrology, a detailed five-year study was conducted on the ecohydrology of hardpan vernal pools in California’s Central Valley. Leveloggers (highly sensitive pressure transducers) were placed in piezometers within three hardpan vernal pools to record water depth every 15 minutes during the entire wet season. Plant species presence/absence and percent cover were recorded each year along with detailed spatial information (position and elevation in pool) so that inundation periods could be correlated with species’ locations within pools.  

Results/Conclusions

Timing of precipitation had a large influence on pool inundation periods. Despite precipitation totals exceeding the previous season, late season rain in 2009 was insufficient to balance higher evapotranspiration and pool inundation periods were shortened by 19-28 days. The magnitude of species’ responses to hydroperiod variability differed among pools within the same time period. Perennial species such as Eleocharis macrostachya declined 25-97% while annual species such as Lasthenia fremontii increased 26-640%. These trends reversed over the following two years as rainfall occurred earlier and in higher amounts. Associations between species presence and pool elevation range did not change substantially despite large annual differences in inundation periods. Thus, when multiple years were compared species did not fall into consistent hydroperiod classes. These annual dynamics highlight the effects that climate change could have on vernal pool systems. Using a water balance model and climate change estimates for the next 100 years, the results of this study predict that pools in the Sacramento Valley could have longer inundation periods with a shift to more perennial species, while those in the San Joaquin Valley could have decreased inundation periods with more annual species and increased invasion by upland or non-native species.