Long Term Evaluation Of Characteristics In An Artificial Northern California Vernal Pool System PDF Download

Vernal pools are ephemeral wetlands that form in depressions underlain with soils that include a water-restricting horizon. These depressions fill with water during the winter in Mediterranean-type climates, and then rapidly dry during the late spring. Vernal pools are diminished in California, due to habitat loss from land-use conversion. The biota endemic to vernal pool habitats includes many rare and endangered species. Creation of vernal pools has been used as mitigation since the mid 1980s. Whether creation can reliably result in vernal pools with the range of biotic characteristics found in natural vernal pools remains unclear. Evaluation of created vernal pool attributes that reflect ecological function over the long-term is needed to inform management of existing mitigation projects and the planning of future projects. The vegetation, hydrology, soils, and plant communities of created vernal pools were compared with natural vernal pools at three mitigation sites in the Great Valley of California that differed in climate and substrate. Vegetation attributes and parameters of hydrologic regime were measured in the first years following construction, and again when the created pools at each site were 12-14 years old. Soil samples were taken during the last year of the study to assess basic soil characteristics. Finally, plant communities of created vernal pools were identified during the last two years of the study period and compared with natural vernal pool communities. The values of vernal pool species attributes increased with increasing maximum water depth and period of inundation. Fluctuations and downward trends in the values of vernal pool species attributes correlated with increased exotic species cover, and exotic species cover was highest in years of lowest annual precipitation. Trends of decreasing maximum water depth, inundation period, and vernal pool species cover, as well as increasing exotic species cover, correlated with release from cattle grazing at two sites. At a third site where light-to-moderate grazing occurred, vernal pools species attributes of created pools matched or were higher than those of natural pools, and maximum water depth was higher in created than in natural pools throughout the study period. Thus, grazing may be of primary importance in the ability of created vernal pools to maintain hydrologic function and vernal pool species attributes over the long-term. Soil texture, color, bulk density, and redox concentrations did not differ between created and natural pools. However, differences in soil pH and depth to hardpan were found between created and natural vernal pools. Natural vernal pool plant communities were found in created pools at two sites, but were those of deeper, longer-inundated habitats compared to the natural pools on-site. This may lead to a greater proportion of deeper, longer-inundated vernal pool habitats over time with continued use of creation as mitigation. Only one natural vernal pool community was identified at one site, where most created pools supported exotics-dominated, non-wetland communities. Site characteristics and management practices of mitigation projects lead to differences in outcomes in ecological function in created vernal pools.