Vernal pools are seasonal wetlands that fill during the winter and dry in the summer. The exotic species that dominate California grasslands are poorly adapted to this seasonal inundation and are mostly limited to pool edges, resulting in ‘islands’ of native flora. Within each pool, distinct vegetation zones emerge across small differences in topography. These zones include the bottom (longest inundation period), transition (moderate inundation), and upland (no inundation). Maintenance of vernal pool flora is a high conservation priority, and livestock grazing has been shown to increase plant diversity within individual pools, particularly in transition zones. However, a large amount of diversity is also distributed across pools, which can host very different communities due to hydrology, chemistry and topography. It is important to know whether grazing reinforces this across-pool spatial heterogeneity or if it makes the composition of pools more similar. To address this, I compared pools that have been grazed continuously and pools that have been fenced off for over 40 years at a site in Sacramento County. For two years, I sampled vegetation in 15 grazed and 15 ungrazed pools, and calculated richness and abundance. PermANOVA and PermDISP were used to compare alpha and beta diversity across treatments.
Results/Conclusions
Similar to past studies, grazing tended to increase alpha diversity of vegetation, although the effect varied across year and pool zone, with the transition and upland zones having a more consistent response to grazing than the pool zone. Native forb species richness and cover were higher in grazed transition and upland zones for both 2015 and 2016. Shannon Weiner diversity of all vernal pool plants (species richness weighted by abundance) was higher in grazed vernal pools in 2016 for all zones, while in 2015, grazing only led to higher alpha diversity in upland zones. While within-pool diversity tended to increase with grazing, grazing tended to make vegetation composition more similar across pools. In the transition zones, between-pool (beta) diversity was lower in grazed pools compared to ungrazed pools. This study highlights that management efforts focused on increasing alpha diversity, may also decrease the variability found across the landscape.