Abstract
Invasive plants continue to threaten native plants throughout California. In the Central valley of California invasive grasses have almost completely replaced the native grasses and forbs that once dominated the region. These grasses form a persistent thatch that has been observed inhibiting the growth and germination of native plants in vernal pool complexes. In this paper I test the hypothesis removing invasive grass thatch in vernal pools will result in an increase in native species abundance and diversity and a decline in invasive plant abundance and diversity in a single flowering season at Mather Regional Park in Rancho Cordova California. I also test for a relationship between water depth and species composition to examine the longstanding belief that inundation is the primary factor limiting the success of invaders in vernal pools. To test the effect of thatch removal I established seven matched plots, each one m2 in area, and randomly assigned them to be the treatment and controls for this study. An additional six plots free of invasion were also chosen for comparison. Thatch was removed during the fall season and vegetation was monitored the following spring. I surveyed the plots eight times during the flowering season in order to account for changes in species composition over time and measured winter. I identified each species and assigned it a percent cover value based on the Daubenmire method. Water depth measurements for each of the plots at the peak of the ponding season using a standard ruler. I used randomized block ANOVAs to test whether there were significant differences in; invasive and native species percent cover, native and invasive species richness, invasive grass and forb percent cover, invasive forb richness, total percent cover, total richness, and percent cover for the four most common species (Plagiobotrys stipitatus, Ranunculus bonariensis, Hypocharis glabra, Erodium botrys) between the three treatment types. I used linear regression to test for relationships between water depth and three dependent variables: native and invasive cover and percent cover of Hypocharis glabra.
Significant differences between the three treatment types were found for native and invasive species richness, invasive forb richness, native and invasive cover, invasive forb cover, cover of Ranunculus bonariensis, Hypochaeris glabra, and Erodium botrys. Significant, strong correlations were found between water depth and both native cover and invasive cover. No relationship between Hypocharis glabra and water depth was found. These results suggest that native and invasive sp~cies composition is affected by both inundation and the accumulation of dead litterfall from invasive grasses.
Additionally, these results suggest that removal of invasive grass thatch in could lead to restorationof native species in compromised .
habitats pools