OOS 22-4
Invasive plant litter inhibits natives and facilitates further invasion in restored vernal pools
Vernal pools, or ephemeral wetlands, commonly found in regions with Mediterranean climates have been rapidly converted to agriculture and land development worldwide. Concerns over continuing rates of habitat loss are elevated as vernal pools often host many rare or endemic plant species specially adapted to a cycle of inundation and desiccation. In California, vernal pool ecosystems have the additional threat of encroachment by invasive plant species which are predominately invasive annual grasses. Many of these invasive species are physically larger than natives, and thus could alter litter accumulation and decomposition rates within vernal pool boundaries. Because of the potential for invasive species to alter litter depths and community composition our study asked: How does the presence of invasive species impact decomposition rates and native species abundance and richness in both restored and naturally occurring vernal pools? We investigated the effects of invasive litter through deploying litter bags and by manipulating litter depths in both restored and naturally occurring pools in a long-term study site in the central valley of California. Within these litter depth manipulations we conducted annual vegetation surveys at peak flowering to determine community composition and species abundance.
Results/Conclusions
We found that overall species richness declined as litter depths increased (P<0.0001). When broken up into native and invasive functional types the native species richness and abundance was drastically reduced with as little as 2 cm of litter, while invasive species abundance was maintained through depths of 7cm. When attempting to understand decomposition rates among invasive and native species we used litter bags filled with a prolific invasive grass (Lolium multiflorum L.) or a native vernal pool grass (Pleuropogon californicus L.). Results revealed that native P. californicus had a significantly higher decomposition rate (P<0.0001) than its invasive counterpart. Additionally, when the Lignin:Nitrogen (L:N) ratios for the most abundant 21 species were analyzed the dominant invasive annual grass species had the highest L:N ratio. Through litter bags and L:N ratios our findings demonstrate invasive grass species as having slower decomposition rates then native species. Overall, our results suggest that once invasive species are established in vernal pools, invasive litter is slower to decompose and reduces native plant abundance through higher litter depths. This effect appears to be reinforcing plant invasion, thus reducing the success of native plants and undermining restoration projects.