Environment is a stronger determinant of exotic plant feedbacks to soil than vegetation type in southern California ecosystems

Background/Question/Methods

Impacts of exotic plant invasion on soil chemical and biological characteristics have been studied for decades, but generalizations across ecosystems remain difficult to make. Previous work has been limited to examination of a few species in one system or one species in multiple systems resulting in species- and site-specific conclusions. This project aimed at addressing the impacts of a suite of exotic species invading multiple ecosystem types. In a comparison of three vegetation types, we hypothesized that systems with plant traits most similar to the invading species would be least altered by invasion but that all ecosystems would experience alterations caused by the novel traits of invading plant species. In addition, we hypothesized that soils would be resilient to these impacts and therefore removal of exotic plants would lead to chemical and biological recovery of soils. We utilized a meta-analytical approach to compare responses to invasion by perennial California grasslands, coastal sage scrub and chaparral shrublands that were all invaded by exotic annual grasses. The responses included soil chemical pools and cycling rates and microbial community structure.

Results/Conclusions

Invasion of exotic annuals altered soil NO₃⁺ availability regardless of the native vegetation type. Coastal sage scrub systems experienced the greatest impacts to soil chemistry whereas grasslands experienced altered microbial community composition. Contrary to expectations, chaparral, which had plant traits that were most different from invading species, was least impacted by annual grass invasion even though the invasion constituted a vegetation type conversion from shrubs to grasses. Changes in chaparral soils as a result of annual invasion are most likely due to differences in the spatial distribution of woody vs. herbaceous roots. While vegetation type represented by exotic plant presence or absence affected the soils at each site in this study, abiotic soil and climatic factors had a stronger role in determining soil chemical and biological characteristics than vegetative inputs alone.