Invasion of perennial exotic fountain grass (Pennisetum setaceum), in a Mediterranean scrub system

Background/Question/Methods

Fountain grass (Pennisetum setaceum) is a perennial C₄ African bunchgrass that is invasive outside its native range and spreading in wildlands in Hawaii and the Southwestern U.S. This species is increasing in California, but little is known about potential habitat suitability or impacts. A climate-matching model based on abiotic characteristics of the native range showed that several habitats in California are suitable for fountain grass establishment, including coastal sage scrub, grassland, and deserts. In order to determine the potential impact of fountain grass invasion on native communities, coastal sage scrub sites in two regions in southern California were analyzed during two growing seasons (2009 and 2010) for cover of native and exotic species, using replicated plots containing 4 cover classes of fountain grass. Soil samples from low- and high-cover areas of fountain grass were analyzed for nutrients. 

Results/Conclusions

Significant reductions in native species cover were found with increased fountain grass cover in both years in Santa Monica Mountains (SAMO) sites and in 2010 in San Diego sites. Native species richness reductions were found with increasing fountain grass cover in both years in SAMO sites but not in San Diego sites. In soil samples from high fountain grass cover areas, nitrate was higher than in low fountain grass cover areas in both years and regions. High fountain grass cover areas also had significantly higher potassium in San Diego in 2009 and SAMO in 2010, and significantly higher percent water content in both regions in 2010. No differences were found in ammonium, phosphorus or percent rock by weight between low- and high-cover areas. Principal Components Analysis (PCA) of abiotic variables associated with sites showed a significant separation by region; SAMO sites were characterized by less surface area covered by rock, but rockier and drier soil, older fires, lower soil phosphorous, and differences in geographic aspect. These site differences, particularly in water content, may indicate a limiting resource that could change or mitigate the impact of fountain grass on native species, and should be investigated further.