Southern California grasslands have largely been type-converted to dominance by exotic annual grasses, leading to displacement of many native grass and forb species. Crimson fountaingrass, Pennisetum setaceum, an exotic perennial C4 species and a relatively new invader to California, is expanding to areas currently occupied by purple needlegrass, Stipa pulchra, a C3 native. We predicted that fountaingrass seedlings might withstand cool season competition in California’s Mediterranean-type climate and establish in Stipa pulchra grasslands due to less competition during the warm, dry summer season, and that interactions might be influenced by density. A field experiment was conducted to examine competitive interactions of the two species from the cool winter season to the warm summer season. As predicted, Stipa produced greater aboveground biomass in the cool season and showed strong intraspecific competition, as well as interspecific suppression of Pennisetum growth, whereas Pennisetum showed no suppression of Stipa. In the warm season, Stipa showed relatively less suppression of Pennisetum, erasing significant differences, and Pennisetum showed increased growth. Results of this study show that C3 Stipa can suppress initial growth of C4 Pennisetum in the cool season, but in warmer months, Pennisetum can overcome this initial suppression at both low and high densities, even within a Mediterranean-type climate with little to no summer rainfall. Thus, in southern California, temporal niche partitioning due to photosynthetic pathway in these two species can allow Pennisetum invasion. Given the similarity in life history and growth form of Stipa and Pennisetum, few options exist for controlling Pennisetum in habitats where Stipa occurs. In these cases, restoration plantings of desirable species are essential in order to reestablish competitive vegetation that will be more resistant to invasion.
Nomenclature: Crimson fountaingrass, Pennisetum setaceum (Forsk.) Chiov.; purple needlegrass Stipa pulchra Hitchc. (synonym, Nassella pulchra (Hitchc.) Barkworth).
Management Implications: Fountaingrass is a warm-season exotic invasive grass that is adapted to extensive areas of southern California where it is able to respond to moisture with opportunistic growth year-round. Remnant native Stipa pulchra communities occur in proximity to populations of invasive Pennisetum setaceum in several coastal and inland areas in southern California. The goal of this study was to investigate establishment-stage competition of Pennisetum and Stipa. In this experiment, Pennisetum was competitively suppressed by Stipa in the early spring portion of the experiment, but ultimately thrived and set seed during the latter summer portion of the study. Results of this study are relevant to first-year grassland invasions, especially in disturbed or patchy areas where recruitment can occur largely by seed. For disturbed grassland areas with a seedbank of Stipa, newly-arrived seedlings of Pennisetum can experience suppression by Stipa in the cool season but have an opportunity to thrive later in the season, once competition from Stipa is reduced. The similar respective life histories and growth forms, as well as the overlap in phenology of these two grasses as seen here does not appear to provide a sufficient window of time to control of Pennisetum using chemical methods without injury to Stipa. Planting competitive summer annual species in these areas might provide additional competitive suppression of Pennisetum as a component of proactive management programs. With relevance to natural systems in southern Ca
