Phenotypic plasticity, precipitation, and invasiveness in the fire-promoting grass Pennisetum setaceum (Poaceae)

Invasiveness may result from genetic variation and adaptation or phenotypic plasticity, and genetic variation in fitness traits may be especially critical. Pennisetum setaceum (fountain grass, Poaceae) is highly invasive in Hawaii (HI), moderately invasive in Arizona (AZ), and less invasive in southern California (CA). In common garden experiments, we examined the relative importance of quantitative trait variation, precipitation, and phenotypic plasticity in invasiveness. In two very different environments, plants showed no differences by state of origin (HI, CA, AZ) in aboveground biomass, seeds/flower, and total seed number. Plants from different states were also similar within watering treatment. Plants with supplemental watering, relative to unwatered plants, had greater biomass, specific leaf area (SLA), and total seed number, but did not differ in seeds/flower. Progeny grown from seeds produced under different watering treatments showed no maternal effects in seed mass, germination, biomass or SLA. High phenotypic plasticity, rather than local adaptation is likely responsible for variation in invasiveness. Global change models indicate that temperature and precipitation patterns over the next several decades will change, although the direction of change is uncertain. Drier summers in southern California may retard further invasion, while wetter summers may favor the spread of fountain grass.