Differential invasion of a wetland grass explained by tests of nutrients and light availability on establishment and clonal growth.
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Phalaris arundinacea (Poaceae) is aggressively invading wetlands across North America. We tested the hypotheses that open canopies and increased nutrients facilitate vegetative establishment in the field, using a phytometer (6 rhizome fragments/plot, 24 plots/wetland). In each of three wetlands, phytometers received three levels of an NPK fertilizer or served as controls. Emergence and survival differed among sites (P=0.0005), but not due to NPK addition. P. arundinacea survival was highest in a wet prairie with a late-developing canopy, but limited by prolonged flooding in one sedge meadow and by an early-growing, dense plant canopy in a second. These patterns were explained in greenhouse experiments, where both flooding (P<0.0001) and heavy shade (P=0.0002) decreased P. arundinacea aboveground biomass by up to 73% and 97%, respectively. Rhizome fragment survival was reduced by 30% under flooded conditions and 25% under heavy shade. We then tested the hypothesis that a clonal subsidy facilitates vegetative expansion into heavy shade. Established clones were allowed access to bare soil under four levels of shade and two levels of NPK fertilizer in a two-factor greenhouse experiment. Young ramets attached to parent clones readily grew into heavy shade, and the high nutrient treatment increased aboveground growth (P<0.0001) and distance of ramet spread (P=0.0051) by nearly 50%. Under low nutrient conditions, root biomass increased by 30% (P<0.0001). P. arundinacea's rapid expansion into a variety of wetland types is likely a function of clonal subsidy, morphological plasticity, and nutrient availability: young ramets that emerge under shaded conditions are supported by parental subsidies; where nutrients are plentiful, P. arundinacea can maximize aboveground growth to capture more light; and where nutrients are scarce, it can increase belowground foraging.
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