Does morphological plasticity of the Phalaris arundinacea canopy increase invasiveness?

Abstract

Morphological plasticity could facilitate invasions of wetland plants into areas that experience increased durations of flooding and eutrophication. We explored canopy plasticity of Phalaris arundinacea, an aggressive invader of wetlands, as it differentially invaded wet prairie mesocosms under 3 flooding durations and 3 levels of nutrient addition. Phalaris grew as a sward with intermittent and early-season flooding but shifted to tussocks under constant flooding. These two growth forms differed by >20% in several canopy ratios. Clones that formed tussocks produced 45% more shoots per unit biomass (P = 0.007) and a 25% higher ratio of total shoot length to biomass (P = 0.04). Lighter-weight shoots supported 33% fewer leaves and, consequently, had 35% less leaf area per shoot height (P < 0.002). Tussocks developed a continuous mat of adventitious roots, with root mats reaching 20.9 ± 0.6 cm in diameter and 4.7 ± 0.3 cm in height over two growing seasons. While forming tussocks, Phalaris tolerated longer durations of flooding and more than doubled its aboveground biomass. Invasions occurred rapidly, with Phalaris exceeding 75% canopy cover and accounting for 66% of the total aboveground biomass under constant flooding. Early-season flooding increased the lateral spread of individual shoots. High nutrient addition produced shoots that were 27% taller and 50% heavier (P < 0.02), with 81% more leaf area (P < 0.0003) than shoots that received no nutrients. Consequently, under early-season flooding with high nutrient additions, Phalaris was primed to invade, nearly doubling its proportion of the total aboveground biomass and exceeding 50% canopy cover during year two.