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The Effects of Nutrient Pulses and Community Diversity on Invasive Plant Success in Virginia

Heckman, R.W. 2008. M.Sc. Thesis. Graduate Faculty, University of Virginia, VA

Abstract

What makes a community susceptible to invasion? Two commonly cited mechanisms are nutrient pulses and community diversity. In both of these, biological differences between native and invasive species can shift the community's competitive balance, especially when one group more effectively utilizes a nutrient pulse. These differences may play a role in invasions into a variety of habitats includ ing deciduous forests and tallgrass prairie. As a biennial Alliaria petiolata (Garlic mustard, Brassicaceae) is active in deciduous forest understories during autumn and win ter, allowing it to utilize nutrient pulses from leaf-litter before its competitors. I m anipulated the timing of leaf-litter addition (fall or spring) and simulated the nutrien t pulse from decaying leaves with artificial fertilizer addition. Fall nutrient-amend ed plots had greater adult biomass than plots receiving spring nutrient additions. In addit ion, silique production per plant and bolting stem was highest in fall litter-amended plo ts. Bienniality may provide a unique opportunity for A. petiolata to spread through deciduous forest understories. Tallgrass prairie is dominated by high nutrient-us e-efficiency warm-season (C4) grasses and is frequently invaded by less nutrient- use-efficient cool-season (C3) forbs. Physiological differences in phenology and nutrient -use efficiency may explain the invasion success of Galium verum (Rubiaceae) , a European C3 forb, in an experimental meadow at Blandy Experimental Farm. Within each of three warm-season grass diversity levels (one, three or five species), I performed fo ur nutrient manipulations: 1) depleted nitrogen (excess carbon introduction), 2) ambient n itrogen, 3) elevated nitrogen (fertilizer ii addition) and 4) fire (early-season burn). I found no effect of native grass diversity on the abundance of exotic Galium, nor did grass diversity significantly alter soil NO3 or NH4 . Elevated nitrogen treatments produced significantly higher Galium biomass and stem density than depleted or ambient nitrogen treatment s. The fire treatment significantly reduced Galium biomass relative to all other treatments. As predi cted, soil plant-available nitrogen drove the pattern of Galium abundance, but early season fire inhibited Galium despite the nutrient release afterwards. Biological differences between exotics and natives in these systems may allow the invasive species to exploit nutrient pulses more effectively and expand their ranges.