Anthropogenic degradation of wetlands often leads to regional biotic homogenization and reduced plant diversity. This reduction is often attributed to the proliferation and dominance of a few generalist, often non-native, species. Biotic resistance from natives can sometimes impede the growth and spread of colonizers, but its dependence on environmental conditions is poorly understood. Based on field and modeling studies, we tested the predictions that (1) biotic resistance declines at higher nitrogen loading and (2) size influences colonization success. In a five-growing season mesocosm experiment, we grew three cattail taxa: Typha latifolia (native, large), Typha angustifolia (non-native, invasive, smallest), and Typha × glauca (hybrid, most invasive, large) as potential colonizers in the presence or absence of pre-established resident vegetation. At two sites differing in climate and growing season length, biotic resistance treatments were crossed with 12 nitrogen levels (inflows 0-45 g N m-2 year-1). Each treatment combination was replicated twice, totaling 48 mesocosms per site. Without residents, colonizers (as total biomass of all three cattail taxa) persisted and expanded clonally across all nitrogen levels. However, their expansion was generally lower when colonizing a pre-established resident community compare to bare ground. The magnitude of biotic resistance, measured as the effect of residents on colonizers' biomass, and its interaction with nitrogen differed between sites. As predicted, biotic resistance decreased with high nitrogen at the northern site, but at the southern site, residents nearly eliminated colonizers. As anticipated, smaller T. angustifolia was a poorer colonizer than the other taxa, while T. × glauca was the strongest colonizer, especially under high nitrogen conditions where biotic resistance was minimal. Our findings partially support the hypothesis that biotic resistance declines with nitrogen loading, indicating that additional research on the factors influencing the magnitude of biotic resistance is needed. Importantly, when combined with our finding that Typha can persist at all nutrient levels when natives are absent, this information could help identify wetlands particularly vulnerable to invasion, especially in environments experiencing concurrent nutrient enrichment and disturbances that expose bare ground.
Keywords: Laurentian Great Lakes; Typha; clonal plants; competition; eutrophication; invasion dynamics; nutrient input.
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