Plant and insect-mediated invasiveness of Phragmites australis and the litter dynamics and biodiversity of six freshwater macrophytes
My thesis involves two distinct projects related to wetland plants. The first evaluates plant traits for their contribution to the success of invasive Phragmites australis in North America and their interaction with herbivores. The second investigates the relative effects of six plant species, with different growth forms, status (native v. nonnative) and tissue quality, on litter dynamics and invertebrate diversity in a New York freshwater wetland. Prevention is the most cost-effective and successful means of managing invasive plants. Predicting future invasions depends on identifying plant traits that facilitate invasive success. We investigated the influence of above-ground growth phenology and increased stem height on the success of invasive Phragmites australis in North America, using a phylogenetically-controlled comparison with a native, non-weedy P. australis subspecies. We also measured the effects of specialist stem-galling Lipara flies and a generalist aphid (Hyalopterus pruni), both nonnatives to North America, on these above-ground traits. Comparisons were made in 1) a common garden at Cornell University, Ithaca, NY, in 2003 and 2004, and 2) a field site at Montezuma National Wildlife Refuge, Seneca Falls, NY in 2003. In the garden, but not the field, nonnative P. australis leaves remained green for about a month longer (native v. nonnative: 2003 = 59.93 v. 85.5 days, P = 0.0002; 2004 = 52.29 v. 87.39 days, P = 0.02). For nonnative P. australis, leaves of the upper canopy consistently lived longer while leaf lifespan in the lower canopy was shorter or the same. Greater investment in high canopy leaves may increase carbon gain efficiency of nonnative P. australis. Nonnative P. australis grew taller in the field but this was mediated by disproportionate Lipara attack rather than plant status (native or nonnative). Lipara attack reduced stem height of all stems but only increased the lifespan of nonnative P. australis? low canopy leaves. Aphids had no significant effect on measured plant traits. Through increased carbon gain, leaf phenology may contribute to P. australis? competitive superiority over its native conspecific. Higher susceptibility of native P. australis to nonnative herbivores may also facilitate nonnative P. australis? competitive superiority. Senesced plant litter from emergent macrophytes fuels freshwater wetland productivity and nutrient cycling. Litter nitrogen content generally has a direct, positive effect on quantity and rate of resource availability to wetland biota. Since plants vary in their nitrogen content, shifts in plant community composition may alter important wetland functions. To study the consequences of changing plant dominance, we compared litter mass loss and invertebrate richness and abundance of six common macrophytes in a central New York freshwater wetland. Plants studied include Typha latifolia L. (broad leafed cattail, Typhacea), T. angustifolia L. (narrow leafed cattail), Phragmites australis (cav.) Trin ex. Steudel (common reed, Poaceae), P. australis subspecies americanus Saltonstall, P.M. Peterson & Soreng, Lythrum salicaria L. (purple loosestrife, Lythraceae), and Phalaris arundinacea L. (reed canarygrass, Poaceae). After nine months, mass loss of most plant species diverged significantly. Plant effect on invertebrate colonization was season and species-specific, with P. arundinacea almost consistently supporting higher invertebrate densities. Although %N differed among some plant species, it was not a good predictor of mass loss or invertebrate abundance and richness. Including plastic drinking straws as a treatment revealed that several invertebrates used litter for substrate rather than food. We conclude that shifts in plant dominance among the six wetland macrophytes investigated could potentially alter wetland function, by changing decomposition rates and the invertebrate community. Net quality of litter resources, which depends on the combined influence of morphology, chemical quality of specific plant organs, and feeding ecology of specific taxa, may be a better predictor of species effects on decomposition and diversity.
Phragmites australis; plant invasion; invasive success; litter dynamics; decomposition; invertebrates; Typha angustifolia; Lythrum salicaria; Phalaris arundinacea; litter bags; Typha latifolia; Phragmites australis americanus