Date of Award
Master of Science
Invasive species, including Lespedeza cuneata (Dum. Cours.) G. Don, threaten the success of grassland restoration through their ability to alter species composition following invasion. Combining the filter framework model of community assembly with the passenger-driver model of non-native species behavior can clarify the broader impacts of invasive species in the communities they invade. Testing this combined theory by exploring the mechanisms by which an invasive functions (above- and belowground) and the response of native plants from different functional groups to these mechanisms can reveal if an invasive functions as a driver and a filter. Observational field surveys were conducted across fifteen L. cuneata-invaded grasslands to compare species composition of plots in which L. cuneata was either present or absent. In a greenhouse experiment, the response of natives in three functional groups—grasses, forbs, and legumes—to aboveground (competition), belowground (soil conditioning), and both above- and belowground interactions with L. cuneata were investigated. Response variables (height, leaf number, root length, biomass, specific leaf area, leaf chlorophyll, and soil pH and conductivity) were measured. Regional analysis of the field surveys did not identify distinct species compositional differences in invaded and uninvaded areas, while within-site analyses revealed differences in half of the sites, suggesting that site specific characteristics could be impacting whether L. cuneata presence corresponds with a fundamental shift in species composition. Regionally, grass and legume abundance was higher in plots not containing L. cuneata than in plots where it was present. The greenhouse experiment illustrated stronger aboveground competitive effects than belowground soil effects, with impacts differing among functional groups. Under pressure of competition with L. cuneata, grasses experienced increases in certain measured traits (height, root length and specific leaf area), while when grown in conditioned soil, grasses produced greater biomass. Additionally, the growth of L. cuneata differed when grown in competition with forbs, grasses, and legumes. Competition with legumes resulted in the most suppressed growth of L. cuneata individuals among the three functional groups. Considering the field survey and greenhouse experiments in conjunction suggests that grasses can benefit from interactions with L. cuneata and have the potential to outcompete and exclude it, while legumes are detrimental to the growth of L. cuneata and compete for niche space, resulting in L. cuneata establishment in legume-poor areas. The forb functional group did not have a significant relationship to L. cuneata presence in either the field surveys or the greenhouse experiment. Lesepedeza cuneata acts as a driver, altering the abiotic and biotic filters to impact species composition, while it does not act as a filter, with native grasses and legumes acting to filter the L. cuneata.
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