Shifts in traits of the invasive plant Sapium sebiferum and their effects on ecosystem carbon and nitrogen processes
Doctor of Philosophy
The evolution of increased competitive ability (EICA) hypothesis posits that release from natural enemies favors exotic plants evolving traits associated with faster-growth and lower herbivore-resistance in the introduced range. Given a trade-off between resistance and tolerance, decreased resistance may translate into increased tolerance to herbivory by invasive plants compared to conspecifics in the native range. These genetic shifts in traits of invasive plants may also have effects on soil and ecosystem processes. To test the EICA hypothesis, we conducted experiments in the native range of Sapium sebiferum using seeds from populations in the native Chinese range (native ecotype) and the invasive North American populations (invasive ecotype). Plants from invasive populations showed greater competitive ability, reduced resistance but increased tolerance to herbivory than those from native populations of Sapium sebiferum when they were competing against each other in the presence of native herbivores. Despite that native specialist beetles Aphthonomorpha collaris preferred invasive populations over native populations of S. sebiferum when beetles had a choice between them in a bioassay trial, plants from invasive populations compensated for leaf damage more efficiently than those from native populations. These results suggest that invasive S. sebiferum has become a faster-growing, less herbivore-resistant and more herbivore-tolerant plant in the introduced range compared to conspecifics in the native range. A greenhouse study showed that invasive North American populations of S. sebiferum differed genetically from native Chinese populations in most plant functional traits. Of 13 measured plant variables, the root to shoot ratio (RSR), total leaf area (TLA) and net CO2 assimilation (A) were identified as powerful traits that contributed the most to the genetic difference between native Chinese and invasive North American population types. These shifts in functional traits resulted in higher soil-plant system CO 2 and N2O emissions for invasive ecotypes than for native ecotypes of Sapium in an outdoor pot experiment. Compared with native ecotypes, invasive ecotypes accelerated soil carbon and nitrogen processes and promoted more nitrogen uptake through soil-plant direct interactions. The results of this study suggest that shift in traits of invasive plants and their effects on ecosystem processes may have implications for their invasiveness.