Metal-hyperaccumulating plants such as Thlaspi caerulescens (Brassicaceae) can uptake extraordinary amounts of heavy metals into their shoots, and have been proposed for use in the clean up of metal-contaminated soils. This type of phytoremediation often involves the introduction of a non-native plant, which may affect the native community, for example, through interspecific competition with native plants. In this experiment, I examine whether T. caerulescens, a zinc- and cadmium-hyperaccumulator native to southern France, competes with Thlaspi montanum var montanum, a congener that is native to my study sites in southwestern Colorado. These sites consist of three abandoned silver mines, where T. montanum has naturally colonized the edges of the metal-contaminated mine tailings. Therefore the results from this study would provide insights into whether phytoremediation of these sites may impact a native plant population. Using field-collected T. montanum seeds and substrate, I conducted a greenhouse competition experiment using a response surface design to vary the total density and the proportion between species independently. I measured initial plant size, weekly growth, and reproductive effort of the plants, metal contents of the field substrate, and metal contents of the above-ground plant biomass.
Results/Conclusions
The two species showed different life history strategies, where the native T. montanum, a perennial, invested in vegetative growth and T. caerulescens, a biennial, invested in reproduction. Analyses of the plant performance and substrate characteristics support previous findings that T. caerulscens is intolerant of copper, producing the fewest seeds at the sites with the highest soil copper levels. Where they did grow well, T. caerulscens accumulated cadmium and zinc, and T. montanum accumulated nickel. Competition coefficients estimated for each species from hyperbolic competition model indicate that T. caerulescens has a negative effect (c >1) on the performance of T. montanum at adjacent and far locations, whereas T. montanum have beneficial, non-significant, or negative effects on T. caerulescens depending on the site and location. These results suggest that, if introduced for phytoremediation, these plants can reproduce rapidly, and may be able to out-compete native plant populations.