Nonnative invasive plants are one of the greatest threats to natural ecosystems worldwide (Vitousek et al. 1996). In fact, their spread has been described as "a raging biological wildfire" (Dewey et al. 1995). Disturbances tend to create conditions that are favorable for germination and establishment of plant species. Nonnative plant species are often characterized as weeds, exotics, and invasives that can exploit such conditions (Rejmánek 1996) and many of them possess traits such as rapid germination, high fecundity, and effective means of seed dispersal (Stohlgren et al. 1998).