Our results indicate the invasibility of riparian plant communities is driven by a combination of factors that determine the success or failure of invasive species establishment—most notably hydrology and temperature. A major objective of this study was to examine the environmental variables that are likely to be altered in future climate change scenarios and to relate these variables to our indicators of diversity and productivity at our sites. We found significant linear relationships between accumulated heat as measured by soil growing degree days (SGDD) and various facets of hydrology (flood power, duration, depth, frequency) and all of our diversity indicators (Shannon diversity, species richness, invasive abundance, and invasive biomass) indicating temperature is a significant driver of community composition at our sites. To assess the predictive power of the environmental variables that we monitored in this study, we developed a series of multivariate models that rank the importance of environmental forcing functions in structuring emergent plant communities. We consistently found the most important environmental predictors of our community indices were those that described aspects of temperature and hydrology with variables describing local soils and watershed land use characteristic typically being less important. Relationships between temperature and species richness have been studied since the founding of ecology as a discipline (Merriam 1894). The relationships between species richness and temperature we see in our study (Figures 1 and 2) are consistent with those seen in classic studies of biogeography where links are found between species and latitude. In recent years, researchers are seeing indications of latitudinal shifts of species distribution in response to climate change (Allen et al. 2002; Walther et al. 2002; Parmesan 2003; Clarke and Gaston 2006). The diversity-temperature trends observed in our study echo the diversity latitude trends seen in many studies of global scale biogeography (where higher latitude is equated with lower temperature). The richness of naturalized alien species decreased as one moves the mid-latitude temperate zone to sub-tropical and finally tropical zone (Holdgate 1986; Sax 2001; Pyšek and Richardson 2006).

At regional and global scales, Shannon diversity is strongly correlated with the number of species (Gentry 1988). This trend is opposite to the observed tendency for overall community species richness where the highest species richness is found in the tropics and decreases as one moves toward the poles (Fischer 1960; Pianka 1966; Stevens 1989).

Future climate scenarios predict alterations of both temperate and hydrology in southeastern riparian plant communities (IPCC, 2001). We found that environmental factors consistently explained half of the variation seen in plant community structure seen at our sites, and variables related to temperature and hydrology were consistently the most important predictor of community composition and diversity.

The invasive species most dominant at our sites are those that originate at high latitudes and are able to tolerate a wide range of climatic conditions. It is likely that the propagules of these species originate in the cooler conditions found in the mountainous headwaters of our study rivers. Thus, the invasive species at our study locations appeared to have a greater competitive advantage at our cold sites most likely due to their broader ecological temperature tolerance.

Agricultural land use was also a significant predictor of community composition at our sites (“Ag” in Figure 2). Thuiller (2007) found that land use and climate change were the most important drivers affecting biodiversity. Thus, management of land use may be a potential tool for mitigating the effects of climate change on hydrology and the sources of invasive species propagules (Miyawaki 2004). In addition, management of watershed land use may be an important tool to offset future increases in water temperature due to shading and effects on temperature, runoff during storms, and groundwater interactions in riparian zones (LeBlanc et al. 1997; Hester and Doyle 2011). 

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