Forest tree biodiversity is correlated with nonnative plant invasion on inventory plots across the Southeastern United States

Background/Question/Methods

Biodiversity may convey numerous functional benefits to forested ecosystems, including the ability to reduce susceptibility to invasion. Identifying indicators of biodiversity associated with susceptibility or resistance to nonnative plant invasion would be beneficial to forest managers. Phylogenetic diversity statistics weight species based on their evolutionary distinctiveness, unlike traditional measures of biodiversity which weight species equally. Phylogenetic community structure statistics quantify the degree to which co-occurring species are clustered on the phylogenetic tree of life. Both types of metrics have relevance for quantifying forest susceptibility and resistance to non-native plant invasion across broad scales. Biodiversity (including phylogenetic diversity) may reduce the susceptibility of ecosystems to such invasion. Phylogenetic community structure, meanwhile, may indicate whether forest communities are vulnerable to invasion. For example, communities of trees that are evenly dispersed across the phylogenetic tree of life should encompass a greater variety of ecological traits, and may therefore be less susceptible to invasion. We describe the associations between several evolutionary diversity metrics and invasive species diversity and cover on approximately 39,000 Forest Inventory and Analysis (FIA) plots across the Southeastern United States, inventoried by the USDA Forest Service.

Results/Conclusions

In general, we found positive relationships between each of the forest tree evolutionary metrics and the degree to which FIA plots were invaded, but this varied across the Southeastern United States. Region-wide, forest tree evolutionary biodiversity was higher on plots that also had invasive plants, and plot-level “invadedness” was positively correlated with evolutionary biodiversity. Among the biodiversity metrics, plot invadedness was most strongly correlated with phylogenetic diversity. These results suggest that, rather than conferring resistance to invasion, forest tree biodiversity in many parts of the Southeast may instead indicate the presence of better environmental conditions for invasive plants. Biodiversity, however, was not well associated with invadedness on better developed stands. In upland areas, meanwhile, plot invadedness was negatively correlated with biodiversity. It is important to note that the inventoried invasive plants were primarily forbs, grasses, shrubs and vines; the relationship between native understory plants and plot-level invasion may be different. Another important consideration is that not all plots were equally exposed to invasive plant pressure. Future work will (1) consider the influence of environmental and socioeconomic factors, (2) assess the relationship between plot invadedness and understory vegetation, (3) analyze the relationship between plot invadedness and forest community trait diversity, and (4) assess the relationship between change in invadedness and change in biodiversity over time.