COS 150-5 - Finding common ground: Assembly processes of native and exotic plants along a soil gradient

Abstract: What determines the abundance of introduced species in natural communities and the susceptibility of communities to invasion remains an unanswered question. Identifying and comparing the assembly processes of co-occuring native and introduced species is useful for answering these questions, as it can tell us whether exotic assemblages form through the same processes as native ones. The forest communities of Long Island (New York State, USA) are particularly useful for such a comparison. These forests range in composition from hardwoods to pine barrens, with a gradient of native composition and invasive plant abundance. Hardwood forests have mesic vegetation and variable levels of plant invasion, while the pine barrens have a stress-tolerant native flora and almost no invasive plants. This variation in composition is attributed to variation in environmental ‘harshness’, in the form of soil quality, a factor that has also been implicated in invasion resistance. Thus, this system provides a natural environmental and compositional gradient, making it ideal for studying the assembly of native and introduced assemblages via soil-based environmental filtering.

Using data from a 1998 forest inventory and soil survey along with resampling of the some of the sites in 2021-22, we quantified the responses of native and exotic assemblages to environmental filtering. We asked whether native and exotic composition changes similarly along soil gradients, and whether these patterns are consistent over time. Our methods involved a combination of constrained ordinations (to quantify relationship between taxonomic composition and soil quality variables), RLQ analysis (to identify the functional traits associated with variation in composition along soil gradients), and regressions on community-weighted trait means (to understand how functional composition changes with soil quality). Preliminary results indicate that the spatial variation in taxonomic composition of both native and exotic species is associated with soil nutrient levels, but the functional compositions (community-weighted trait means) of the two groups differ in their relationship with the soil environment. Specifically, native functional composition is correlated with soil nutrients and pH, but exotic functional composition does not show any such association. These results indicate that exotic species may not follow the same assembly rules as native species, and are thus less responsive to variation in soil quality than native plants. This contradicts our expectations but supports the conventional notion that exotic species, particularly invasive ones, are fundamentally different from co-occuring native species.