Methodological approaches for assessing ecosystem functional impacts from woody invasive species
Invasive plants are known to cause ecological disturbances, resulting in reductions in biodiversity and alterations of soil properties, among other impacts. For example, invasive plants can affect soil carbon-nitrogen ratios due to abundant biomass production and tissue chemistries. This pilot project sought to develop a suite of metrics to quantify ecosystem functional impacts due to assemblages of invasive shrubs and vines at a preserve near Mystic, Connecticut. We specifically aimed to quantify whether such assemblages were functionally similar to morphologically analogous extant native shrubs and vines. Therefore, we tested the relative utility of different metrics of plant community functional diversity, and both vertical and horizontal structure, including comparing visual and digital measurements of ground and canopy cover. We also experimented with various laboratory protocols for measuring soil carbon and nitrogen levels, among other edaphic parameters. Vegetation and soil sampling occurred across the site and its varying habitat types using transects, and stratified nested plots were used along a forest edge where invasive assemblages were being controlled to make functional comparisons at a finer scale. While the faculty investigator was primarily responsible for researching methods, undergraduate students played a critical role in testing and refining laboratory and field protocols.
Through this pilot project we determined that some plant community metrics were more valuable than others (e.g. the collection of biomass samples), and which were inefficient and not particularly informative in the context of functional differences. In particular, we found that ground cover estimation using digital image analysis was as accurate and 55% less variable than visual estimations; we also found that relative plant canopy density could be approximated using leaf area index measurements. Additionally, organic carbon levels in soils from the forest edge (mean = 11.8%) were significantly (P < 0.05) higher than those in grassland soils, but lower than those from the forest. Overall, our results suggested that by using selected metrics we could test for plant community functional impacts and soil nutrient cycling changes due to woody invasive species assemblages. While we encountered particular challenges carrying out laboratory soils analyses at our primarily undergraduate institution due to a lack of support for equipment maintenance, ultimately this type of collaborative and iterative research proved beneficial for both faculty and student researchers. Students especially gained valuable experience in applying ecological concepts, coordinating research efforts with each other, and integrating data from separate parts of the project.