Evaluating Proactive Cultural Methods And Herbicide For Control Of Invasive Vegetation In Freshwater Wetland Mitigation

Wetlands are disproportionally susceptible to invasions by non-native plant species which degrade ecosystem functions. Frequent disturbances associated with wetland creation and restoration facilitate the spread of invasive propagules, thus challenging invasive species’ performance standards and the efficacy of compensatory wetland mitigation. Traditional invasive management methods, such as the use of non-selective herbicides, are often ineffective yet widely exercised to control invasive vegetation, like Arthraxon hispidus (joint-head grass) and Phalaris arundinacea (reed-canary grass). Given the regulatory mandates under Section 404 of the Clean Water Act pertaining to invasive species in mitigation wetlands, there is significant demand for effective control strategies of invasive plants while promoting native vegetation communities.We conducted a field experiment at a wetland mitigation site in Catlett, Virginia, over two growing seasons (GS-1 and GS-2) to evaluate the efficacy of various cultural management strategies (defined as ecologically informed approaches that modify environmental conditions or manipulate ecosystem processes to control invasive species) alongside traditional herbicide treatments in controlling Arthraxon and Phalaris. Our experimental design replicated management- related disturbances within 1.5m2 plots by utilizing mowing and tilling, followed by broadcast seeding of a standard volume native herbaceous wetland seed mix. Stress-inducing treatments included high carbon-to-nitrogen (C:N) ratio soil amendments using wood chips to induce a nitrogen limitation, soil amendments with aluminum sulfate (Al₂(SO₄)₃) to reduce phosphorus availability, plot shading via overhead canopy structures to reduce photosynthetically active radiation (PAR) and induce light limitation, high-density native herbaceous seed mix to promote interspecific competition with native congeners, and herbicide, both individually and in partial combinations. Treatment performance was assessed over two growing seasons through measurement of vegetation abundance (relative cover plus relative density) of focal invasives coupled with environmental variables. Our findings demonstrate the impact of environmental variables on the dynamics of invasive-dominant communities in a wetland mitigation system following disturbance. The abundance of Arthraxon was positively correlated with cation macronutrients (e.g., K, Mg) and negatively correlated with shade, indicating a preference for high-nutrient, high-light environments. Conversely, Phalaris abundance showed a marginally significant positive correlation with shade, likely reflecting its tolerance to higher moisture and cooler conditions. Hydrology was identified as a significant indirect factor affecting vegetation community assemblage in both focal species’ experimental populations. Our results reinforce the necessity for replicable long-term studies to evaluate the effectiveness of resource-limiting strategies in various wetland mitigation contexts and among problematic invasive species with contrasting life history strategies.