Shane C. Lishawa

Water Level Decline Promotes Typha X glauca Establishment and Vegetation Change in Great Lakes Coastal Wetlands

Climate change is predicted to reduce Laurentian Great Lakes water levels, altering coastal wetland ecosystems and potentially stimulating invasive macrophytes, like Typha X glauca. Recent prolonged low water levels, which climaxed in 2007, created conditions comparable to those predicted by climate change science. In 2008, we examined ecosystem and plant community properties in 14 intact northern Great Lakes coastal wetlands and compared community data with data from a 1987–1989 high-water period, before T. X glauca invasion. In 2008, T. X glauca occurred in 50% of wetlands and 16% of plots; was associated with reduced Floristic Quality and increased soil organic matter, soil nutrients, and leaf litter (all p < 0.05); and plant community composition had shifted and was more homogeneous than in 1988 (both p < 0.05). Additionally, T. X glauca was more dominant when growing behind barrier beach ridges, which form in high-water conditions and persist in low-water, than in lake-exposed marshes (p < 0.05), revealing a physiographic mechanism for increased dominance. Beach ridges protect T. X glauca from wave and seiche energy, and as water levels decline, these energy-insulating microtopographic features will likely stimulate further invasion and dominance by T. X glauca, even in high quality wetlands.

Herbicide management of invasive cattail (Typha × glauca) increases porewater nutrient concentrations

Invasive wetland plants are the primary targets of wetland management to promote native communities and wildlife habitat, but little is known about how commonly implemented restoration techniques influence nutrient cycling. We tested how experimental mowing, herbicide application, and biomass harvest (i.e., removal of aboveground biomass) treatments of Typha-invaded mesocosms altered porewater nutrient (NO3−, NH4+, PO4−3) concentration and supply rate, vegetation response, and light penetration to the soil surface. We found that while herbicide application eliminated the target species, it also reduced native plant density and biomass, as well as increased porewater nutrient concentration (PO4−3, NO3−) and supply rates (N, P, K) up to a year after treatments were implemented. Because herbicide application promotes nutrient enrichment, it may increase the likelihood of reinvasion by problematic wetland invaders, as well as cause eutrophication and deleterious algal blooms in adjacent aquatic systems. Our data suggest that biomass harvest should be considered by managers aiming to reduce Typha abundance without eradicating native diversity, avoid nutrient leaching, as well as possibly utilizing biomass for bioenergy.

Mechanical Harvesting Effectively Controls Young Typha spp. Invasion and Unmanned Aerial Vehicle Data Enhances Post-treatment Monitoring

The ecological impacts of invasive plants increase dramatically with time since invasion. Targeting young populations for treatment is therefore an economically and ecologically effective management approach, especially when linked to post-treatment monitoring to evaluate the efficacy of management. However, collecting detailed field-based post-treatment data is prohibitively expensive, typically resulting in inadequate documentation of the ecological effects of invasive plant management. Alternative approaches, such as remote detection with unmanned aerial vehicles (UAV), provide an opportunity to advance the science and practice of restoration ecology. In this study, we sought to determine the plant community response to different mechanical removal treatments to a dominant invasive wetland macrophyte (Typha spp.) along an age-gradient within a Great Lakes coastal wetland. We assessed the post-treatment responses with both intensive field vegetation and UAV data. Prior to treatment, the oldest Typha stands had the lowest plant diversity, lowest native sedge (Carex spp.) cover, and the greatest Typha cover. Following treatment, plots that were mechanically harvested below the surface of the water differed from unharvested control and above-water harvested plots for several plant community measures, including lower Typha dominance, lower native plant cover, and greater floating and submerged aquatic species cover. Repeated-measures analysis revealed that above-water cutting increased plant diversity and aquatic species cover across all ages, and maintained native Carex spp. cover in the youngest portions of Typha stands. UAV data revealed significant post-treatment differences in normalized difference vegetation index (NDVI) scores, blue band reflectance, and vegetation height, and these remotely collected measures corresponded to field observations. Our findings suggest that both mechanically harvesting the above-water biomass of young Typha stands and harvesting older stands below-water will promote overall native community resilience, and increase the abundance of the floating and submerged aquatic plant guilds, which are the most vulnerable to invasions by large macrophytes. UAVs provided fast and spatially expansive data compared to field monitoring, and effectively measured plant community structural responses to different treatments. Study results suggest pairing UAV flights with targeted field data collection to maximize the quality of post-restoration vegetation monitoring.