Successional dynamics of a 35 year old freshwater mitigation wetland in southeastern New Hampshire

Abstract

The long-term ecological success of compensatory freshwater wetland projects has come into question based on follow-up monitoring studies over the past few decades. Given that wetland restoration may require many years to decades to converge to desired outcomes, long-term monitoring of successional patterns may increase our ability to fully evaluate success of wetland mitigation projects or guide adaptive management when needed. In Portsmouth, New Hampshire a 4 ha wetland was constructed in an abandoned gravel quarry as off-site compensatory mitigation for impacts to a scrub-shrub swamp associated with property expansion. Building upon prior evaluations from 1992 and 2002, we conducted a floral survey in 2020 to compare results with prior surveys to document vegetation successional trends over time. In addition, we monitored the avian community throughout the growing season as a measure of habitat quality. The plant community mirrored documented successional trends of freshwater wetland restoration projects as native hydrophytes dominated species composition. Plant species composition stabilized as the rate of turnover, the measurement of succession, declined by nearly half after 17 years. Researchers should consider long-term monitoring of specific sites to better understand successional patterns of created wetlands as we documented long time frames required for the development of scrub-shrub swamps, red maple swamps, and sedge meadows. High species richness was attributed to beaver activity, topographic heterogeneity from Carex stricta tussocks, and the seed bank from the application of peat from the original wetland. Habitat heterogeneity of open water, herbaceous cover, and woody vegetation supports a diverse avian community including 11 wetland dependent species. Although the mitigation project has not created the full area of lost scrub-shrub swamp after 35 years, it has developed a structurally complex habitat and diverse avian community that effectively provides the functions and values of the impacted system.

The Effects of Legacy Sediment Removal and Floodplain Reconnection on Riparian Plant Communities

“…Changes to the composition of the herbaceous vegetation community are also likely influenced by the substantial decrease in woody basal area and species richness, an expected consequence of the excavation accompanying these projects (Violin et al 2011;Wood et al 2022). At these sites, the combination of a higher water table, increased overbank flooding, and decreased shading provides an environment that favors hydrophytic, native, and graminoid species immediately after restoration (McKown et al 2021). The rapid herbaceous layer vegetation response of these sites was expected as riparian vegetation often responds quickly to restoration involving extensive floodplain remodeling that increases floodplain connection (Fraaije et al 2019;Kail et al 2015;Lorenz et al 2018;Pedersen et al 2007b;Sarneel et al 2019).…”

Effects of biodiversity on functional stability of freshwater wetlands: a systematic review

“…Freshwater wetlands (FWs) are ecosystems formed by the interaction between freshwater rivers, lakes and land, mainly including riverine wetlands, lakes, marshes and floodplains. FWs not only provide suitable habitats for many plants and animals ( McKown et al, 2021 ), but also play an important role in nutrient cycling, water purification and biodiversity maintenance ( Li C. et al, 2022 ; Yu et al, 2023 ; Li et al, 2024 ). FWs have four the ecological services categories: provisioning, regulating, cultural and supporting services ( Keddy et al, 2009 ).…”

Assessing Spectral Band, Elevation, and Collection Date Combinations for Classifying Salt Marsh Vegetation with Unoccupied Aerial Vehicle (UAV)-Acquired Imagery

“…Remote sensing provides a means to monitor and map vegetation species within salt marshes, but there are many options available to implement this technology [22,[48][49][50][51][52][53]. The remote sensing of salt marshes has typically been completed using satellite or aerial platforms.…”