Globally, peatlands are vulnerable to degradation via drainage, with consequences for ecosystem structure and function such as increased fire vulnerability, soil oxidation, and altered vegetation composition. Peatland function is largely dependent on hydrologic regimes and their influences on the accumulation and properties of peat soil. Therefore, an understanding of soil-hydrology interactions is needed to inform management in drained peatlands, including expansive systems such as the Great Dismal Swamp (GDS; Virginia and North Carolina, USA) where hydrologic restoration is underway. Two physically distinct soil layers have been observed at GDS, the upper layer thought to be a result of past drainage and the lower layer more representative of an undisturbed state. To understand the occurrence and consequences of these distinct layers, we integrated continuous water level data, peat profile characterization, and analyzed soil physical and hydraulic properties. The transition from upper to lower peat soil layers typically occurred at depths below contemporary water level observations, suggesting that the upper layer may be a result of historical drainage with limited recovery following hydrologic restoration. We also found distinct differences between the properties of the two layers, where upper layers had lower fiber and organic matter contents and higher bulk densities. Further, upper layers had higher proportions of macropores, resulting in an overall lower water retention capacity. These differences in layer properties suggest the upper layer is more susceptible to drying, increasing fire vulnerability, oxidation, and shifts in vegetation composition that do not support current management objectives. / Master of Science / Peatlands provide many valuable ecosystem services, including carbon storage, water quality maintenance, and habitat provision. However, peatlands have been subjected to centuries of drainage (i.e., lowered water levels) to support timber harvesting, land conversion, and other land use actions. Drainage and the resulting drier conditions can lead to soil carbon loss, increased fire vulnerability, and changes in vegetation communities. Additionally, peatland drainage has consequences for peat soil properties and their role in ecosystem services. In an effort to restore peatland ecosystem services, hydrologic restoration, usually in the form of water control structures, is often implemented to reduce drainage and reestablish historical water levels. To guide restoration practices, research is needed to understand how drained peat soils respond to such hydrologic management. In this study, we investigated peat soil profiles, current water level regimes, and soil properties at the Great Dismal Swamp (Virginia and North Carolina, USA), a drained peatland currently undergoing hydrologic restoration. We found a visibly distinct upper soil layer, which we suggest developed as a result of past drainage and with little recovery under restored, wetter conditions. We also found that this upper layer has altered soil properties and thus is more vulnerable to drying, with implications for ecosystem function such as fire vulnerability, carbon sequestration and vegetation composition. Together, our findings will help inform restoration and water level management at GDS and our understanding of drained peatlands more broadly.

Forested peatlands of the Great Dismal Swamp (GDS) have been greatly altered since colonial times, motivating recent restoration efforts. Community structure and function were hydrologically altered by 19th and 20th century ditches installed to lower water levels and enable early timber harvesting. Contemporary forest communities are comprised of maturing remnants from selective timber harvesting that ended in the early 1970s. Red maple (Acer rubrum) has become the dominant species across GDS, encroaching on or replacing the historical mosaic of cypress (Taxodium spp.)/tupelo (Nyssa spp.), Atlantic white-cedar (Chamaecyparis thyoides), and pocosin (Pinus spp.). Moreover, peat soil has been exposed to more unsaturated conditions resulting in carbon loss through decomposition and increased peat fire frequency and severity. Installation of ditch control structures aim to control drainage and re-establish historical hydrology, vegetation communities, peat accretion rates, and fire regime. To help inform restoration and management, we conducted two complimentary studies to test hypotheses regarding hydrologic influences on vegetation, peat depths, and peat fire vulnerability. First, we found thicker peat, lower maple importance, and higher species richness at wetter sites (e.g., higher mean water levels). In our second study, we evaluated the integrated effects of peat properties and water level dynamics on peat fire vulnerability. We found decreased burn vulnerability with increased wetness, suggesting that the driest sites were always at risk to burn, whereas the wettest sites never approached fire risk conditions. Together our findings demonstrate strong hydrologic controls on GDS ecosystem structure and function, thereby informing water level management for restoration goals. / Master of Science

The Great Dismal Swamp (GDS) is a forested peatland located in the Atlantic Coastal Plain. Once a mosaic of wetland communities, disturbances (e.g., timber harvesting and ditching) have resulted in altered hydrologic regime, homogenized forest communities, and increased peat subsidence. In response, hydrologic restoration and forest management aim to enhance community composition and function. To help inform these efforts, we investigated variability and drivers of forest communities by surveying vegetation composition and structure, hydrologic indicators, and soil properties at 79 monitoring plots across GDS. Data were augmented with modeled water levels and peat depths. Our results demonstrate red maple (Acer rubrum) dominance across GDS, which decreases tree density, richness, and diversity. However, hierarchical cluster analysis identified four community types: Gum (G), Maple-Gum (M-G), Sweetgum-Maple (SG-M), and Maple (M). These communities differed in tree composition and structure; differences in other growth forms (shrubs, herbaceous, and regeneration) were limited. Modeled water levels failed to explain vegetation differences, but community associations with soil properties suggest that communities exist along a hydrologic gradient. Specifically, the G community likely exists on wetter sites whereas SG-M communities occur at drier locations. Maple-dominated communities (M and M-G; 78% of plots) likely occur across broader hydrologic gradients, explaining their dominance. However, more characterization of hydrology (i.e., time-varying water levels and soil moisture) and other drivers (e.g., site history and soil hydraulics) is needed to further explain community variation. As such, we propose future strategies for long-term monitoring to inform ongoing hydrologic restoration and forest management efforts. / Master of Science