High peat carbon stocks across a hydrological gradient in a cedar swamp in the Lake Simcoe watershed, southern Ontario, Canada

Background/Question/Methods

Forested wetlands contribute to landscape-scale biogeochemical cycling and potentially to climate regulation, but they are less well constrained in terms of soil carbon dynamics as compared to other wetland types. This is partly due to their large spatio-temporal complexity and diverse vegetation communities. Standing out among forested wetlands are Eastern white cedar (Thuja occidentalis) swamps, a peat-forming wetland found in the North American Great Lakes region, where large soil carbon stocks have been documented in previous studies. Contemporary observations suggest that altered hydrological regimes, deer browsing pressure, and other anthropogenic impacts are causing these systems to decline despite studies showing their apparent stability throughout the Holocene. The objective of this research is to analyze a series of cores in a cedar swamp peatland to characterize the influence of spatial and temporal variation on key soil properties including peat type, bulk density, carbon and nitrogen content, and areal carbon stocks. To address this, we established a series of plots along a 120-m transect spanning a hydrological gradient in a cedar swamp peatland located in the Herrell Ecological Preserve in the Lake Simcoe watershed, part of the Great Lakes-St. Lawrence mixed forest zone of southern Ontario, Canada.

Results/Conclusions

At each plot, we measured water table depth, surveyed vegetation, and collected peat cores for the analysis of soil properties, peat type, and radioisotope dating to measure peat carbon accumulation. Our results confirm the high peat carbon stocks reported at other Eastern white cedar swamp sites, with peat depths ranging from 80-140 cm in plots with a 50-65% abundance of Eastern white cedar and a subdominant, 25-35%, abundance of red maple (Acer rubrum). Soil bulk densities were somewhat higher than typical for northern bog or fen peat (0.14 g cm-3) and mean organic matter content generally exceeded 75%, with some variability linked to water table depth. Assuming 50% of organic matter is carbon, our estimates for carbon density of the Herrell Preserve cedar swamp averaged 57 kg C m-2. This value is more than twice that of the mean ecoregion-based estimates for areal carbon density, which include above- and below-ground plant biomass as well as soil carbon, demonstrating the outsized importance of cedar swamps in regional carbon accounting. By sampling across the peatland environmental gradient, these results highlight the importance of addressing underlying processes that can help elucidate landscape-scale carbon turnover rates and the spatial-temporal complexity of these systems.