Perched Peatlands: insights into eco-hydrologic roles of peatlands in water limited boreal environments

Peatlands are integral to sustaining landscape eco-hydrological function in water-limited boreal landscapes and serve as important water sources for headwater streams and surrounding forests, and recently for mega-scale watershed construction associated with resource extraction. Despite the regional moisture deficit of the Boreal plains, peatlands and margin swamps exist on topographic highs where low permeability (clogging) layers occur proximal to the surface and are apparently isolated from surface water and local and regional groundwater inputs. The water generating mechanisms (external water sources, internal feedback mechanisms) that enable peatland formation with such delicate water balances in these unique hydrogeologic settings are not well known, and have large implications for understanding the eco-hydrologic role of natural peatlands as well as direct peatland construction in drier boreal landscapes.

A multi-year sampling campaign was conducted to collect hydrometric, geochemical (DOC, pH, major cations and anions), and isotopic (D/H, ¹⁸O/¹⁶O) data from a small isolated peatland-margin swamp complex. We explored the relative roles of margin swamps in buffering water loss and generating perched groundwater, shading and wind protection from adjacent forests, snow redistribution in and around the peatland, and wetland feedbacks on maintenance of peatland moisture and ecosystem function. Long-term (18 year) records of water table gradients between the peatland and an adjacent forest combined with 3 year high intensity water balance calculations show the peatland to be a source of water to adjacent forests during this period and illustrate the dominance of autogenic wetland feedbacks over allogenic controls (external sources) in peatland development at this location. Contrasts in water storage due to the morphometry of the clogging layer appear to the dominant determinants of peatland and swamp form and function. Layers of decomposed peat and fine textured mineral soils in margin swamps with low water storage potential promoted frequent soil saturation and anoxia, limiting forest vegetation growth and water uptake, further enhancing wetland vegetation, water conservation and generation within the wetland complex. Shading and wind protection from adjacent forests appear to influence soil frost duration and atmospheric demand to further reduce evapotranspiration losses contributing to a slight moisture surplus in the wetland complex relative to the adjacent forest. Understanding the water balance and moisture surplus controls in isolated peatlands sheds light on the relative role of allogenic and autogenic controls on peatlands with implications for: 1) assessing regional eco-hydrological roles of peatland and forestland covers, 2) predicting landscape-scale response to environmental change and land use, and 3) directing landscape scale reclamation or large reconstruction projects over a range of geologic settings in water-limited boreal regions.