The Engineering of Peatland Form and Function by Beaver (Castor spp.)

Northern peatlands are significant contributors to global biogeochemical cycles. In Canada alone, peatlands cover over a tenth of the land surface and store over half of the country’s terrestrial carbon. Their ability to function as a carbon sink is inextricably linked to hydrological conditions defined by an intricate web of feedbacks from numerous autogenic (internal) and allogenic (external) drivers. Research over the last forty years has been focused on understanding the importance of each driver, as such knowledge is necessary to foresee how these landscapes might respond climate change. However, one external driver - the activity of beaver (Castor canadensis in North America and C. fiber in Eurasia) - has received little attention, even though beaver have inhabited northern peatlands for many thousands of years. Identified as a keystone species and ecosystem engineer, beaver can alter the physical, hydrological, and biogeochemical function of landscapes on a scale comparable to that of humans. Thus, the primary goal of this dissertation was to enhance our understanding of how beaver activity alters peatland function and transforms these landscapes over time. To achieve this goal, the associated impacts of beaver activity were studied over numerous scales, mostly in the montane peatlands of Alberta, Canada, via the collection and analyses of field data comprised of different physiographic, hydrological and soil variables. It was found that the activities of beaver have profound impacts on peatland landscapes. Beaver activity changes the physical appearance of peatlands, with the construction of berm-like dams that persist for long periods even after dams breach and/or are abandoned. Peat, excavated from the surrounding area, was a primary dam building material, and dams often extended far beyond the stream channel and inundated large surface areas. Peat excavation added complexity to the pond shape and bathymetry, but despite the physical complexity of beaver ponds, it was found that relationships between quickly measured field attributes allows for reliable estimates of surface water storage in these features. In addition to storing large volumes of surface water, Beaver dams/ponds had significant impacts on hydrological processes in the peatlands studied. Just as in mineral soil environments, beaver ponds acted as sinks for mineral and organic sediments. Furthermore, a multi-year study in a Rocky Mountain fen, showed that the beaver dams connected the peatland to the stream, thereby raising and stabilizing shallow ground water tables within 150-m proximity. Such findings have implications for peat formation in affected areas because plant community and carbon sequestration are tightly linked to water table behavior. Beaver ponds also had significant impacts on underlying soils. Regional sampling of peatland beaver meadows found they were depleted in organic matter. Deeper inspection of this phenomenon through multi-proxy analysis and paleo-reconstruction of peat cores revealed that beaver-meadow soils accumulated the least amount of peat over time compared to areas unaffected by beaver. This phenomenon is likely a result of sediment deposition, which increases the bulk density of peat bulk volumes and may enhance turnover and decomposition when ponds wash out. Unlike beaver ponds built in mineral soil environments, the peatland beaver ponds studied here were not associated with an accumulation of organic matter. Overall, this research shows that beaver activity can alter the appearance of peatlands and exert control over processes fundamental to their function as a carbon sink. This activity leaves a legacy beyond cyclic pond creation and abandonment that contributes to the spatial complexity of the landscape. Beavers deserve greater inclusion in peatland conceptual models and further research is needed in the peatlands beavers are known to inhabit