Cumulative Effects of Beaver Ponds and Forest Harvest on Streamwater Chemistry in Boreal Watersheds

Abstract

Canada’s boreal forests provide many important ecosystem services, including but not limited to biomass production, habitat provisioning, and soil and water protection. As forests are an important part of Canada’s economy and landscape, it is crucial that they are managed sustainably. In the province of Ontario, sustainable forest management is largely based on emulating natural disturbances. With respect to stream biogeochemistry, beaver ponds are one of the most common and significant disturbances against which to compare.   Thus, there is a need to further our understanding of how beaver ponds and forest management interact and modify the effects of one another. This study examined a suite of surface water chemistry variables, with a focus on the bioaccumulative neurotoxin methylmercury, in 28 headwater catchments across 3 years in northwestern Ontario. Some were undisturbed; some were impacted by active or abandoned beaver ponds; some were undergoing active harvest, while others were harvested previously; and some were impacted by active or abandoned beaver ponds and new or previous harvest.  Forest harvest impacts on organic carbon, suspended sediments, and mercury fell largely within the range of natural variation seen at undisturbed sites, except where significant soil and water disturbance from stream crossings occurred upstream. Pond impacts were highly variable, but more strongly related to catchment characteristics (such as mean slope and channel length) than to pond characteristics (such as shape and in-pond vegetation cover). Though downstream impacts were found to be greatest at a new pond in a catchment undergoing active harvest, pond and harvest impacts were not consistently additive in catchments where both occurred. Understanding the impacts of beaver ponds in conjunction with forest harvest is important for predicting the ultimate effectiveness of management decisions aimed at protecting terrestrial-aquatic ecosystems commonly affected by both disturbances.