Legacy Forest Harvesting Impacts on Phosphorus Transport Dynamics in Hardwood Dominated Canadian Shield Catchments: Evaluating Changing Source Availability and Source Channel Connectivity
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Forests are critical to the safe provision of drinking water across the globe and provide over 4.1 trillion USD per year in savings to drinking water treatment costs. These supplies are threatened by climate change-exacerbated landscape disturbances such as wildfire, which can severely impact forest hydrologic and biogeochemical cycles. Potential increases in the transport of sediment and associated nutrients to streams networks are especially concerning because they can propagate downstream and lead to conditions that challenge drinking water treatment operations. Excess phosphorus loading is especially concerning in drinking water sources as it can cause the proliferation of cyanobacteria and other algae that can clog filtration processes or produce toxins that most plants are not equipped to treat, thereby leading to service disruptions or complete outages. Forest harvesting has been proposed as a potential mitigation strategy to lower fuel loads and reduces the risk of catastrophic wildfire, preventing negative impacts on water quality and treatability. However, this strategy needs to be adopted with caution as forest harvesting has the potential to increase phosphorus transport to stream networks, presenting a potential threat to downstream drinking water treatment operations and, thereby exacerbating the problems that they are supposed to mitigate. Accordingly, the objectives of this study were to evaluate the impacts of legacy forest harvesting on the physical processes (i.e., source channel connectivity and source availability) that control phosphorus transport from terrestrial to aquatic environments. This work was conducted on hardwood dominated, Canadian Shield catchments within the Turkey Lakes Watershed (TLW) of Ontario, Canada. A before-after-control-impact (BACI) study design was used to evaluate the immediate and legacy impacts (1-12 years after harvesting) of multiple forest harvesting strategies (i.e., clear-cut, shelterwood cut, and selection cut) on total phosphorus (TP) concentration and yield. Additionally, the hydrologic source areas contributing to stream flow were evaluated using end member mixing analysis (EMMA) in a legacy clear-cut (24 years after harvesting) and control catchment. Phosphorus sources in those catchments were evaluated by measuring TP and soluble reactive phosphorus (SRP) in these source areas and analyzing soil samples for total particulate phosphorus (TPP) and phosphorus fractions throughout the soil profile. These results were used to explain the legacy impacts of harvesting on seasonal and event-based TP and SRP concentrations draining the clear-cut and control catchments. Results from the BACI study and seasonal and event sampling demonstrate that harvesting had a small but significant impact on TP stream concentrations. However, as many of these differences were below the detection limit (< 1 µg l-1) the results are not practically significant. EMMA showed that stream water chemistry corresponds most with shallow groundwater and wetland groundwater chemistry in both catchments, suggesting that legacy forest harvesting has little impact on the primary water sources contributing to stream flow. Additionally, legacy forest harvesting appeared to have little impact on phosphorus source availability as there were few differences in phosphorus concentrations between the legacy clear-cut and control catchment within those source areas. Further evidence of this is demonstrated by the lack of differences in TPP and phosphorus fractions within the mineral soils between the two catchments. Finally, wetlands were identified as a major source of phosphorus delivered to streams as they are hydrologically connected with the stream channel and have measurable phosphorus concentrations. As legacy forest harvesting appears to have little long-term impacts on either hydrologic processes or phosphorus source availability its expected that little impact was observed on stream phosphorus concentration and yield. Results of the study suggest that forest harvesting may be a suitable land management strategy that promotes source water protection in hardwood-dominated Canadian Shield forested watersheds.
Cite this version of the work
Robert Fines (2023). Legacy Forest Harvesting Impacts on Phosphorus Transport Dynamics in Hardwood Dominated Canadian Shield Catchments: Evaluating Changing Source Availability and Source Channel Connectivity. UWSpace. http://hdl.handle.net/10012/19428