Long-Term Responses of Nutrient Budgets to Concurrent Climate-Related Stressors in a Boreal Watershed
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The climate of the circumpolar Boreal forest is changing rapidly, resulting in a growing frequency of wildfires and changing precipitation patterns. These climate-related stressors may influence the cycling of nutrients within, and overall ecosystem condition of, Boreal watersheds. However, long-term perspectives of concurrent climate-related impacts on the cycling of nutrients in watersheds are rare. We present multi-decadal terrestrial and lake mass budgets of nitrogen, phosphorus and carbon within a headwater Boreal Shield watershed that was recovering from an extensive wildfire while experiencing measureable increases in annual precipitation. We used these budgets to quantify associations between nutrient retention in each ecosystem and changes in metrics defining landscape recovery after wildfire or precipitation. The terrestrial watershed retained over half of all nitrogen and phosphorus delivered to it by the atmosphere. Strong nutrient retention occurred despite ongoing landscape recovery from wildfire, measurable increases in precipitation, a forest tent caterpillar defoliation and rising atmospheric deposition. A downstream headwater lake was also a strong and consistent sink of nitrogen and phosphorus, highlighting a whole-watershed resistance to environmental disturbances. However, carbon was strongly lost downstream from the terrestrial ecosystem in close and positive association with precipitation, resulting in a darkening of the headwater lake over time with implications for the functioning of its ecosystem. Long-term mass budget monitoring of a Boreal catchment has provided insight into the resistances and dynamic changes within a northern watershed exposed to concurrent wildfire and increasing precipitation conditions.
Keywordsnutrient budget wildfire climate change tree-ring Boreal forest lake defoliation watershed landscape change precipitation
Funding and logistical support for this study was provided by the International Institute for Sustainable Development Experimental Lakes Area Inc., the Government of Ontario and historically by the Government of Canada. In particular, we thank the gracious support from Matt McCandless, Mark Lyng, Roger Malot, Dr. Sonya Havens, John Shearer, Mike Dwyer, Dr. Michael Stainton and IISD-ELA field site staff. J. Tardif received funding assistance from the Natural Sciences and Engineering Research Council and the University of Winnipeg. Numerous field, laboratory and support workers all contributed to the extensive data set used in this study and are graciously thanked by the authors. Ken Sandilands provided meteorological and hydrological data sampling information for development of mass budgets of the L239 watershed. France Conciatori, Tess Macdonald and Martin Hadad supported collection of tree cores from the field. Annie Wheeler assisted in assessment of tree species coverage in the L239 watershed. We would particularly like to thank the two anonymous reviewers whose suggestions improved the manuscript.
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