Potential impact of forest harvesting on lake chemistry in south-central Ontario at current levels of acid deposition

The potential impact of harvesting on lake chemistry was assessed for ~1300 lakes in south-central Ontario using a critical loads approach based on the steady-state water chemistry (SSWC) model. The critical load of acidity is currently only exceeded by bulk sulphate deposition in 9% of the lakes if harvesting does not occur. However, the percentage increases to 23%, 56%, and 72% under potential harvesting scenarios that assume wood-only (stem without bark), stem-only, or whole-tree harvesting, respectively. This increase in exceedance of critical load is due to the much lower base cation concentrations in lakes resulting from base cation removals during harvest. For example, only 0.3% of lakes will have Ca2+ concentrations <50 μequiv.·L–1 if harvesting does not occur, whereas 52% of lakes will have Ca2+ concentrations <50 μequiv.·L–1 if whole-tree harvesting occurs. Harvesting clearly has an enormous potential impact on lake chemistry, which will become more apparent as exchangeable base cation pools in soil decline and acid inputs can no longer be buffered.