Canadian Forest Service Publications
Canadian Experiences in Development of Critical Loads for Sulphur and Nitrogen. 2006. Watmough, S.A.; Aherne, J.; Arp, P.A.; DeMerchant, I.; Ouimet, R. Pages 33-38 in C. Aguirre-Bravo, P.J. Pellicane, D.P. Burns, and S. Draggan, editors. Monitoring Science and Technology Symposium: Unifying Knowledge for Sustainability in the Western Hemisphere Proceedings RMRS-P-42CD, September 20-24, 2004, Denver, CO. USDA Forest Service, Rocky Mountain Research Station, Fort Collins, CO. 990 p.
Issued by: Pacific Forestry Centre
Catalog ID: 27006
Critical loads are a broad-scale modelling approach designed to assess the potential risk of pollutants to ecosystems. A description of the methodology for estimating critical loads (sulphur and nitrogen) for acid deposition (CL(A)) for upland forests in eastern Canada is presented, using a case study in central Ontario. In eastern Canada, CL(A) have been calculated for upland forests, with the objective of maintaining the molar ratio of base cations to aluminium in soil solution above 10. In the current approach, nitrogen (N) dynamics including N fixation, N immobilisation and denitrification have been set to zero. Further, critical load estimates presented in this study do not include nutrient removals through harvesting, and dry deposition input is estimated to be 20 percent of wet (1994 to 1998) deposition. Critical loads were calculated separately for Ontario, Québec and the Maritime Provinces (New Brunswick, Nova Scotia and Newfoundland) using the same methods, but using different soil and forest databases. Mean area-weighted critical loads among provinces are similar, ranging between 273 eq ha--1 yr--1 (Newfoundland) and 512 eq ha--1 yr--1 (Ontario). Preliminary estimates indicate that more than 50 percent of the upland forest area in Ontario and Québec and between 10 (Newfoundland) and 33 percent (Nova Scotia) of upland forest in the Maritimes receive acid deposition in excess of the critical load. Current efforts are being directed toward improving the accuracy of critical load estimates and current exceedances using better estimates of dry deposition and harvesting removals, and investigating the linkage between exceedance of the critical load and adverse biological effects.