Nitrogen deposition in Canadaʹs Boreal Shield: implications for the Kyoto Protocol

The Boreal Shield ecozone is of substantial economic importance in Canada with regard to forestry, mining, hydro electric power generation and tourism. It contains 106 million hectares of timber productive forest land and is being subjected to many anthropogenic stresses, such as acid rain, and increased UV.B radiation from stratospheric ozone depletion. From 1961 to 1990 the average annual temperature of the eastern portions of the ecozone were constant or slightly decreasing whereas at the same time the western areas experienced a warming trend. The south central parts of the ecozone are subjected to acid deposition including nitrogen and since these forest systems tend to be nitrogen limited, increased forest growth might be expected. Several studies have shown that tree growth has decreased, even though, atmospherically deposited nitrogen is almost entirely being taken up in the terrestrial ecosystems. There are studies suggesting that some watersheds may be approaching nitrogen saturation. All areas of the ecozone are subjected to the increasing atmospheric concentrations of carbon dioxide so the range of temperature and nitrogen deposition values offers some potential for further studies to determine the relative ecological effects of these various stresses.