Assessing the geochemical balance of managed boreal forests

A sustainable forest management system requires that a balance must be reached between ecosystem nutrient losses and gains in the course of a rotation. In order to determine the influence of stand characteristics (species composition, density, site potential productivity), method of forest harvesting (stem-only versus whole-tree) as well as rotation length on nutrient losses induced by biomass harvesting, a geochemical balance (nutrient inputs minus outputs) was computed from published information and forest inventory databases for the southern portion of the boreal forest of Quebec. Losses were compared with potential nutrient gains that varied according to soil types. We provided a tool for assessing the risk of having a negative nutrient budget (outputs > inputs) that forest managers can use with information that is already available to them. This exercise was conducted for five commercial tree species, namely paper birch (Betula papyrifera Marsh.), aspen (Populus trenmloides Michx.), balsam fir (Abies balsamea (L.) Mill.), jack pine (Pinus banksiana Lamb.), and black spruce (Picea mariana (Mill.) B.S.P.), grouped into four site indexes and three stand density classes. Strong differences in nutrient exportation in biomass appeared between stands of different compositions as well as between stands of the same species but of different classes of productivity or density. The most nutrient-demanding tree species were trembling aspen and balsam fir. As expected, whole-tree harvesting caused a greater drain on nutrient reserves than stem-only harvesting, but this effect varied strongly with tree species and was greatest for balsam fir and lowest for jack pine. Harvesting the forest before or after the age of financial maturity, which might be desirable under some circumstances, generally created a lesser nutrient drain but this was at the expense of biomass production. Aspen was an exception to this rule showing a greater nutrient drain for stands harvested prior to the age of financial maturity. Implications for the development of indicators of sustainable forestry and for future research are discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.