Long-term base cation mass balances for Swedish forests and the concept of sustainability

Data from the Swedish Forest Inventory was used to calculate mass balances for base cations Ca, Mg and K for Swedish forests. Using lysimeter and forest survey soil analyses to estimate present base cation leaching from the root zone reveals that weathering plus base cation deposition is not sufficient to support both, the present base cation leaching rate and the present rate of uptake caused by stem growth. Calculations suggest that 96% of the productive forested area may have higher rates of removal than supply for one or more base cation. Under a best-case scenario, assuming less pollution, the present growth rate and 100% efficiency in uptake of available nutrients, the area with more removal than supply would still be at least 30% of the total area. Forest soils are being depleted at a rate where the exchangeable reservoirs have high risk of being severely depleted in the next few decades in central and southern Sweden. During 1983–1985 the depletion rate is calculated to be, on the average, 0.33 keq ha −1 year −1. The weathering rate and present base cation deposition can sustain growth at a level where (80–85)×10 6m 3 stemwood year −1 can be harvested. Any harvested growth beyond this volume must be sustained by artificial means. For whole-tree harvesting without base cation return, the calculations indicate that it would significantly increase the base saturation depletion rate to an average of 0.62 keq ha −1 year −1, and risk depletion of the soil in less than one-to-two rotation periods almost anywhere in Sweden. The calculations stress the importance that sustainable forest management must include the management of nutrient fluxes and reservoirs.