Long-term changes in forest growth: Potential effects of nitrogen deposition and acidification

In spite of numerous experimental studies, it has, so far, not been possible to link historic changes in forest growth to acid deposition at regional scales, partly due to difficulties in modeling the ecological complexity of forests. We analyzed radial increment data from increment cores from > 31 000 spruce forest plots in southern Norway from 1954-1996. Using a combination of a bio-stratification model to control confounding factors, and a catchment model for acidification, we demonstrate for the first time a spatial and temporal co-variation between forest growth and both nitrogen deposition and acidification, as indicated by acidity critical loads exceedances. Increases in growth during the 1960-1970s, followed by a subsequent decline in the 1980-1990s, were best explained by combined actions of acidification, nitrogen deposition and climatic stress on forest growth. While forest conditions vary primarily with natural growing conditions, the results suggest that boreal forests are sensitive to moderate levels of nitrogen and sulphur deposition where acidity critical loads are low, and that effects may be observed over relatively short time scales.