Forest growth and critical air pollutant loads in Scandinavia

We investigated radial increment in relation to natural growing conditions and critical loads exceedance for 8577 Norway spruce forest inventory plots in Norway, Sweden, and Denmark. Tree age, growing season, stand density and site productivity accounted for about 50% of the variation in increment, underlining the importance of natural variation for forest growth. We developed a model for assessing increment in comparable forest stands in relation to exceedance of critical loads for nitrogen. Increment was 8-17% lower in exceeded areas. Increment declined with increasing exceedance of critical loads for nitrogen. These results demonstrate that natural growing conditions may conceal important patterns in forest vitality correlated to pollution loadings. Although forest condition thus varies primarily with natural stressors, air pollution may have weakened forest health in exceeded areas in parts of Scandinavia. As air pollution is likely to enhance natural stressors for tree growth, geographic patterns of forest damage are likely to continue to be highly variable and complex.