Effects of nitrogen deposition:: Results of a temporal-spatial analysis of deciduous forests in South Sweden

We studied permanent plots in deciduous forests in two provinces of South Sweden, Skane (127 plots) and Smaland (116 plots). Vegetation data were sampled in 1983 and 1993 and used to calculate weighted averages of Ellenberg indicator values for light, reaction (pH) and nitrogen. Soil samples were collected from all plots in 1993 for determination of pH, organic matter content and potential net nitrogen mineralization (minN). Data on minN were also available for 66 plots in Skane from 1983. Nitrogen deposition is considerably higher in Skane than in Smaland. The same holds true for total minN, minNH(4)(+), minNO(3)(-), and the proportion of total minN due to nitrate (nitrification ratio). According to the comparison of average Ellenberg indicator values between 1983 and 1993, the stands had become darker (not in the heavily managed plots), more acid and nitrogen rich, except at the most acid sites where only minor changes could be observed. The nitrification ratio was significantly higher in 1993 than in 1983. Mean soil nitrification ratios were determined for the most common species in both regions. The means were higher in Skane than in Smaland. It is likely that nitrogen deposition causes an increase in nitrification rate and nitrification ratio. This ratio is a significant determinant of the species composition in deciduous forests.