Effects of nitrogen deposition on field layer vegetation in south Swedish oak forests

The influence of atmospheric nitrogen deposition on mixed-oak forest vegetation along a deposition gradient in south Sweden was studied using multivariate methods. The main vegetation gradients were largely controlled by soil acidity, nitrogen supply, canopy composition and the location of the sample plots. The results suggest that nitrogen deposition has affected the field layer vegetation directly by increased nitrogen availability and indirectly by accelerating soil acidification. A close correlation between nitrogen deposition and longitude/latitude does not allow a sharp separation between effects of deposition and other geographically controlled factors. However, according to time series studies, 20 of the 30 field layer species that are most closely associated with high nitrogen deposition in our study have increased in frequency in areas with high deposition during the past decades. This group of field layer species includes many species generally considered as nitrophilous, but also several acid-tolerant species.