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

The influence of atmospheric nitrogen deposition on mixed-oak forest vegeta tion along a deposition gradient in south Sweden was studied using multivar iate methods. The main vegetation gradients were largely controlled by soil acidity, nitrogen supply, canopy composition and the location of the sampl e plots. The results suggest that nitrogen deposition has affected the fiel d layer vegetation directly by increased nitrogen availability and indirect ly by accelerating soil acidification. A close correlation between nitrogen deposition and longitude/latitude does not allow a sharp separation betwee n effects of deposition and other geographically controlled factors. Howeve r, 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 decad es. This group of field layer species includes many species generally consi dered as nitrophilous, but also several acid-tolerant species.