1 Institut Fédéral de Recherches sur la Forêt, la Neige et le Paysage, Zürcherstr. 111, CH-8903 Birmensdorf
2 Institut de biologie végétale appliquée IBA/IAP, Sandgrubenstrasse 25/27, CH-4124 Schönenbuch
Emissions of nitrogenous pollutants reached a peak in the mid-1980s and have decreased since then, but they remain at a high level. Long-term observation sites were established between 1984 and 1995 within three networks in Switzerland (IAP, LWF and NITREX), following growing concern that the vitality of forests might be affected by air pollution. Here we assess the nitrogen status of selected sites and its changes over the last 15 to 25 years. Nitrogen deposition at the sites was either obtained from models (IAP) or from throughfall measurements (LWF and NITREX). Indicators of the nitrogen status of a site included nitrate leaching from the rooting zone (43 sites), tree foliar concentrations of nitrogen and other nutrients (>100 sites), and the species composition of ground vegetation (59 sites). Nitrogen deposition has significantly decreased on eight out of twelve sites during the last 15 years. However, except in the Alps, deposition remains within or above the range of empirical critical loads, which correspond to the deposition levels above which the ecosystem might be negatively affected. Nitrate leaching can be locally high, depending on deposition rates, but also on other factors such as soil properties. Tree foliar concentrations in nitrogen and phosphorus have tended to decrease during the last 15 to 25 years. The comparison of vegetation surveys, first carried out in 1984, then repeated in 2003/2004, indicated an increase in nitrophilous species (IAP), but on a more recent and shorter period (1994/1998–2011), on a smaller number of sites (LWF), we found no significant changes.
Keywords: forest ecosystems, nitrogen deposition, foliar analyses, nitrate leaching, ground vegetation, long-term observation sites