Nitrogen deposition increases drought sensitivity in Swiss forests

A network of forest monitoring plots was established in Switzerland in 1984, covering gradients in drought, nitrogen deposition, ozone, age, altitude and soil chemistry. Data are available from 95 plots with European beech (Fagus sylvatica) and 76 plots with Norway spruce (Picea abies).

The results of this long-term forest monitoring programme show that nitrogen (N) deposition (range 8.5-81 kg N ha-1 yr-1 ) clearly increased the drought sensitivity of beech stem increment. The mortality of beech was increased after drought and when foliar P concentrations were low. As the latter may be a consequence of high N deposition this is an indirect N effect. Under drought conditions, spruce mortality increased significantly with high N deposition, as a consequence of bark beetle infestations.