BIOLOGICAL RESPONSE OF 5 FOREST ECOSYSTEMS IN THE EXMAN PROJECT TO INPUT CHANGES OF WATER, NUTRIENTS AND ATMOSPHERIC LOADS

In five coniferous forest ecosystems in Europe, water and nutrient sup ply, as well as atmospheric loads, were manipulated for 3 or 4 years. Water supply was optimised and nutrients were added according to tree demand in optimal proportions relative to the ambient N supply. Tree g rowth was strongly enhanced by optimal water supply but not further en hanced by nutrient additions. The nutritional balance in trees was imp roved for P and K. The increased water and nutrient supply retarded ne edle shedding in autumn and diminished root production. To date, the m anipulated decrease in N input to the soil has decreased the N content in needles in one stand. Water additions tended to lower N contents a t two sites. Large applications of N increased N content in needles ev en though the N nutrition was already optimal. Liming with dolomite st imulated tree growth only in the nutrient-poor stand, but has generall y increased Ca content in needles. The number of species and cover of understorey vegetation has increased considerably by liming and, in so me cases, by water addition. Ecosystem manipulation experiments have b een shown to be a useful tool for the quantification of the growth eff ects of traditionally limiting factors. Additionally they give indicat ions of the effects on forest ecosystem processes of future changes in atmospheric loads.