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 supply, 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 growth was strongly enhanced by optimal water supply but not further enhanced by nutrient additions. The nutritional balance in trees was improved for P and K. The increased water and nutrient supply retarded needle shedding in autumn and diminished root production. To date, the manipulated decrease in N input to the soil has decreased the N content in needles in one stand. Water additions tended to lower N contents at two sites. Large applications of N increased N content in needles even though the N nutrition was already optimal. Liming with dolomite stimulated tree growth only in the nutrient-poor stand, but has generally increased Ca content in needles. The number of species and cover of understorey vegetation has increased considerably by liming and, in some cases, by water addition. Ecosystem manipulation experiments have been shown to be a useful tool for the quantification of the growth effects of traditionally limiting factors. Additionally they give indications of the effects on forest ecosystem processes of future changes in atmospheric loads.