EFFECTS OF DIFFERENT FINE-ROOT DISTRIBUTI ONS AND FINE-ROOT BIOMASS ON DROUGHT STRESS AND NEEDLE LOSS IN SPRUCE STANDS - RESULTS FROM SIMULATION CALCULATIONS ON WATER UTILIZATION

Chronic needle and leaf losses of major tree species, called ''novel f orest damage'', are widespread in Central Europe. Air pollutants are o ften blamed for this kind of damage. The influence of air pollutant de position, soil chemical changes and the increasing nitrogen availabili ty may cause a shallowing of the root system and decreasing fine root biomass. We hypothesize chat the changes in the root system cause decr easing drought susceptability of the trees and needle or leaf losses. Using computer models for the watercycle in a mature Norway spruce sta nd over a period of 18 years we quantified the influence of different rooting patterns on the transpiration and the risk of drought stress. Here me compared the dry year 1976 with the wet year 1981. Ln 1976 the simulated xylem water potentials indicate severe drought stress under conditions of shallow rooting and low root. biomass. Under these root ing conditions the drought stress in 1976 would have been avoided, if the trees had lost about 30% of the needle mass. During the wet year 1 981 rooting patterns also influenced xylem mater potentials, but sever e drought stress did not occur.