ASSESSMENT OF THE EXPOSURE AND LOADS OF ACIDIFYING AND EUTROPHYING POLLUTANTS AND OZONE, AS WELL AS THEIR, HARMFUL INFLUENCE ON THE VITALITY OF THE TREES AND THE SPEULDER FOREST ECOSYSTEM AS A WHOLE

Within the framework of the Dutch Priority Program on Acidification, 1 0 yr of research was conducted in a Douglas fir stand at Speulder fore st. Research was conducted to establish the loads and levels of acidif ying and eutrophying pollutants and ozone, to determine forest vitalit y characteristics and follow growth parameters and nutrient status in time and to determine the effects of reduction of loads and levels by manipulation experiments. Results indicate that during the last 20 yr critical levels for air pollutants have hardly been exceeded except fo r ozone, which slightly affected assimilation. Elevated nitrogen depos ition has caused several adverse effects including (i) inhibited mycor rhizal development, leading to a decreased base cation and phosphorus uptake; (ii) elevated foliage/root (fine and coarse) ratios, making th e forest more sensitive to drought and windthrow; (iii) elevated nitro gen and arginine concentrations in the foliage, associated with relati ve base cation and phosphorus deficiency, and (iv) elevated nitrate le aching polluting the groundwater. High inputs of acidity have caused e levated ratios of Al to base cations, affecting fine root (uptake) and depletion of the readily available Al pool, thus affecting the long-t erm sustainability. Despite these effects, forest vitality, in terms o f defoliation/discoloration, is reasonable and forest growth even incr eased in response to nitrogen. The exceedances of critical loads for n itrogen and acidity, however, implies a (large) risk for the long-term sustainability of the Speulder forest.