MODELING EFFECTS OF SOIL ACIDIFICATION ON TREE GROWTH AND NUTRIENT STATUS

Understanding the effects of soil acidification on tree growth require s understanding the nutrient relations of trees and stands, notably th e uptake of nutrients by the roots in relation to soil conditions. Alt hough a substantial amount of research has been carried out on nutrien t relationships, both on plant and stand scale, changes in nutrient up take as a result of soil acidification are hard to predict. This poses serious problems for attempts to model nutrient uptake by roots in re lation to changes in soil chemistry induced by acidification and nitro gen enrichment. Very detailed mechanistic models of root uptake have b een developed, but the extrapolation of rhizosphere models developed u nder controlled, laboratory conditions to field situations is cumberso me. On the other hand, general models of nutrient dynamics very often lack the sensitivity that is required to describe the reaction to grad ually changing site conditions. This renders difficult the linkage bet ween critical loads derived from soil criteria, and tree and stand rea ctions such as allocation and growth. In the models applied to Selling , most emphasis is on soil chemistry, with only few models accounting for feedback mechanisms between soil conditions and tree growth. From the model results presented during the workshop, it would appear that nitrogen and magnesium are the key elements in Selling, but such concl usion is biased as much the same assumption also underlies the guiding concepts on which the models are based. From the models presented at the workshop, no clear consensus emerged on the predictions of the con sequences of changes in soil chemistry. At this stage, there seems to be a clear need for additional experimental results on nutrient transp ort in soil, on decomposition under changed soil conditions, and on nu trient uptake in the case of competition between different ions. In ad dition, more detailed information on the response of uptake kinetics a nd biomass allocation in case of reduced nutrient supply, would improv e deterministic models of nutrient relations of trees. From such exper imental information, theoretical understanding can be derived, and per spectives for generalization and modelling can be drawn.