Modelling effects of soil acidification on tree growth and nutrient status

Understanding the effects of soil acidification on tree growth requires understanding the nutrient relations of trees and stands, notably the uptake of nutrients by the roots in relation to soil conditions. Although a substantial amount of research has been carried out on nutrient relationships, both on plant and stand scale, changes in nutrient uptake as a result of soil acidification are hard to predict. This poses serious problems for attempts to model nutrient uptake by roots in relation to changes in soil chemistry induced by acidification and nitrogen enrichment. Very detailed mechanistic models of root uptake have been developed, but the extrapolation of rhizosphere models developed under controlled, laboratory conditions to field situations is cumbersome. On the other hand, general models of nutrient dynamics very often lack the sensitivity that is required to describe the reaction to gradually changing site conditions. This renders difficult the linkage between critical loads derived from soil criteria, and tree and stand reactions such as allocation and growth. In the models applied to Solling, 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 Solling, but such conclusion 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 consequences of changes in soil chemistry. At this stage, there seems to be a clear need for additional experimental results on nutrient transport in soil, on decomposition under changed soil conditions, and on nutrient uptake in the case of competition between different ions. In addition, more detailed information on the response of uptake kinetics and biomass allocation in case of reduced nutrient supply, would improve deterministic models of nutrient relations of trees. From such experimental information, theoretical understanding can be derived, and perspectives for generalization and modelling can be drawn.