Effects of stresses on forest growth in models applied to the Solling spruce site

Forests are rarely limited by only a single pollutant or environmental factor, yet experimental investigations are usually limited to one or two stress factors and to short durations. Simulation models provide a means of exploring the long-term effects of multiple natural and anthropogenic stresses on forest health. Sixteen simulation models were applied to a Norway spruce site at Solling (Germany) for a workshop comparing forest-soil-atmosphere models. Five of these models were applied under various environmental conditions to quantify forest damage at Solling and to analyze interactions of multiple stresses. The models agreed that the forest damage caused by drought stress and nutrient deficiency was more important than that caused by short-term, direct air pollution stress. The models did not agree on the magnitude of some effects because the models differed in complexity, in the choice of stresses and processes modelled, and in the assumed mechanisms of stress effects. Stress interactions in the models were explored by comparing the effects of single stresses with those of multiple stresses. Multiple stresses tend to be less damaging than the sum of individual stresses, but more damaging than the single most damaging stress. However, the exact form of the effects of multiple stresses is not predicted by the mechanism of the stress as implemented in the model, even in the most simple models, due to feedback mechanisms and integration over time. is a lack of knowledge about long-term effects of direct stress and a lack of data of manipulation experiments to validate the simulations. Future research should be focused on long-term effects of SO2 and O3 exposure and on manipulation of field plots.