THE INFLUENCE OF STAND CHARACTERISTICS, T OPOGRAPHY, SITE AND WEATHERON TREE HEALTH IN THE FOREST DISTRICT ROTHENBUCH

This article tries to evaluate, by different regression models, the da ta of a visual forest health inventory, which has been carried out sin ce 1983 in the forest district Rothenbuch (Spessart). As explanatory v ariable (regressors) characteristics of stand, topography, site and we ather were considered, as well as the state of tree health of the prev ious year (''last year value''). Separately for five tree species, set s of regressors are established which are able to determine the degree of defoliation in a significant manner. Within these lists the regres sor ''last year value'' is dominant, explaining (maximum) 61-74% of th is year's defoliation. Further, variable related to stand, site and we ather play an important role. For beech, oak and spruce there is a hig hly significant influence of age, with lower damage in younger stands (<50 years old), and greater damage in old stands (>100 years old) for beech and spruce and with high damage in middle aged stands (50-100 y ears old) for oak. For beech in thinner stands too and after fructific ation significant higher defoliation can be observed. Unfavourable soi l conditions such as thick humus layer, low pH etc. in tendency lead t o higher defoliation, but here the tree species do not behave uniforml y. With respect to weather, after dry years one has to expect higher t ree damage with high transpiration stress due the to dry and hot atmos phere having a greater impact than water deficit in the soil. Topograp hy is of minor importance and indicates higher defoliation at higher e levation (for oak and larch) and less steep slopes (for oak and spruce ). In total the regressors related to stand, topography, site and clim ate explain in addition to the regressor ''last year value'' only abou t 4 to 12% of defoliation, depending on tree species.