TREE ROOTING PATTERNS AND SOIL-WATER RELATIONS OF HEALTHY AND DAMAGEDSTANDS OF MATURE OAK (QUERCUS-ROBUR L AND QUERCUS-PETRAEA [MATT] LIEBL)

At three sites in northwestern Germany, which represent the centres of the present oak damage, root distribution and biomass beneath healthy and damaged trees of mature pedunculate oak (Quercus robur L.; Neuenb urg site) and sessile oak (Q. petraea. [Matt.] Liebl.; Lappwald and Sp rakensehl sites) were investigated, and soil texture, bulk density, du ration of waterlogging periods and the water available in the mineral soil were determined. For Neuenburg and Sprakensehl, the available soi l water was related to leaf water parameters determined in a separate investigation. At the clayey and hydromorphic sites of Neuenburg and L appwald, the measurements were performed in each one healthy and one d amaged part of the site, which differed in the number of oaks with cro wn damage. In the damaged stand of Neuenburg, the clay content of the subsoil was higher than in the healthy stand, and the soil water avail ability was reduced especially in dry periods. Compared to healthy oak s of the healthy stand, the density of finest plus fine roots as well as the biomasses of finest roots were lower beneath damaged oaks of th e damaged stand. With decreasing relative available soil water (actual ly available water in relation to water available at the saturation st ate), the relative leaf water content decreased in damaged, but not in healthy oaks. At Lappwald, similar differences in soil water availabi lity between the healthy and the damaged stand were found, but had no effect on the distribution or biomass of the roots. At the sandy site (Sprakensehl), the available soil water decreased drastically during a dry period, and predawn leaf water potentials of both healthy and dam aged oaks declined with decreasing relative available soil water. Howe ver, the damaged oaks were not inferior to the healthy ones with respe ct to root density and biomass. It is concluded that, in the damaged s tand of Neuenburg, the high clay content of the subsoil, which results in prolonged periods of waterlogging, in sharp changes from waterlogg ing to drought and decreased water availability in dry periods, is the reason for the reduced biomass and density of roots of the pedunculat e oak. Thus, in northwestern Germany, unfavourable soil water relation s are considered as a factor contributing to crown damage of peduncula te oak at hydromorphic sites, but not to damage of sessile oak.