Plant and Soil 203: 145–158, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 145 Tree rooting patterns and soil water relations of healthy and damaged stands of mature oak (Quercus robur L. and Quercus petraea [Matt.] Liebl.) Frank M. Thomas 1 and Günter Hartmann 2 1 Universität Göttingen, Albrecht-von-Haller-Institut für Pflanzenwissenschaften, Abt. Ökologie und Ökosystem- forschung, Untere Karspüle 2, D - 37073 Göttingen, Germany ∗ and 2 Niedersächsische Forstliche Versuchsanstalt, Abt. Waldschutz, Grätzelstr. 2, D - 37079 Göttingen, Germany Received 20 January 1998. Accepted in revised form 30 June 1998 Key words: clay content, mineral soil, oak decline, Quercus, root biomass, root density, water availability Abstract 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.; Neuenburg site) and sessile oak (Q. petraea [Matt.] Liebl.; Lappwald and Sprakensehl sites) were investigated, and soil texture, bulk density, duration of waterlogging periods and the water available in the mineral soil were determined. For Neuenburg and Sprakensehl, the available soil water was related to leaf water parameters determined in a sep- arate investigation. At the clayey and hydromorphic sites of Neuenburg and Lappwald, the measurements were performed in each one healthy and one damaged part of the site, which differed in the number of oaks with crown damage. In the damaged stand of Neuenburg, the clay content of the subsoil was higher than in the healthy stand, and the soil water availability was reduced especially in dry periods. Compared to healthy oaks 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 the damaged stand. With decreasing relative available soil water (actually available water in relation to water available at the saturation state), the relative leaf water content decreased in damaged, but not in healthy oaks. At Lappwald, similar differences in soil water availability 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 damaged oaks declined with decreasing relative available soil water. However, the damaged oaks were not inferior to the healthy ones with respect to root density and biomass. It is concluded that, in the damaged stand of Neuenburg, the high clay content of the subsoil, which results in prolonged periods of waterlogging, in sharp changes from waterlogging to drought and decreased water availability in dry periods, is the reason for the reduced biomass and density of roots of the pedunculate oak. Thus, in northwestern Germany, unfavourable soil water relations are considered as a factor contributing to crown damage of pedunculate oak at hydromorphic sites, but not to damage of sessile oak. Introduction In northern Germany, increased dieback and decline of pedunculate (Quercus robur L.) and sessile oak (Q. petraea [Matt.] Liebl.) have occurred during the last decade (Hartmann et al., 1989). As was the case ∗ FAX No:: +49 (0)551 395701. E-mail:fthomas@gwdg.de in earlier occurrence of such damages, they were at- tributed to the combined effects of climatic extremes (winter frost, summer drought) and insect defoliation (Hartmann and Blank, 1993). Among the site factors investigated in a large-scale regional survey by CIR aerial photography, the site water conditions showed the closest relation to the visible crown damage, which plso7615.tex; 7/10/1998; 9:46; p.1 JS: DISC: CP: PIPS No. 184382 (plsokap:bio2fam) v.1.15