The impact of soil aeration on oak decline in southwestern Germany

In middle-European silviculture, oak species are largely used for restoration of wind-thrown spruce stands on dense or badly aerated soils. However, vitality of mature oak stands have decreased in the last decade. Symptoms of fine root degeneration as well as soil structure deficiencies in the upper layer have been observed. This study tested the working hypothesis that deficiencies in soil gas permeability reduce fine root formation and thereby reduce stress tolerance of trees. Topsoil gas diffusivity, root density and oak vitality were assessed for 36 oak stands with pedunculate oak (Quercus robur L.) and sessile oak (Quercus petraea [Matt.] Liebl.). The relationship between topsoil gas diffusivity, soil respiration and soil CO2 concentration were also investigated. Evidence that root density decreases significantly with decreasing soil gas permeability was found, which is representative for oak stands at southwest Germany. Heavily damaged oak stands have been found only at sites suffering from soil aeration deficiencies. Although, we observed decreased soil respiration in compacted soils, CO2 concentration in soil was up to three times higher on these sites. High Soil CO2 concentrations indicate insufficient soil aeration rather than high biological activity. Insufficient soil aeration is apparently an important factor causing oak decline. It cannot be, therefore, concluded from the oak's ability to open up dense subsoils that they can be used for the restoration of stands with compacted topsoils. (C) 2002 Elsevier Science B.V. All rights reserved.