The impact of soil aeration on oak decline in southwestern Germany

Abstract

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.

Introduction

In middle-European silviculture, Quercus species (especially Quercus robur L., pedunculate oak) are regarded as one of the key species for restoration of wind-thrown spruce stands on dense, badly aerated soils (e.g. Otto, 1979, Petri, 1993, Moser, 1994). In silvicultural textbooks, one finds the designation of such stands as “coercive oak stands” (Mayer, 1984). However, the extent of oak damage increased dramatically all over Europe in the last decade (Fischer and Hartmann, 1999) even on such “coercive oak stands”. In Germany, the percentage of damaged oaks $\text{(}\text{leaf}\text{loss}\text{>25\%)}$ increased from 5% in 1983 to 40% in 1990. Until now, no recovery is visible (Kronauer, 1999).

The impacts of biotic factors such as insect or fungal attacks, weather conditions such as frost and drought as well as soil factors such as water availability, nutrient supply and soil chemical properties on oak decline have been thoroughly investigated. All results indicate that oak decline cannot be explained by one single factor. Different combinations of site factors are likely responsible for oak decline (Rosel and Reuther, 1995, Hartmann, 1996, Thomas and Buttner, 1992, Thomas, 1995, Thomas and Kiehne, 1995, Heinsdorf, 1996).

In spite of important factors such as insect attacks or drought periods having only a short impact on oak stand health, the situation has not noticeably improved in the last decade. It was hypothesized that primary factors of oak decline may be found in the rootspace (Blaschke, 1994, Thomas and Hartmann, 1998). To our knowledge, no data is available on how oak vitality is affected by soil aeration constraints.

An assumed 25–50% of carbon fixed by forests is consumed by root respiration (Qi et al., 1994). Oxygen for root respiration must be taken from the ‘free’ atmosphere. Because the soils air-filled porous space can be regarded as the atmosphere’s ‘dead end’, all gas fluxes triggered by partial pressure gradients have to pass and are controlled by the diffusivity of the soil surface. Even in deeper soil regions, the roots cannot be supplied with oxygen if the soil surface ‘interface’ is smeared or compacted. Similarly, CO₂ emission from the soil is inhibited.

The smearing and compacting of the upper soil layer is usually caused by skidding. A large percentage of productive area in many deciduous stands show traces of vehicle movement. However, loss of soil structure and porosity have various causes. The actual soil structure is not rigid. Rather, it is a result of dynamic loosening (e.g. biological activity) and compacting processes (e.g. rain splash). The biological activity of the soil is influenced by various factors. The soil structure as well as soil acidification by deposition determines the crucial boundary condition for soil organisms. As a result, the floating equilibrium between pore-creating factors and pore-destroying factors may shift to a state with less porosity (Hildebrand, 1987).

Investigations involving potted plants in the laboratory (Hildebrand, 1983, Hildebrand, 1986, Schack-Kirchner, 1994) and artificial soil aeration experiments (Murach et al., 1993) showed that fine root growth of beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) can be strongly reduced by restricting aeration.

While studying the oak death phenomenon, we observed compacted soil structure combined with a high percentage of dead roots. This gave reason to test the working hypothesis that loss of topsoil gas permeability causes a reduced fine root formation. With a reduced root system, the tree will lose potential reactions to other adverse site factors. Tree recovery after “natural” damaging events are, therefore, reduced. With regard to the choice of oak species for restoration purposes, this question has important implications due to the use of heavy machinery during the clearance of wind-thrown areas. The planting procedure often creates dense topsoils in the whole area.