The impact of soil aeration on oak decline in southwestern Germany
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 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.
Section snippets
Investigation sites
The relationship between rooting, oak vitality and soil aeration was investigated using 36 oak stands in Baden-Württemberg, Germany. The measuring plots were randomly distributed in regions where oak stands exceed 10% of the total forest area (Fig. 1). According to the high diversity of site factors in Baden-Württemberg, the investigated test sites included a wide range of substrate and soil properties. The dominant soil types (WRB) included Haplic Luvisol at 17 sites and Dystric Cambisol at 10
Gas-diffusion coefficient and soil respiration
The relationship between air temperature, soil gas diffusivity and soil respiration at the intensive approach stands is illustrated in Fig. 3. Using these three variables, a bi-variate linear regression model was calculated and plotted as a plane in a three-dimensional system (Fig. 3). As expected, the respiration rate increased significantly with temperature. There was also a significant (p<0.01) dependence from topsoil diffusivity. Soil respiration increased with increasing DS/D0.
Gas-diffusion coefficients and soil air CO2 concentration
The
Discussion
Soil respiration is an integrative measurement of root and of rhizosphere organism respiration. Fixed energy is consumed in both cases. In this way, soil respiration is an energy turnover indicator of root soil space. This energy turnover decreased with decreasing gas diffusivity in the topsoil at the investigated sites. Simultaneously, CO2 concentration in the soil air increased with reduced energy turnover. The capacity of gas transport between soil and atmosphere depends on the gas-diffusion
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2019, Soil and Tillage ResearchCitation Excerpt :This general causality between soil structure and rootability was also supported by our findings of high fine root densities measured on both control plots as well as the median strip without recent compaction damage. An increased air permeability and soil porosity supply fine roots with oxygen (Gaertig et al., 2002) and lead to enhanced root growth. As we did not distinguish the fine roots origin, a possible influence of the flanking scots pine plantation (at a distance of 8–10 m) cannot be excluded totally and might also be the reason of a higher fine root density measured on the deeper layer of the unplanted control plot.