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Abstract "Heft 40"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: P. Trüby

Heft 40

Thorsten Gaertig

Bodengashaushalt, Feinwurzeln und Vitalität von Eichen

Freiburg im Breisgau 2001

ISSN 0344-2691


The study at hand tests the working hypothesis, whether structural loss in the topsoil of oak stands leads to aeration deficiencies in the root zone. The metabolic activity and growth rate of fine roots decreases either from insufficient transfer of oxygen out of the atmosphere to the roots or from insufficient discharge of carbon dioxide from the root zone into the atmosphere. The supply of water and nutrient elements to the tree trunk and crown is not optimally provided with a reduced root system. Loss of vitality and reduced regeneration ability after other damages could also have repercussions for this trend.

The relationship between soil aeration and fine root density and the oak vitality (Quercus robur L. and Quercus petraea [ Matt.] Liebl.) was investigated in two spatial integration levels in order to test the hypothesis.

In five oak stands with very differently structured topsoils (from crumbly to hard packed), the influence of aeration deficiencies on gas fluxes and root penetration was thoroughly examined (Process Study). For this purpose, soil CO2 concentration, gas-diffusion coefficient, soil respiration, and fine root density were measured at these sites. The objective of this level was the investigation of causal correlations between soil aeration, soil biological activity and root penetration.

In a second level, the aeration condition, soil-chemical condition, root density and oak vitality were investigated using 38 oak stands in Baden-Wuerttemberg (Inventory Study). The aim of this level was the evaluation of the influence of soil aeration on root penetration as well as the health of oaks on a regional level.

It was revealed in the process study that soil biological activity declined with decreased topsoil gas-diffusivity. In topsoils with a relative apparent gas diffusion coefficient below 0.06, the CO2 produced could not be sufficiently discharged. CO2 concentrations in a soil depth of 5 cm increase to values over 0.6 %. In oak stands with better-aerated topsoils, a more intensive fine root development and a more favorable relationship of root length to root mass were observed.

The inventory study shows that surface soils from the examined oak stands of Baden-Wuertemberg range from acidic to strongly acidic and are sufficiently supplied with primary nutrient elements. Half of all gas-diffusion measurements taken from forest topsoils (0-5 cm) show aeration deficiencies (Ds/Do<O.O6). A high percentage of coarse pores in the primary root area is only to be found in sites with high calcium or aluminium saturation.

The pH value does not have a direct influence on the soil aeration or the fine root development of oaks. Root density is substantially controlled by the gas permeability of the mineral topsoil. The higher the gas permeability of topsoil, the more intensively can soil be opened up by fine roots. The vitality of oaks assessed by crown structure, decreases with topsoil gas permeability. 75 % of the topsoil gas diffusion measurements of "damaged" oak stands show aeration deficiencies in the top 5 cm, whereas only 25 % of the "vital" samples indicate insufficient gas exchange. In soil depths below 25 cm, the fine root density of "damaged" oak stands are significantly reduced compared to the fine root density of the "vital" oak stands. Therefore it can be concluded that the "typical oak characteristic" of being able to root into deep soil horizons disappears if those horizons are not connected to atmospheric gas exchange. In the investigated oak stands, this disconnection is the result of insufficient topsoil gas permeability (plastic foil effect). In other studies observed loss of vitality in oaks growing on seasonally water-logged sites could be traced back to aeration deficiencies of the sub soil. During wet periods, the total pore space is filled with water. As a result, all paths for gas flux in the subsoil are blocked and respiration-required gas exchange with the atmosphere is brought to a standstill. In dry periods subsoil, weakly rooted, can no longer contribute to water supply. The oaks react to water stress with leaf and twig loss, which increases their predisposition to other damages.

For forest practice two important consequences arise from this study:

1. Extensive vehicle movement leads to formidable disturbances in oak ecosystems. With regard to the aeration function the protection of topsoil structure is of major concern for forest harvesting.

2. Planting oak species in order to open up and recover compacted top soils, which in practice is predicted on their ability to grow into dense sub soils is questionable. Moreover it can be suggested that establishment of oak stands on seasonally water-logged sites in order to "restore" these sites will be unsuccessful.

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