Physiologische, anatomische und chemische Aspekte der Regulation der Wurzelwasseraufnahme bei Rotbuche, Kiefer und Birke auf zwei unterschiedlich wasserversorgten Standorten

Abstract

1. Surface-related root water uptake rates were assessed for three tree species (Fagus sylvatica, Betula pendula, Pinus sylvestris) that are important for forestry and ecology. The measurements were done on mature trees using miniature sap flow gauges according to the heat balance method. The measurements took place in two different periods: a dry vegetation period (2003) and a wet vegetation period (2004) at two stands varying in water supply.2. The aim was to find out, in which way fine roots of the relatively drought sensitive beech and the relatively drought tolerant birch and pine are different in respect to physiological, morphological, anatomical and chemical properties and if they are capable of adjusting their fine root system to dry locations in order to ensure sufficient water uptake.3. The surface dependent water uptake rates of roots showed a large time-dependent variability caused mainly by climatic factors like vpd and global radiation and less by soil water content.4. All three tree species showed higher surface-related water uptake rates at the dryer location compared to the significantly wetter location, caused mainly by higher vpd and higher water potential gradients inside the trees.5. Under comparable environmental conditions, the roots of the three species showed significant differences in surface-related water uptake capacity, varying by a factor of 2 to 10. With > 2000 g H2O m-2d-1 pine had a much higher uptake rate than birch (approx. 1000 g H2O m-2d-1). Beech root water uptake rates were 500 g H2O m-2d-1 or less. Based on these results we conclude that species with a relatively high fine root biomass and surface area, like beech, can have lower specific uptake rates than species with less fine root biomass like pine.6. In the dry summer of 2003 all three species had their steepest decrease of root water potential. The least negative water potential was measured in pine, whereas most negative values were found in birch. Beech root water potential was usually intermediate with the exception of a striking decrease in late summer at the driest site.7. The strong seasonal and interannual differences in water uptake can well be explained by variations in root water potential. Pine had the highest uptake of the three species, despite of the smallest decrease in potential. This could lead to the conclusion that pine has a higher radial conductivity than beech or birch. However this should be confirmed by further laboratory experiments.8. A tendency of difference in the amount of periderm layers and suberin was observed between the roots of the three species. The drought-tolerant pine was the species with the least suberin in its secondary root tissue. Thus, suberin amounts do not seem to correlate with the adaption to drought. However the significant higher amount of suberin per periderm layer in the roots of beech and birch, as observed in the dry summer of 2003, can be interpreted as an adaption to dry soil conditions.9. This leads to the following conclusions: Fine roots of central European drought tolerant and drought-resistant tree species are not systematically different in respect to their anatomy and their suberin content. However some components of suberin could be considered as species-specific biomarkers for fine roots. In the three tree species the most significant response to drought was a decrease in root water potential. Thus, water uptake of fine roots in natural surroundings is mainly controlled by the potential gradient between root xylem and soil.