Soil Moisture and Diurnal Variation of Sap Flow in Birch and Norway Spruce - Effects of the Summer Drought 2003

2003

S. 59

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200107819

Pure stands of Norway spruce (Picea abies Karst L.) outside their natural range, especially when growing on heavy soils with stagnic properties at low elevation, are very susceptible to ecological risks because of their shallow rooting system. In order to minimise management risks, a change of tree species composite towards a higher percentage of deciduous trees is required. On such sites with extreme soil moisture regimes the consequences of such changes for the water budget are important to know. The investigation was performed at a site near Fürstenfeld (in the south east of Austria) in a stand with pioneer species, following a large scale wind-throw of a mature spruce forest. Soil hydrology parameters were recorded in two birch and two spruce eco-groups. Sap flow of 11 birch and 10 Norway spruce trees was measured during the growing season 2003. The results for summer 2003 show differences in water use of birch and Norway spruce. Initially the soil moisture in the birch eco-groups decreased more rapidly than in the spruce eco-groups. But with continuance of the drought pahse the soil water content under both tree species became converged more and more. The general patterns of the diurnal variation of the sap flow showed differences between birch and spruce. The sap flow of the birch trees had a maximum level which was reached in the morning and remained constant until early aternoon. In contrast the maximum of the sap flow of the spruce trees was reached later and did not remain on a constant level. The drought in August 2003 showed different effects on the sap flow of birch and spruce. The reaction of birch in the daily pattern of sap flow was obviously less distinctive than that of spruce. Half of the spruce trees showed a strong restriction of the sap flow in the afternoon, the rest showed the normal daily variation. The reasons for this differentiation are not clear. Differences in the rooting systems of the trees or different social positioning within the stand could be responsible for that.