Water relations of spruce trees in low-elevation pure stands

Monocultures of Norway spruce (Picea abies) on sites originally covered by broadleaf forests are generally assumed to need restoration. The effects of these monocultures and their restoration on the ecosystem and on the socio-economic system have been investigated. The investigations have been focussed to the 2800 km forests of the eastern Austrian Flysch-and the Molasse- zone, where 32% of the forest area are secondary spruce stands. This multi-disciplinary research has been performed by teams covering different disciplines, from eco-physiology, entomology, forest pathology, growth and yield, wildlife and soil science to economics and political science. The research was performed on different levels depending on the disciplines: on the tree level, the stand level and on a regional level. The results are as follows: Beech uses far more water than spruce and is less able to restrict its water use in periods of drought. Thus in drought periods there is less water available for spruce in mixed stands than in pure spruce stands. Spruce roots more shallow than beech, and even more shallow when in mixed stands. In mixed stands it responds stronger to competition, falls back in its social position and dies earlier than in pure spruce stands. Finally only dominant spruce trees are left in mixed stands. Decomposition of the litter of spruce is much slower than for broadleaves, and a high proportion of organic matter and nutrients is fixed in the soils below spruce stands. Tree species mixture does not alter total nutrient stores in the poor soils of the Molasse, while the respective stores are higher below mixed stands in the Flysch-zone. Generally the top soil below mixed stands exhibit higher calcium contents, due to the deeper rooting beech. During drought periods the amount of volatiles from spruce trees is enhanced. The pattern of the emitted monoterpenes in combination with high release of ethanol, increase beetle attraction. Associated with bark beetles, blue stain fungi impede sap flow in spruce trees. Another fungus, Sirococcus congenius was identified as the main cause of spruce shoot blight and crown dieback. Manifestation of the symptoms additionally depends on the Ca and Mg nutrition of the host tree. All together these susceptibilities and instabilities of spruce can be recognised in the fact that 80% of the salvage cut is from spruce trees. Nevertheless even including these risks, if all spruce stands were converted to broadleaf stands, the capital of the forest enterprises, on the average, would be reduced by 27% in this region. Large scale conversions of former spruce dominated forests to broadleaved forests, after 15 years, changed the habitat quality for roe deer. When most of the stands reach thicket or pole stage, the ratio between protection- coverage and availability of forage worsens, susceptibility to browsing increases and hunting becomes more difficult.