Current drought effects on forest trees in Switzerland (4)
Drought intensity reaches a new (last?) peak in this summer. Forest trees of all regions investigated are heavily affected according to our drought-indicator TWD (tree water deficit). The values are far above the TWDs measured in the past eight years and indicate a very high drought stress.
The network TreeNet detects with point dendrometers how much tree stems shrink during dry periods. This so-called tree water deficit (TWD) indicates how much trees are suffering from dry conditions. The higher the value is the more a tree is thirsty. The figure shows that the situation accentuated in all three regions in the Alps and in the lowlands. Analyzed were beech, spruce, pine, fir and oak trees. The TWDs are weighted with a species-specific factor in order to account for different TWD amplitudes of different species.
Spruce is the species showing the greatest deviations from the average TWDs measured since 2011. This species is known to respond very sensitive to heat and drought which is clearly visible in the current TWD data. We expect negative implications for the whole tree physiology incl. reduced growth, shedding of needles and in extreme cases tree death.
Beech shows TWDs above the average of the last years in the lowlands of Switzerland (Mittelland). The rain of the past days in the Vallais (Alp 
Wallis) led to a decrease of the TWD in this region. Beech trees with already brown leaves are reported from all parts of Switzerland. Particularly sun-exposed forests and trees along the forest edges are particularly affected.
See also the earlier reports about the development and interpretation of tree water deficits in trees:
TreeNet Switzerland collects continuous data on stem radius fluctuations measured with point dendrometers from about 250 trees all over Switzerland and estimates drought and growth indicators for Swiss forest ecosystems. We closely collaborate with the Long-term Forest Ecosystem Research Programme (LWF/WSL), the ETHZ and the Institute for Applied Plant Biology (IAP).
For more information about the actual drought in Switzerland refer to drought.ch