Current drought effects on forest trees in Switzerland (5)

Rain over the weekend reduced the tree water deficit (TWD) in most trees of all species. However, we are far from an average situation. Most trees are still showing TWDs much higher than the average of the past years. Drought stress is still very high and more rain is needed to compensate the water deficit.

Tree water deficits (TWD) of Swiss forest trees in three different regions. Dots indicate the average of the past 5 days. Bars indicate the average of the past 7 years.

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 drought-stress situation improved with rain over the weekend in all 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.

Tree water deficits (TWD) of spruce trees in three different regions. Dots indicate the average of the past 5 days. Bars indicate the average of the past 7 years.

Spruce is the species still showing the greatest deviations from the average TWDs measured since 2011 in the Valais (Alp Wallis) and in the Swiss lowlands (Mittelland). Close to the average of the past years is the TWD of spruce in the northern part of the Alps (Alp Nord).

Spruce 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.

Tree water deficits (TWD) of beech trees in three different regions. Dots indicate the average of the past 5 days. Bars indicate the average of the past 7 years.

TWDs of beech are still above the average of the past years in the lowlands of Switzerland (Mittelland) and in the Vallais (Alp Wallis). The drought-stress of the trees in the Vallais even increased. The amount of rain was not enough to recover the trees in the past 5 days. 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. The leaf senescence of beech is up to two months earlier than in past years.

See also the earlier reports about the development and interpretation of tree water deficits in trees:

See further the media reports about drought effects on forest trees in Switzerland.

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

16 August 2018, RZ and MH