Rain over the weekend brought some stress relief to forest trees in Switzerland, particularly in the Northern Alps and in the Valais (see situation on July 16). In these two regions, the tree water deficit-induced stem shrinkage (TWD, Baumwasserdefizit) decreased and the current TWDs returned to a range of values measured also in the years before (since 2011).
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 TWDs generally decreased since the last evaluation on July 16. This means the lack of water in forest trees and thus drought stress has decreased.
However, the situation in the lowlands of Switzerland (Mittelland) remains exceptional in terms of very high TWD. Particularly spruce trees show TWDs in a range not measured before yet.
The figure shows three regions of Switzerland: the region in the northern part of the Alps (Alp Nord), the Valais, a valley located in the central Alps, and the lowlands north of the Alps (Mittelland). The red columns indicate the mean tree water deficit (TWD) of the five days from 19-24 of July 2018 and the orange columns indicate the mean value of the same days in the years 2011-2017. The black symbols indicate the range of values occurring during this time period. The figure includes several tree species.
The figure shows TWDs of different tree species within each of the three regions. In the northern Alps fir, pine, and spruce currently are within the TWD range of the years before. This region shows the greatest reduction of TWD compared to a week ago. TWD of beech, oak, and pine in the central Alpine valley Wallis are still above the average TWD of the past years but also to a lesser extend than a week ago. TWDs of spruce in the Swiss lowlands (Mittelland) are far higher than the TWDs measured in the past years. Here the situation has become even more accentuated than a week ago. We conclude that spruce trees in the Mittelland are currently affected most by drought.
The lack of rain and the consequent drought effects are very heterogenous over the Mittelland. The area of Basel (northwest of Switzerland) seems to be particularly dry at the moment (20min.ch). Unfortunately, both TreeNet stations in this area are in revision and currently do not send data.
TreeNet Switzerland collects continuous data on stem radius fluctuations measured with point dendrometers from 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