Trees in the Northern Alps and in the Valais show distinctly higher tree water deficit-induced stem shrinkage (TWD) than in the years before. We conclude that the investigated tree species in these areas are more heavily affected by drought than in any of the years measured before (since 2011). Forest trees in the Swiss lowlands are not strongly affected by drought yet.
TreeNet detects how much tree stems shrink during dry periods with point dendrometers. 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. We compared the average TWDs of the past five days (11-16 July 2018) with TWDs of the same period of the years before (starting in 2011) in order to quantify current levels of drought stress.
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 11-16 July 2018 and the orange columns indicate the mean value of 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 (Tanne), pine (Föhre), and spruce (Fichte) currently show higher mean TWDs than in the years before. The same is true for the species beech (Buche), oak (Eiche), pine (Föhre), and spruce (Fichte) in the central Alpine valley Wallis. Somewhat increased but not yet beyond the range of past years are TWDs measured in the Swiss lowlands (Mittelland).
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