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Are beech trees from inner alpine dry valleys adapted to drought?

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Approximately every fourth forest tree in central Switzerland is a beech – and in the Jura every third. This makes the beech the most common tree species in the lower-lying regions (the foothill and lower montane altitude zones) of our country. But there might be trouble in store in future: beeches are poorly equipped for future climate changes, as they do not cope well with extremely dry summers. Ecophysiologists Marcus Schaub and Matthias Arend have taken an interest in beech trees that have long grown in dry locations, but which are also adapted to the cli­matic conditions of the Swiss winter. Could they perhaps serve as pro­genitors for future generations of beech tree when conditions in central Switzerland become drier?

In spring 2011, Arend replanted young beeches from the dry forest populations of the Rhone and Rhine valleys into the Model Ecosystem Facility (MODOEK) in Birmensdorf – a testing plant at which soil dryness can be precisely controlled. For the purposes of comparison, Arend also replanted beeches from considerably damper forest populations, also from the Rhone and Rhine valleys. In the summers of 2013 and 2014, half the trees were exposed to an artificial, controlled summer drought, while the other half grew in damper con­ditions. The researchers investigated how the trees grew during and after the drought, if their metabolism changed and how they developed in different seasons. As expected, the photosynthetic rates of the trees from dry forest populations diminished less, and made quicker recoveries after the drought. Their yearly stem growth was also less affected. In an astonishing result, trees of all origins demonstrated a higher photosyn­thetic rate after the drought than trees that had always had enough water. Thus, they were able to compensate for some of the loss suffered. “This was something we hadn’t counted on,” says Schaub.

Forest Policy 2020’s precautionary adaptation strategy recommends that forest managers use trees from species and origins that tolerate dry conditions well. This experiment demonstrates that some Swiss beech trees have the origins necessary for this. (Birgit Ottmer, Diagonal 2/16)