Why so many trees are already losing their leaves

Why so many trees are already losing their leaves
Credit: Felix Mittermeier / Pexels

Even though it is still the middle of the summer season, many trees are already starting to lose their leaves – a phenomenon experts are calling "drought stress."

Belgium is experiencing an "early autumn," in which trees are losing their leaves several months earlier than usual in response to the recent drought, according to Kris Verheyen, a forestry professor at the University of Ghent (UGent).

"The fact that trees get rid of their leaves is a defence strategy against drought," he told VRT. "A lot of water evaporates through the leaves, so by shedding them, trees go into protection mode to keep up with the moisture."

However, as trees also need those leaves for photosynthesis, shedding them early also has a downside. "If [the photosynthesis process] does not happen, trees absorb fewer sugars and do not replenish their food stocks. In the long term, this leads to exhaustion and reduced resistance."

'Complete exhaustion of the trees'

In the long run, that lack of resistance could ultimately cause trees to disappear from Belgium. Flanders, for example, is no longer a good environment for the Norway spruce, which has almost completely disappeared or died off in the region, according to Verheyen.

"Due to the drought, the spruces were weakened enormously, which gave European spruce bark beetles, small beetles that love coniferous trees, free rein," he explained.

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In previous years, the leaves also already started falling in the summer months. Last year, however, they did not because it was a very wet summer. "That definitely did the trees some good, and it was necessary as well because in the long run, this drought stress threatens to cause complete exhaustion of the trees."

However, not all trees suffer equally from the drought. Beech trees, for example, are very sensitive to it: it is mainly their leaves that are falling now, while oak trees are more drought-resistant.

The soil on which trees grow also plays a role, said Verheyen. "Certain types of soil retain water better. Loam soils, for example, are better at this than sandy soils."

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