The trees appear to already have entered autumn, but it’s not the time of year that is causing them to lose their leaves earlier than usual, it’s what naturalists are calling ‘drought stress’.
The precarity of Belgium’s groundwater levels is a perennial subject these days, but this year in particular has been unusually dry. And that leads the trees to turn on their natural defences.
When the tree detects a shortage of water in the ground, it shuts down the mechanisms which allow it to control water loss by evaporation. That leads to the loss of leaves.
This past dry summer was not an isolated event, but simply the latest of a line, said the Flemish nature conservancy organisation Natuurpunt. The importance of that is that one dry summer leaves the tree less well-prepared for whatever Nature may deliver the following year, and so on down the years.
One sign of drought stress is the premature colouring of the leaves in their autumn colours, a result of the tree losing leaves as a protection against drought.
Trees grow less in dry years, and are more vulnerable to pests and diseases. There have been examples of trees simply dying of what seems to be thirst, but that is difficult to prove conclusively because the cause is usually a combination of factors, exacerbated by the dry conditions, according to Natuurpunt.
Drought affects different tree species differently, Natuurpunt explains.
“Beeches suffer a lot from the drought because they have a superficial root system. In addition, they also suffer greatly from the direct impact of solar radiation as the leaves and bark can burn. After all, beech is a real forest tree that is adapted to a lot of shade.”
Limes are currently also well advanced in their autumn colouring. Coarse birch trees are also very sparse and summer oaks can suffer a lot from a too dry soil.”