“The brown leaves you see now are not normal”

Is the impression deceptive, or are the leaves in the forest turning brown rather than usual?
Constantine Zohner: The forest actually looks different than usual, we are experiencing a special year. The leaves usually don’t discolor that early. Something stands out this year.

That would?
The brown leaves you see now are not normal. Usually there is a transition from green to yellow and possibly red – the beautiful autumn colors. But we don’t see that at the moment. This year the leaves have turned brown and wilted.

Are specific trees or all deciduous trees affected?
It seems that many trees have been affected, but only a few species. However, we do not yet have precise information about this. The situation can differ from forest to forest. This emphasizes the important role of the mixed forest. If we had monocultures, the whole forest could be affected by drought.

Why are the leaves wilting so early this year?
Various factors play a role in this.

In mid-August it was very hot in Switzerland.
Yes, and before that it was very cool for a while. The shift from wet weather with low temperatures to extreme heat and dryness is certainly a factor why the leaves are already wilting. The high light intensity and the ozone level may also have played a role.

Do bark beetles and other pests also have something to do with wilting leaves?
High ozone levels, pollution, heat, drought: these are all factors that lead to increased stress for the trees. This stress, caused by climate change, makes the trees more susceptible to disease. Therefore, bark beetle infestations are increasingly observed in our forests.

When do the leaves usually change color?
End of September.

That’s only a few weeks away.
Yes, we are way too early. At the end of September, the forests are usually still quite green. The strongest decrease in the green color and the transition to yellow mainly takes place in October.

What are the consequences of premature leaf fall?
As a result, the forests can absorb less CO2 from the air. And if a tree has years of stress damage, it is possible that it will die. There will be major impacts on the ecosystem as the forest becomes less productive and more susceptible to disease. Birds and insects that live in the trees will also feel the effects.

Do we have to get used to the fact that our forests will turn brown as early as August in the future?
That is an interesting question, because the answer to it is not so simple.

To explain!
It used to be assumed that climate change would slow down leaf fall. Because plants need a cold stimulus to break down the chlorophyll. So if it is warmer for longer in autumn, this may mean that the trees are green longer. But as it turns out, there are factors that work against a longer growth phase.

For example, the drought and the UV exposure you mentioned earlier.
Precisely. But not alone. The activity of plants in the spring also plays an important role. If they become active early in the year, they are more likely to tire. For the past 60 years we have found that the spring budding of the plants has been delayed by two weeks. This causes them to stop growing a little earlier in the fall. Due to the interplay of all these factors, we do not see any major changes in the autumn. However, there are increasingly extreme years – such as now – in which the leaves fall earlier than usual. The stress years will tend to increase.

Are there immediate measures to protect the trees?
The most important measure is to strengthen the mixed forest. If a tree species has extreme problems, it’s easier for the forest to deal with it if it’s diverse. The more diverse a forest is, the more resilient it is.