tree survival-- 5/22/24

Today's selection -- from The Power of Trees by Peter Wohlleben. The coping strategies of trees:

“In hot, dry summers, trees have big problems. They cannot escape to the shade, and they cannot take a sip of water to cool themselves down. Indeed, they cannot react quickly in any way. And because they're so slow, it's all the more important for them to choose the right coping strategy. But what is the right strategy, and what happens when a tree makes a mistake?

“I had a ringside seat to observe this from the academy I established in Wershofen in the German state of Rhineland-Palatinate to educate people about forests and help support struggling forests around the world. One side of North Street is lined with horse chestnuts. In the dry summer of 2020, the horse chestnuts behaved much like many trees in Europe that year: their leaves began to take on the colors of fall in August, which is far sooner than normal. 

“Horse chestnuts have been having a particularly challenging time in many parts of Europe for years. Shortly before 2000, horse-chestnut leaf miners advancing northward reached the trees in Wershofen. These small, light-brown moths are native to Greece and Macedonia, the horse chestnuts' original homeland. Up until the moths' arrival, like many other imported plants, the horse chestnuts in Wershofen had been leading charmed lives. Although the ecosystems in countries like Germany are not perfect for these trees—it really is a bit too cold for them this far north–nevertheless, our chestnuts settled in nicely. The parasites that plagued them back home had not yet made it to the trees' new location and being slightly colder in winter was a small price to pay for a life without leaf miners. Then, about forty years ago, things started to change. That was when the moths began to follow their prey north, and eventually they, too, arrived in Wershofen.

“Leaf miners do just as their name suggests: the caterpillars ‘mine’ tunnels in leaves. The female moths lay their eggs on the surface of the leaves, and after they hatch, the caterpillars eat their way inside. Small brown, wavy lines show where the moth babies have been happily chowing down—happily, because living inside a leaf offers good protection from hungry birds. The mined sections of the leaves dry out, and the caterpillars continue to eat. As summer progresses, the foliage looks increasingly ragged, especially as the first round of egg laying is often followed by a second.

“The leaves on the trees along North Street were therefore already damaged when, after several hot days, the drought settled in. In a situation like this, chestnuts react just as all trees do: they shut down photosynthesis and wait. The trees have even less of an idea than we do of how long a dry period like this might last and therefore it makes sense for them not to panic right away.

Botanical illustration (1885)

“The trees' first response is to close the millions of tiny mouths, the stomata, located on the undersides of their leaves. Trees use these tiny mouths to breathe, just like we do, and, just like us, trees expel water vapor with every breath.

“The water vapor cools their surroundings as it evaporates, and the green giants actively manage this process to make hot summer days more bearable.

“When the roots signal all the moisture in the soil has been used up, the trees close these countless mouths. However, when the stomata are closed, photosynthesis stops because carbon dioxide is no longer entering the leaves. Without water and carbon dioxide, the trees can no longer use sunlight to produce sugar. At this point, the trees begin to consume the sugar reserves they were hoping to increase so they could survive their long winter sleep.

“Despite the shutdown, the trees continue to lose a minimal amount of moisture through their leaves, roots, and bark. If the drought continues, they now employ a second strategy: they discard some of their leaves. Like their fellow green giants, the chestnuts lose their leaves from top to bottom, and the first leaves to fall are those way up in the canopy, farthest from the roots. Trees expend a great deal of energy transporting water to their crowns, and because they can no longer stockpile sugar, they need to conserve energy. If dropping the topmost leaves doesn't do the trick, and if the rain still doesn't come, the trees continue to drop leaves gradually from the top down until finally, as early as August, they stand there with completely bare branches.

“In 2020, almost none of the beeches, oaks, or chestnuts around Wershofen had resorted to this final step. Only a few individuals had given up. Perhaps these trees were particularly anxious and wanted to play it safe. Or perhaps they were growing in especially dry spots. Whatever the reason, by August, the branches on these trees had no leaves left on them at all.”



Peter Wohlleben


The Power of Trees: How Ancient Forests Can Save Us if We Let Them


Greystone Books


barns and noble booksellers
Support Independent Bookstores - Visit

All delanceyplace profits are donated to charity and support children’s literacy projects.


Sign in or create an account to comment