1. When Trees Make Mistakes 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.
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. The chestnuts in particular really couldn''t afford to lose their leaves, as by summer they had already been weakened by leaf miners. Their leaves, covered in brown patches where the caterpillars had fed, were producing limited quantities of sugar, which meant the trees were starving. Next on the chestnuts'' list of challenges was the elevation at which they were growing. North Street lies about 2,000 feet (600 meters) above sea level. The elevation and the harsh conditions in the Eifel, a low mountain range in central Germany, make for a short growing season and the window to produce sugar is tight. Trees need to produce enough not only to keep going day to day, but also to see them through their winter rest period and get them kick-started the following spring. In the Eifel, far from their original home, this was already a struggle for the chestnuts.
And now they were dealing with the third dry summer in a row, during which every last drop of water in the soil had clearly been used up. Under normal circumstances, when trees encounter conditions like these, they simply advance their seasonal shutdown and drop their leaves in September--which is what the beeches in my forest typically do. Although the beeches look dead, they leaf out again in the spring and try to make up for time lost the previous year. Most of the chestnuts do this too. The anxious ones that dropped their leaves in August had definitely jumped the gun and employed this survival strategy too soon. On August 31, 2020, the weather gods had a change of heart. In a small area on the northern edge of the Eifel, clouds darkened the sky. It rained for hours, dumping about a quarter of an inch of rain (60 liters of water per square meter).
This wasn''t nearly enough to replenish water reserves in the drought-stricken soil, but the upper few inches got a bit damp, and I hoped it would be enough to give the trees some respite. The panicky chestnuts'' reaction over the next few days took me by surprise and initially made no sense: they began to blossom. A tree that is already short on sugar shouldn''t be putting energy into reproduction, especially since producing blossoms isn''t going to pay off this late in the year. Even if the flowers were fertilized, there would be no time to develop seeds and fruit before the onset of winter. I was on my way back to the forest academy with a group of forest guides in training when they drew my attention to the chestnuts'' behavior. We stopped to take a closer look, and it immediately became clear what was going on. Not only were the trees blooming, but they were also unfurling fresh new leaves. This solved the mystery.
The chestnuts were starving. With fresh green growth on their branches, they could now stock up on sugar and fill their storage spaces. Clearly, the trees couldn''t tell if they were growing only leaf buds or all the buds on their branches, including those that produce blossoms. And this was what we were witnessing. I made a short video with my cell phone and uploaded it to my social media page for discussion. I soon learned some chestnuts in other areas were resorting to the same strategy. An internet search revealed that a few horse chestnuts had unfurled fall blossoms in previous years. I wasn''t totally convinced by the explanations that were being offered, which suggested the stress of climate change and attacks by leaf miners and fungi had brought the trees to the brink.
The trees, then, were blossoming in the fall in a final, desperate effort to reproduce. On the face of it, the explanations sounded logical. However, they assumed trees are unable to differentiate between seasons. Blossoms in fall won''t result in fruit because the few weeks that remain until winter are nowhere near long enough for fruit to ripen. Any tree that pursues such nonsensical behavior squanders energy and adds to its predicament. Scientists have known for decades that trees calibrate their behavior based on day length and temperature. In other words, they follow the seasons as accurately as we do--and they do so without needing a calendar. And this is where the next head-scratching assertion pops up: chestnuts are mixing up their seasons.
The summer drought that interrupted their uptake of water and therefore their ability to photosynthesize&n.