Can trees count? How do they know when to shed their leaves?

In many latitudes, forests drop leaves in the fall and leaf out in the spring, and we take this cycle for granted. But if we take a closer look, the whole thing is a big mystery, because it means that trees need something very important: a sense of time. How do they know that winter is coming or that rising temperatures aren’t just a brief interlude but an announcement that spring has arrived?

It seems logical that warmer days trigger leaf growth, because this is when frozen water in the tree trunk thaws to flow once again. What is unexpected is that the colder the preceding winter, the earlier the leaves unfurl. Researchers from the Technical University of Munich (TUM) tested this in a climate-controlled laboratory.

The warmer the cold season, the later beech branches greened up – and at first glance, that doesn’t seem logical. After all, in warm years, lots of other plants – wild flowers, for example – often start to grow in January and even begin to flower, as we are constantly reminded by media headlines. Perhaps trees need freezing temperatures to get a restorative sleep in winter and that’s why they don’t get going right away in the spring. Whatever the reason, in these times of climate change, this is a disadvantage, because other species that are not so tired and grow their new leaves more quickly will be a step ahead.

How often have we experienced warm spells in January or February without the oaks and beeches greening up? How do they know that it isn’t yet time to start growing again?

We’ve begun to solve the puzzle with fruit trees, at least. It seems the trees can count! They wait until a certain number of warm days have passed, and only then do they trust that all is well and classify the warm phase as spring. But warm days alone do not mean spring has arrived.

Shedding leaves and growing new ones depends not only on temperature but also on how long the days are. Beeches, for example, don’t start growing until it is light for at least thirteen hours a day. That in itself is astounding, because to do this, trees must have some kind of ability to see. It makes sense to look for this ability in the leaves. After all, they come with a kind of solar cell, which makes them well equipped to receive light waves. And this is just what they do in the summer months, but in April the leaves are not yet out.

We don’t yet understand the process completely, but it is probably the buds that are equipped with this ability. The folded leaves are resting peacefully in the buds, which are covered with brown scales to prevent them from drying out. Take a closer look at these scales when the leaves start to grow and hold them up to the light. Then you’ll see it. They’re transparent! It probably takes only the tiniest amount of light for the buds to register day length, as we already know from the seeds of some agricultural weeds.

Out in the fields, all it takes is the weak light of the moon at night to trigger germination. And a tree trunk can register light as well. Most tree species have tiny dormant buds nestled in their bark. When a neighbouring tree dies and falls down, more sun gets in, which in many trees triggers the growth of these buds so that the tree can take advantage of the additional light.

And how do trees register that the warmer days are because of spring and not late summer? The appropriate reaction is triggered by a combination of day length and temperature. Rising temperatures mean it’s spring. Falling temperatures mean it’s fall. Trees are aware of that as well. And that’s why species such as oaks or beeches, which are native to the northern hemisphere, adapt to reversed cycles in the southern hemisphere if they are exported to New Zealand and planted there. And what this proves as well, by the way, is that trees must have a memory. How else could they inwardly compare day lengths or count warm days?

In particularly warm years, with high fall temperatures, you can find trees whose sense of time has become confused.

Their buds swell in September, and a few trees even put out new leaves. Trees that get in a muddle like this have to suffer the consequences when delayed frosts finally arrive. The fresh growth has not had time to get wood – that is, to get hard and tough for winter – and the leaves are defenceless anyway. And so the new greenery freezes, and that must surely hurt. Worse, the buds for next spring are now lost and costly replacements must be grown. If a tree isn’t careful, it will deplete its energy supplies and be less prepared for the coming season.

Trees need a sense of time for more than just their foliage. This sense is equally important for procreation. If their seeds fall to the ground in fall, they mustn’t sprout right away. If they do, two problems present themselves. First, the delicate shoots won’t have time to get woody, which means they will freeze. Second, when the weather is cold, there is very little for deer to eat and they would be only too happy to pounce on the fresh, green growth. So it’s better to sprout in the spring along with all the other plants. Therefore, seeds register cold, and only when extended warm periods follow hard frost do the baby trees dare to come out of their protective coverings.

Many seeds don’t possess a sophisticated counting mechanism like the one used to trigger leaf growth, and that’s why it works so well when squirrels and jays bury beechnuts and acorns an inch or so deep in the soil. Down here it doesn’t warm up until true spring arrives. Light seeds, such as the seeds of birches, have to pay more attention. With their little wings, they always land on the surface of the soil and just lie there. Depending on where they come to rest, they might end up in bright sunlight, and therefore, these little ones must be able to wait and register the appropriate day length just as their parents do.

Excerpted with permission from The Hidden Life of Trees: What They Feel, How They Communicate – Discoveries from a Secret World, Peter Wohlleben, translated from the German by Jane Billinghurst, introduced by Pradip Krishen, Penguin Allen Lane.