The Hidden Life of Trees: What They Feel, How They Communicate—Discoveries from a Secret World by Peter Wohlleben

Author:Peter Wohlleben
Language: eng
Format: mobi
ISBN: 9781771642491
Publisher: Greystone Books
Published: 2016-09-12T23:00:00+00:00

ASPEN

23

— A SENSE OF TIME —

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.54 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.55 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.

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