The Hidden Life of Trees: What They Feel, How They CommunicateDiscoveries from a Secret World
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— WOODY CLIMATE CONTROL —
TREES DO NOT enjoy extreme changes in temperature or moisture, and regional climates do not spare anything, not even large plants. But have you considered whether trees might be able to exert their influence once in a while? My Eureka moment on this subject came in a little forest growing on dry, sandy, nutrient-deficient soil near Bamberg, Germany. Forest specialists once claimed that only pines could flourish here. To avoid creating a bleak monoculture, beeches were also planted so that their leaves could neutralize the acid in the pine needles to make them more palatable to the creatures in the soil. There was no thought of using the deciduous trees for lumber; they were considered to be so-called service trees. But the beeches had no intention of playing a subservient role. After a few decades, they showed what they were made of.
With their annual leaf fall, the beeches created an alkaline humus that could store a lot of water. In addition, the air in this little forest gradually became moister, because the leaves of the growing beeches calmed the air by reducing the speed of the wind blowing through the trunks of the pines. Calmer air meant less water evaporated. More water allowed the beeches to prosper, and one day they grew up and over the tops of the pines. In the meantime, the forest floor and the microclimate had both changed so much that the conditions became more suited to deciduous trees than to the more frugal conifers. This transformation is a good example of what trees can do to change their environment. As foresters like to say, the forest creates its own ideal habitat.
As I have just explained, as far as calming the wind is concerned, this is certainly possible, but what about budgeting water? Well, if hot summer air cannot blow-dry the forest floor because the soil remains deeply shaded and well protected, then that too is possible. In the forest I manage, students from RWTH Aachen discovered just how great the temperature differences can be between a coniferous plantation that is regularly thinned and a beech forest that has been allowed to age naturally. On an extremely hot August day that chased the thermometer up to 98 degrees Fahrenheit, the floor of the deciduous forest was up to 50 degrees cooler than that of the coniferous forest, which was only a couple of miles away. This cooling effect, which meant less water lost, was very clearly because of the biomass, which also contributed shade. The more living and dead wood there is in the forest, the thicker the layer of humus on the ground and the more water is stored in the total forest mass. Evaporation leads to cooling, which, in turn, leads to less evaporation. To put it another way, in summer an intact forest sweats for the same reason people do and with the same result.
Incidentally, you can indirectly observe trees sweating by looking at houses. You often find Christmas trees with intact root balls that people did not want to discard, all nicely planted by the house and in the best of health. They grow and grow, and sooner or later they get much larger than the homeowners anticipated. Usually, they are planted too close to the side of the house, and some of their branches might even extend out over the roof. And this is when you can see something like sweat stains. These are unpleasant enough when we get them under our arms, but for houses there are more than merely visual consequences. The trees sweat so profusely that algae and moss colonize their moist facades and roof tiles. Rainwater, slowed down by the plant growth, no longer drains away so easily, and dislodged patches of moss clog the gutters. The stucco crumbles over the years because of the damp and has to be replaced prematurely. People who park their cars under trees, however, benefit from the way trees even out extremes. When there are freezing temperatures, people who park their cars out in the open have to scrape ice from their windows, whereas cars parked under trees often remain ice free. Apart from the fact that trees can negatively affect the exterior of buildings, I find it fascinating how much spruce and other species influence microclimates in their vicinity. Consider how much greater the influence of an undisturbed forest must be.
Whoever sweats a lot must also drink a lot. And during a downpour, you have the opportunity to observe a tree taking a long swig. Because downpours usually happen at the same time as storms, I can’t recommend taking a walk out into the forest. However, if you, like me (often because of work), are outside anyway, then you can observe a fascinating spectacle. Mostly, it is beeches that indulge in such all-out drinking binges. Like many deciduous trees, they angle their branches up. Or you could say, down. For the crown opens its leaves not only to catch sunlight but also to catch water. Rain falls on hundreds of thousands of leaves, and the moisture drips from them down onto the twigs. From there, it runs along the branches, where the tiny streams of water unite into a river that rushes down the trunk. By the time it reaches the lower part of the trunk, the water is shooting down so fast that when it hits the ground, it foams up. During a severe storm, a mature tree can down an additional couple of hundred gallons of water that, thanks to its construction, it funnels to its roots. There, the water is stored in the surrounding soil, where it can help the tree over the next few dry spells.
Spruce and firs can’t do this. The crafty firs like to mix in with the beeches, whereas the spruce usually stick together, which means they’re often thirsty. Their crowns act like umbrellas, which is really convenient for hikers. If you’re caught in a shower and stick close to the trunks, you’ll hardly get wet at all, and neither will the trees’ roots. Rainfall of up to 2.5 gallons of water per square yard of forest (that’s a pretty good downpour) gets completely hung up in the needles and branches. When the clouds clear, this water evaporates and all this precious moisture is lost. Why do spruce do this? They have, quite simply, never learned to adapt to water shortages.
Spruce are comfortable in cold regions where, thanks to the low temperatures, the groundwater hardly ever evaporates. For instance, they like it up in the Alps just below the tree line where particularly heavy downpours ensure that drought is never an issue. There are heavy snowfalls, though, which is why the branches grow out horizontally or angled slightly down so that they can lean on each other for support when the snow piles up. But this means that water doesn’t run down the tree, and when spruce are growing in drier areas at lower elevations, then this winter adaptation is of no use to them. The majority of the coniferous forests we have in Central Europe today were planted, and people put the forests in places that made sense to them. In these places, the conifers are always suffering from thirst, and all the while their built-in umbrellas are intercepting one third of the rain that falls and returning it to the atmosphere. Deciduous forests intercept only 15 percent of the rain that falls, which means they are profiting from 15 percent more water than their needle-bedecked colleagues.
BEECH
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— THE FOREST AS —
WATER PUMP
HOW DOES WATER get to the forest, anyway, or—to take one step further back—how does water reach land at all? It seems like such a simple question, but the answer turns out to be rather complicated. For one of the essential characteristics of land is that it is higher than water. Gravity causes water to flow down to the lowest point, which should cause the continents to dry out. The only reason this doesn’t happen is thanks to supplies of water constantly dropped off by clouds, which form over the oceans and are blown over land by the wind. However, this mechanism only functions within a few hundred miles of the coast. The farther inland you go, the drier it is, because the clouds get rained out and disappear. When you get about 400 miles from the coast, it is so dry that the first deserts appear. If we depended on just this mechanism for water, life would be possible only in a narrow band around the edge of continents; the interior of land masses would be arid and bleak. So, thank goodness for trees.
Of all the plants, trees have the largest surface area covered in leaves. For every square yard of forest, 27 square yards of leaves and needles blanket the crowns.43 Part of every rainfall is intercepted in the canopy and immediately evaporates again. In addition, each summer, trees use up to 8,500 cubic yards of water per square mile, whic
h they release into the air through transpiration. This water vapor creates new clouds that travel farther inland to release their rain. As the cycle continues, water reaches even the most remote areas. This water pump works so well that the downpours in some large areas of the world, such as the Amazon Basin, are almost as heavy thousands of miles inland as they are on the coast.
There are a few requirements for the pump to work: from the ocean to the farthest corner, there must be forest. And, most importantly, the coastal forests are the foundations for this system. If they do not exist, the system falls apart. Scientists credit Anastassia Makarieva from Saint Petersburg in Russia for the discovery of these unbelievably important connections.44 They studied different forests around the world and everywhere the results were the same. It didn’t matter if they were studying a rain forest or the Siberian taiga, it was always the trees that were transferring life-giving moisture into land-locked interiors. Researchers also discovered that the whole process breaks down if coastal forests are cleared. It’s a bit like if you were using an electrical pump to distribute water and you pulled the intake pipe out of the pond. The fallout is already apparent in Brazil, where the Amazonian rain forest is steadily drying out. Central Europe is within the 400-mile zone and, therefore, close enough to the intake area. Thankfully, there are still forests here, even if they are greatly diminished.
Coniferous forests in the Northern Hemisphere influence climate and manage water in other ways, too. Conifers give off terpenes, substances originally intended as a defense against illness and pests. When these molecules get into the air, moisture condenses on them, creating clouds that are twice as thick as the clouds over non-forested areas. The possibility of rain increases, and in addition, about 5 percent of the sunlight is reflected away from the ground. Temperatures in the area fall. Cool and moist—just how conifers like it. Given this reciprocal relationship between trees and weather, forest ecosystems probably play an important role in slowing down climate change.45
For ecosystems in Central Europe, regular rainfall is extremely important because water and forests share an almost unbreakable bond. Streams, ponds—even the forest itself—all these ecosystems depend on providing their inhabitants with as much stability as they can. A good example of an organism that doesn’t like a lot of change is the freshwater snail. Depending on the species, it is often less than 0.08 inches long, and it loves cold water. They like it to be no more than 46 degrees Fahrenheit, and for some freshwater snails the reason for this lies in their past: their ancestors lived in the meltwater that drained off the glaciers covering a large part of Europe in the last ice age.
Clean springs in the forest offer such conditions. The water comes out at a constantly cool temperature, for these springs are where groundwater bubbles to the surface. Groundwater is found deep underground, where it is insulated from outside air temperatures, and therefore, it is as cold in summer as it is in winter. Given that we no longer have any glaciers, this is the ideal replacement habitat for today’s freshwater snails. But that means the water has to bubble up year round, and this is where the forest comes into play. The forest floor acts as a huge sponge that diligently collects all the rainfall. The trees make sure that the raindrops don’t land heavily on the ground but drip gently from their branches. The loosely packed soil absorbs all the water, so instead of the raindrops joining together to form small streams that rush away in the blink of an eye, they remain trapped in the soil. Once the soil is saturated and the reservoir for the trees is full, excess moisture is released slowly and over the course of many years, deeper and deeper into the layers below the surface. It can take decades before the moisture once again sees the light of day. Fluctuations between periods of drought and heavy rain become a thing of the past, and what remains is a constantly bubbling spring.
Although it has to be said, it doesn’t always bubble. Often it looks more like a swampy-squishy area, a dark patch on the forest floor seeping toward the nearest little stream. If you take a closer look (and to do that you must get down on your knees), you can make out the tiny rivulets that betray the existence of a spring. Now, to find out whether this is indeed groundwater or just surface water left over from a heavy shower, reach for your thermometer. Less than 48 degrees Fahrenheit? Then it is indeed a spring. But what kind of a person carries a thermometer around all the time? Another option is to take a walk when there’s a hard frost. Puddles and rainwater will be frozen, while water will still be seeping out of a spring. This then is where the freshwater snails call home, and here they enjoy their preferred temperature year round. And it is not only the forest floor that makes this possible. In summer, a microhabitat like this could warm up quickly and overheat the snails. But the leafy canopy throws shadows that block out most of the sun.
The forest offers a similar and even more important service to streams. The water in a stream is susceptible to greater temperature variations than spring water, which is continuously replaced with cool groundwater. These streams contain animals such as salamander larvae and tadpoles, which are just waiting for their life outside the stream to begin. Like the freshwater snails, they need the water to remain cool so that oxygen doesn’t escape, but if the water freezes solid, the baby salamanders will die. It’s a good thing deciduous trees just happen to solve this problem. In winter, when there is very little warmth in the sun, bare branches allow a lot of warmth to penetrate. The movement of water over the uneven bottom also protects the stream from suddenly freezing. When the sun climbs higher in the sky in late spring and the air is noticeably warmer, the trees unfurl their leaves, closing the blinds and shading the running water. Then in the fall, when temperatures drop once again, the sky reopens above the stream when the trees drop all their leaves. It’s tougher for streams that flow under coniferous trees. It’s bitterly cold here in winter, and sometimes the water freezes solid. Because it warms up only gradually in spring, this habitat is just not an option for many organisms. But pitch-dark streams like this are rarely found in Nature, because spruce don’t like having wet feet and, therefore, usually keep their distance. It’s most often plantations that cause this conflict between coniferous forests and the denizens of streams.
The importance of trees for streams continues even after death. When a dead beech falls across a streambed, it lies there for decades. It acts like a small dam and creates tiny pockets of calm water where species that can’t tolerate strong currents can hang out. The nondescript larvae of the fire salamander are just such creatures. They look like small newts, except they have feathery gills. They are finely stippled with dark markings and have a yellow spot where their legs meet their bodies. In the cold water of the forest, they lie in wait for the tiny crawfish they love to eat. These little guys need crystal-clear water, and the dead trees look after this as well. Mud and floating debris drop to the bottom of the tiny dammed pools, and because stream flows are so low, it gives bacteria more time to break down harmful substances. And don’t worry about that foam that sometimes forms in these pools after heavy rains. What looks like an environmental disaster is, in fact, the result of humic acids that tiny waterfalls have mixed with air until they turn into froth. These acids come from the decomposition of leaves and dead wood and are extremely beneficial for the ecosystem.
When it comes to the creation of small pools, forests in Central Europe have become less dependent on dead trees falling down. They are increasingly getting help from an animal that has recently made a comeback after being nearly eradicated. This animal is the beaver. I have my doubts whether the trees are really happy about this, for this rodent, which can weigh more than 60 pounds, is the lumberjack of the animal world. It takes a beaver one night to bring down a 3-to-4-inch-thick tree. Larger trees are felled over the course of multiple work shifts. What the beaver is after are twigs and small branches, which it uses for food. It stockpiles enormous quantities in its lodge to last the winter, and as the years pass, the lodge grows by many yards. The branches also camouflage the entrances
to the tunnels that lead into the lodge. As an added security feature, the beaver builds these entrances underwater so that predators can’t get in. The rest of the living space is above water and therefore dry.
Because water levels can fluctuate wildly with the seasons, many beavers also build dams, blocking streams and turning them into large ponds. Beaver ponds slow the flow of water from the forest, and extensive wetlands form in the areas around the dams. Alders and willows like to grow here; beeches, which cannot stand having wet feet, die off. But the upstart trees in the feeding zone around the lodge don’t get to grow old, for they are the beavers’ living larder. Although beavers damage the forest around them, they exert a positive influence overall by regulating water supplies. And while they’re at it, they provide habitat for species adapted to large areas of standing water.
So, as we close this chapter, let’s return once again to the source of water in the forest—rain. Rain can put you in the most wonderful mood while you are out walking, but if you’re not wearing the right clothes, it can be unpleasant. If you live in Europe, mature deciduous trees offer a very special service to help you: a short-term weather forecast brought to you by chaffinches. These rust-red birds with gray heads normally sing a song whose rhythm ornithologists like to transcribe as “chip chip chip chooee chooee cheeoo.” But you’ll hear that song only on a fine day. If it looks like rain, the song changes to a loud “run run run run run.”