17
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.