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Bill BrysonA Walk in the Woodspp. 155 |
For all its mass, a tree is a remarkably delicate thing.
For all its mass, a tree is a remarkably delicate thing. All of its internal life exists within three paper-thin layers of tissue, the phloem, xylem and cambium, just beneath the bark, which together form a mois sleeve around the dead heartwood. However tall it grows, a tree is just a few pounds of living cells thinly spread between the roots and leaves. These three diligent layers of cells perform all the intricate science and engineering needed to keep a tree alive, and the efficiency with which they do is one of the wonders of life. Without noise or fuss, every tree in a forest lifts massive volumes of water - several hundred gallons in the case of a large tree on a hot day - from its roots to its leaves, where it is returned to the atmosphere. Imagine the din and commotion, the clutter of machinery, that would be needed to a fire department to raise a similar volume of water to that of a single tree. And lifting water is just one of the many jobs the phloem, xylem and cambium perform.
They also manufacture lignin and cellulose, regulate the storage and production of tannin, sap, gum, oils, and resins, dole out minerals and nutrients, convert starches into sugars for future growth (which is where the maple syrup comes into the picture), and goodness knows what else. But because all this is happening in such a thin layer, It also leaves the tree terribly vulnerable to invasive organisms. To combat this, trees have formed elaborate defense mechanisms. The reason a rubber tree seeps latex when cut is that this is its way of saying to insects and other organisms, ´Not tasty. Nothing here for you. Go away.´ Trees can also deter destructive creatures like caterpillars by flooding their leaves with tannin, which makes the leaves less tasty and so inclines the caterpillars to look elsewhere. When infestations are particularly severe, some trees can even communicate the fact. Some species of oak release a chemical that tells other oaks in the vicinity that an attack is under way. In response, the neighbouring oaks step up their tannin production the better to withstand the coming onslaught.
By such means, of course, does nature tick along. The problem arises when a tree ncounters an attacker for which evolution has left it unprepaired, and seldom has a tree been more helpless against an invader than the American chestnut against Endothia parasitica. It enters a chestnut effortlessly, devours the cambium cells and positions itself for attack on the next tree before the tree has the faintest idea, chemically speaking, what hit it. It spreads by means of spores, which are produced in the hundreds of millions in each canker. A single woodpecker can transfer a billion spores on the flight between trees. At the height of the American chestnut blight, every woodland breeze would loose spores in uncountable trillions to drift in a petty, lethal haze onto heighbouring hillslides. The mortality rate was 100 per cent. In just over thrity-five years the American chestnat became a memory. The Appalachians alone lost four billion trees, a quarter of its cover, in a generation.
A great tragedy, of course. But how lucky, when you think about it, that these diseases are at least species specific. Instead of a chestnut blight or Dutch elm disease or dogwood anthranoce, what if there was just a tree blight - something indisciminate and unstoppable swept through whole forests? And in fact there is. It’s called acid rain.