manipulated as a kind of joke, like those old postcards that show watermelons as big as
barns or an ear of corn that entirely fills a wagon under the droll legend "A TYPICAL
IOWA FARM SCENE." But this is simply the way it was--the way it was over tens of
thousands of square miles of hill and cove, from the Carolinas to New England. And it is
all gone now.
In 1904, a keeper at the Bronx Zoo in New York noticed that the zoo's handsome
chestnuts had become covered in small orange cankers of an unfamiliar type. Within days
they began to sicken and die. By the time scientists identified the source as an Asian
fungus called Endothia parasitica, probably introduced with a shipment of trees or infected
lumber from the Orient, the chestnuts were dead and the fungus had escaped into the
great sprawl of the Appalachians, where one tree in every four was a chestnut.
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 moist sleeve around the dead heartwood. However tall it
grows, a tree is just a few pounds of living cells thinly spread between 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 it 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 for a fire department to raise a similar volume of water.
And lifting water is just one of the many jobs that 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 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 neighboring 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
encounters an attacker for which evolution has left it unprepared, 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 one flight between
trees. At the height of the American chestnut blight, every woodland breeze would lose
spores in uncountable trillions to drift in a pretty, lethal haze on to neighboring hillsides.
The mortality rate was 100 percent. In just over thirty-five years the American chestnut
sean pound
(Sean Pound)
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