Summer World_ A Season of Bounty - Bernd Heinrich [97]
There must be a good reason why many northern trees shed their leaves whereas others keep them. My hypothesis regarding which trees do shed and which don’t depends on a conflict of selective pressures that trees must face in the northern hemisphere in areas where there is a lot of precipitation: a large leaf surface area is needed to intercept solar radiation and to absorb carbon dioxide in the summer; but the same leaf surface is a liability in the winter because snow loading could collapse the tree.
What we observe now is a result of evolution over hundreds of millions of years. But the selective pressures that have acted on some features in the past are now unlikely to occur every year and may be seen only rarely. Instead, they are probably witnessed only at bottlenecks. One such event occurred in New England on 26 October 2005, near our home in Vermont. The following journal entry for that date describes what happened:
It rained all day yesterday, and temperatures were dropping gradually: to 40, 39, and 35°F by evening. The sky stayed dark. Flocks of geese passed over. I woke up in the dark, and the light switch did not respond. I then looked out—SNOW! I went back to bed and waited for daylight before brewing a cup of coffee and stepping outside for a closer look. It was still snowing, and the outside thermometer then read 29°F. I saw devastation—the result of a confluence of rather precise temperature changes, wind directions, clouds, and all this weather in relation to the timing of the leaf shedding of the trees. A perfect timing, complete with proper experimental controls, had produced a rare natural experiment.
At that time, near the end of October, many trees—red oak, quaking aspen, apple, black locust, and silver maple—still retained their full complement of green leaves. Other deciduous trees, including white ash, elm, and red maple, had lost all theirs. Some of the sugar maples, black cherry, and white birches were bare, but a few still had branches that retained most of their leaves, by now golden.
The effect of the snow on individual trees was dramatic but unrelated to the species as such. Trees that retained their leaves paid a steep price. Those that had shed their leaves suffered no damage. The thin, young maples and oaks in the woods around our house were snapped in half or bent to the ground. Similarly, old sugar maples with heavy trunks had huge limbs broken off, and many of their other limbs were bent and ready to snap. The black cherry next to the house had retained its leaves; and while I was getting wood out of the shed for our stove, three of its huge limbs cracked and fell. One after another, they came crashing to the ground. From the nearby woods, I heard what sounded like muffled rifle shots followed by dull thumps: tall poplars were falling. The red oaks that had suffered the least damage during the great ice storm of 8 January 1998 were now hit the worst. Healthy oak trunks a foot or more in diameter had bent and shattered. Limbs two or more feet thick lay heaped on the ground. In sharp contrast, no trees or limbs that had shed their leaves (these trees included some maples, cherries, poplars, and oaks) were damaged. As I later learned, the same scene was enacted over a large part of northern New England, especially at the higher elevations.
Trees face the same problem of timing at the beginning of the summer, as was shown at the same location on 30 May 1996. As is typical by the end of May, all the trees had just