THE NATURALIST’S NOTEBOOK: A glacier’s gift
The story of New England’s forests
I’m from Vermont. My state has many trees and a few people. When you combine those two things, you get delicious maple syrup. In October, you also get hordes of tourists — the so-called “leaf-peepers.” Vermont calls itself the Green Mountain State, but it is really now, when the mountains are red and orange, that the forest gets the most attention. With winter approaching, trees pump the precious chlorophyll from their leaves and store it safely in their roots, revealing other leaf pigments that were previously obscured by green: the carotenoids (yellow/orange) and anthocyanins (red).
The autumn transformation starts with occasional splashes of color. Spots of scarlet appear on the leaves of the red maples, then whole trees seem to burst into flame. Next, the sugar maples glow golden orange. On the mountain slopes, birches and aspens add a stroke of yellow, and are matched by white ash and silver maple in the valleys. Red oaks join in, asserting a sober chestnut-red. The beeches become rainbows, as the outermost leaves of each tree turn orange-brown, the middle leaves change to yellow, and the innermost leaves preserve a bright spring-green. Finally, a few roadside sumacs are left to redden the landscape as it settles into winter stillness.
Colorful trees and rich forests have only recently come to New England. Only 20,000 years ago, a glacier covered the entire Northeast in ice up to a mile thick. As it oozed southward under the pressure of its own weight, it scoured vegetation and earth from the ground and gouged large chunks out of mountains. About 10,000 years ago, the climate warmed and the glacier receded, leaving behind a barren wasteland. As the glacier melted, it kindly replaced all the rocks and detritus it had picked up in its headlong advance. This is the origin of those giant boulders you may find lying around in the forest; they were chiseled out by the glacier and left behind.
Trees colonized the newly exposed mountains. Some slopes faced north and received less sunlight, making them cold and damp. In the darkest ravines, the hemlocks — giant conifers that can live up to a thousand years — took root. On the windy upper slopes, the birches and poplars settled, trees from the far north. Birch bark peels off in sheets, enabling the tree to rid itself of a pesky fungus. This attribute makes it an ideal material for dishes and canoes. Poplars (also called aspens) are known for bending in the wind, an adaptation for enduring harsh northern blizzards and heavy snowfall. The bark of poplars is slightly green from chlorophyll, which is used for photosynthesis even when the cold of winter makes it impossible for leaves to grow.
The sunny south-facing slopes, covered with rocks by the glacier, were warm and dry — the perfect place for forest fires. We humans flatter ourselves for having “invented” fire, but natural fires are a common occurrence over much of the world, and are actually necessary to the survival of many species. Trees like oaks, pines, and hickories are specially adapted to resist fire. They have thick bark and large tough seeds that sometimes don’t sprout at all unless they’re lightly burned first. Now that people have started extinguishing forest fires, some of these trees are having a hard time competing.
The beautiful American beech is another tree that colonized these south-facing slopes. It isn’t a fire-resistant species: it just wanted to stay warm. Beeches come from the tropics, and this species is the northernmost of its kind. However, it still looks like a rainforest tree. It has huge, broad branches, perfect for climbing, and long-tipped leaves that channel the rain away. The smooth, pale gray bark, which lovers sometimes write on, is also an adaptation to the tropics, where it stops the tree from being overrun by vines and other plants looking for a foothold. Unfortunately, smooth bark is a terrible idea in a northern winter, since it splits easily in the cold. Almost every other tree here has ridged bark, which can expand and contract with temperature changes.
The valleys of New England were vast lakes only a few thousand years ago, filled with meltwater from the retreating glaciers. When the water receded, it left behind rich soil that is now perfect for farming. Trees such as the White Ash and American Elm grow in these soils. Ashes are straight, tall trees, with exceptionally hard wood that is used for making baseball bats and fancy furniture. Elms are stately, with spreading branches, and were once planted in gardens and parks, but have now become very rare as a result of an invasive fungus called Dutch elm disease.
The quintessential New England tree is the maple. Different species grow in each habitat. In rich lowland soils, the dominant tree is the large sugar maple. Around rivers, it is replaced by the silver maple, a drooping, elegant tree whose leaves look silver from underneath. In bogs and poor soils, there is the red maple, while on hillsides the striped maple offers its huge leaves to hikers who need toilet paper. At the very tops of mountains lives the tiny mountain maple, rarely bigger than a sapling.
As the trees of New England change color, think about glaciers as you admire the carotenoids. Even at MIT, fall foliage is quite spectacular. The banks of the Charles are planted with exotic Japanese zelkovas, which turn a bright red-brown. In Killian Court, the red maples are scarlet and the elms are yellow. There are even sugar maples on the far side of Next House, with orange leaves and the promise of syrup.
But, of course, the best maple syrup comes from Vermont.