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Teasing the Trees from the Forest

Teasing the Trees from the Forest
Illustration by Adelaide Tyrol

With trees stripped to their skeletons, winter is a good season for noticing the shapes that make hardwood silhouettes so distinctive. It’s a relief to put away the hand lens and twig guides and instead tease out how biology shapes the larger-scale differences between tree species.

At first, it may be hard to see the trees for the forest. All of those long, straight trunks with high, branching crowns can look very much alike. Look up into the crowns, though, and you’ll find that branching patterns run basically true to species. Some branches, like those of maples and ashes, emerge in pairs opposite one another; most others alternate. Birches and hophornbeam have fine twigs; walnuts and hickories, coarse. The branches of mature elms, locusts, and cherries wander around at crazy angles, while those of white ashes and sugar maples head up, up, up.

Best of all for identification, many patterns are simple to remember because they suggest images of one kind or another. Aspen branches bend upward like coat hooks; elms have fish skeletons diving from their pendant branches. Pull down a beech branch and notice how much the twigs, branchlets, and branches, lying all in one plane, resemble the brooks, streams, and rivers of a watershed.

Even easier to recognize are the branching patterns that are dead giveaways for human personality types. Take black cherry and white ash, for example. These are people you know and like, but that you’d never invite to the same dinner table. They make good examples, too, of how biology affects shape.

The white ash has an engineer’s mindset. Its few, coarse twigs emerge opposite each other on symmetrical branches, giving a tinker toy-like uniformity to its parts. This tree clearly has a plan, and that is to go UP as efficiently as possible. In this case, the image happens to describe the ash’s biology pretty well. It’s a tree that is adapted to grow quickly in canopy gaps or in openings where the soil is fertile. Here, where many other trees are also rushing to fill the sunny gap, fast vertical growth is a necessity, so a lion’s share of the ash’s resources goes right to the top. In a feedback loop that is partly hormonal, rapid growth at the tip perpetuates itself and inhibits the production of branches below.

Because lower branches are doomed to be quickly shaded out and die, species like the ash sometimes invest instead in leaves that act as cheap, “disposable branches.” These large, feather-shaped, compound leaves have a central stalk that acts as a twig and leaflets that are dead ringers for leaves. They live for a season and then drop, leaving the ash with its characteristic blocky winter silhouette.
This growth strategy has sent white ash down several career paths. With few branches come few knots, a trait beloved by firewood splitters. And since wood that grows fast is light in weight, white ash bats have always been major players on the baseball diamond.

At the other end of the personality chart lies that disheveled beauty, the black cherry. Its crown looks like a neglected hairdo. Twigs of every length and thickness angle off every which-a-way. Compared to the ash, whose branches rise reliably at 35 or 55 degrees from vertical, the cherry is a little lax, to say the least.

Like the ash, though, the black cherry’s apparently random branching pattern is determined by biological function - in this case, the location of its flowers. Flowers and fruit develop in clusters at the tips of branches, permanently ending growth there. The following spring, the next buds back take over as leaders, but instead of continuing straight on, they jut out at various angles to fill any canopy gaps. As this repeats year after year, some branches flowering and others not, the canopy becomes a crazy quilt of twigs. This type of zigzag branching pattern, in which the main stem stops growing and a side bud takes over, is called sympodial growth. It produces the graceful branches of the beech as well as the wildly snaky crowns of the locust and the elm. Birches, elms, willows, basswood, hornbeam, and hophornbeam also branch sympodially.

How genetically defined and simple this large-scale identification sounds! But of course the real ash and the real cherry share another human trait: individuality. Is a neighboring tree now shading the branches on your right side? Shed them, even if your branching pattern now looks alternate. Did a sapsucker kill your leader? Send out a basket of lateral branches to replace that central stem. Growing in the open? Branch early and often.

Identifying a tree by its architecture is not infallible. But it’s pretty reliable - AND, you can do it without taking off your bifocals a hundred times a day. That’s a big plus.


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