Naturally Curious: Reading Tree Shapes; Environment Plays a Big Role
A white birch growing in the open. (Mary Holland photograph)
A white birch in a forest. (Mary Holland photograph)
It would be nice if each the shape of each species of tree was so distinctive that you could identify it just by looking at its silhouette. However, as a general rule, the shape of the crown, branches and trunk of trees within a given species can differ significantly, often making it quite difficult to identify a tree by its shape. Even so, a tree’s shape can reveal many things about the factors that contribute to its form, including age, climate, soil conditions, branching pattern and the tree’s environment. It’s helpful to know the basics of how a tree grows in order to appreciate the process involved in shaping it.
Branches and roots can grow longer only at their tips. For a few weeks every year in New England during the spring and summer this growth is very rapid. Growth also occurs in a tree’s girth, but it is a different kind of growth and produces different results. A layer of cells called cambium produces cells that make the trunk, branches, twigs and roots grow fatter, as opposed to longer. This growth allows the tree to support the extra height and spread that it gains every year. This form of growth explains why, once a branch has grown out from the trunk, it will always be at the same height above the ground, as the trunk can only get fatter, not longer.
The shape of a tree is influenced by many factors. Young trees tend to be symmetrical and have narrow crowns that spread with age. This typical cone shape which many young trees have is due to the fact that the crowns of young trees often have a distinctive leading shoot which outgrows the branches below it. As a tree ages, the growth of this leading shoot decreases and the crown then becomes rounded or even flat.
Soil and climate play a big part in a tree’s growth. Trees growing on fertile, moist sites tend to grow faster than trees of the same species on poor or dry sites. Trees growing at timberline on high mountains tend to be sprawling and irregularly shaped due to deep snows, poor soils, a short growing season and constant buffeting by strong winds. As trees grow, their trunks and branches do not always expand at the same rate everywhere. If trees grow in an exposed, windy site, the wood tends to grow faster on the side facing away from the wind than it does on the windward side. Trees growing where there tends to be a lot of snow typically have short branches which easily shed the snow. In addition to soil and climate, injuries due to wind, insects, fire or ice storms can significantly alter the shape of trees.
Probably the most influential factor affecting the growth and shape of a tree is its environment. Did it grow in an open field, unhampered by neighboring trees, or in a crowded forest? Competition for light is fierce in a forest, and trees hemmed in on all sides tend to grow mainly upward in an attempt to reach the light. Forest-grown trees tend to have fairly slender, long, straight and unforked trunks that are free of branches for half or more of their length. Their crowns are relatively short and narrow. The same species of tree grown in the open has a very different form. It often has a relatively thick, often crooked and sometimes forked trunk, with low branches and a large, wide-spreading crown. It is these trees that grow in the open that tend to develop characteristic shapes which are typical of a few species such as American elm (vase-shaped), sugar maple (egg-shaped) and basswood (pyramid-shaped). In large part these shapes can be attributed to the angle of the tree’s branching, which, for a small number of species, is fairly consistent.
For the most part, however, it can be difficult to identify most trees by their shape, no matter where they grow. The tree’s age, the soil conditions, climate, location or branching pattern will affect two trees of the same species differently, even if they’re growing in the same environment. Although a tree’s shape may not allow you to narrow it down to a given species, it can provide you with a lot of information about the conditions under which it grew.
Mary Holland is the author of “Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England” and “Milkweed Visitors.” She has a natural history blog which can be found at www.naturallycuriouswithmaryholland.wordpress.com.