Trees in Winter – Background

Mysteries abound in the winter woods. Besides animal tracks in the snow, we might find nipped-off twigs, gnawed branches, debarked trunks – signs made by animals feeding on trees in winter. To figure out which animal has been doing the eating, we often need to identify the tree species. But, without their leaves at this time of year, trees can be mysteries to us as well. Animals that feed on woody plants have no trouble recognizing which twigs, buds, and bark make the best meals, but for us identifying trees in winter requires a close look and attention to detail, the skills of a good detective.

At the tips of branches, twigs offer many clues to a tree’s identity. Take a closer look at a twig in winter, and you will see that it has buds along the sides and at its tip. These are essential to a tree’s annual growth cycle because they contain the beginnings of leaves, flowers, and shoots for the coming growing season. The buds begin to form in the spring but are microscopic initially. By mid-summer, they are fully formed, perched behind the stem of each green leaf. They remain on the twigs after the leaves have fallen from the tree in autumn and stay dormant all winter, until the days grow longer and the temperatures warm up. Then they begin to swell and open, unfurling as new leaves, flowers, and stems in springtime.

Because they are often very distinctive, buds can help us solve the mystery of which tree is which. Their size, shape, color, texture, and orientation make up pieces of the puzzle. Terminal buds (or end buds) are those at the tip of a twig and can be in clusters (oak, pin cherry) or single as in aspen, birch, and many other trees. Terminal buds might be long and pointed (beech), short and rounded (ash, sumac), off-center (elm, basswood), or flanked by two side buds as in maple and ash. Buds can be different colors – red in basswood and red maple, brown in sugar maple, oak, and beech. Bitternut hickory has bright yellow, velvety buds, apple buds are gray and fuzzy, and black ash buds are smooth and black.

Bud scales protect the bud from damage and from drying out. Sometimes they overlap like shingles; sometimes they meet like a duck’s bill. The number of scales varies from none or one to many. Oak buds have five rows of scales, striped maple have just two bud scales, and willow have a single, hood-like scale protecting each bud. Some trees, such as butternut, witch hazel, and bitternut hickory, have fuzzy buds with no scales. Buds shed their scales as the twig elongates, leaving a series of rings on the twig like threads on a screw, called a bud scale scar. The distance between the terminal bud and the closest bud scale scar shows how much the twig grew in the previous year. In this way, each twig displays the history of its growth. Sometimes longer sections indicate good growing conditions; often they indicate where the twig is located on the tree or the age of the tree. Young trees can grow very fast, and so can twigs in the upper and outer parts of the tree. Shaded twigs and those on old trees may grow very slowly or hardly at all.

Lateral buds (or side buds), located along the side of the twig, may grow into side branches or leaves. Their location is a helpful clue for identifying the tree, as they can be arranged in one of three patterns: opposite, alternate, or whorled, but only two kinds of full-sized trees in our forests have pairs of buds that grow opposite one another: maple and ash. Most other deciduous trees are alternate-branching, with buds that appear to zigzag or be staggered up the branch. Whorled branching, seen on catalpas and some evergreens like pines, is when buds and twigs are arranged in a circle around a branch. The same branching patterns seen on twigs and buds can also be seen on the larger branches.

A tree’s leaf scars offer more clues to identification. Leaf scars form where leaves fall off the tree in autumn. The leaves are shed in the fall to prevent water from leaving the tree through the process of transpiration. Since water is lacking in winter, the tree needs to conserve as much moisture as possible. A cork-like layer of cells, called an abscission layer, forms where each leaf falls off the twig, leaving a scar shaped like the stem of the leaf in cross-section. Leaf scars come in a variety of shapes such as semicircles (elm), U-shaped (sumac) or elliptical (birch). Some leaf scars, like those of American sycamore, completely encircle the bud.

In addition to distinctive leaf scars, each tree species also has distinctive bundle scars. These formations of dots within the leaf scars are the remains of the bundles of tubes that carried water and nutrients in and out of the leaves. Bundle scars may be few or many. They may be in a line, scattered, or grouped within the leaf scar. The bundle scars of white ash are lined up in a curve like a smile, while those of butternut and black walnut have the appearance of a monkey face.

The bark of young twigs is another feature that can aid in identification. Red maples have reddish bark on the newest twigs while those of sugar maple are a rich brown – the color of maple syrup. Some willows have yellow twigs, mountain maple twigs are bright red, and striped maple twigs can range from red to green to black. Lenticels are tiny, corky bumps on the bark of a twig that allow it to breathe. Often they look like pale dots and aren’t visible on older bark. Birch and cherry have small lenticels on the twigs that persist and become horizontal lines on the older branches and trunks.

Yet another clue is the pith or inner core of the twig. This food-storing tissue often has a distinctive shape and color. It is star-shaped in oak, round in ash. The pith of red elderberry is bright orange; butternut pith is chocolate brown and has chambers you can see if you cut the twig lengthwise. The pith may be a tiny dot in the center of the twig or fill most of the space. The twigs of sumac and elderberry, with their large soft piths, can be hollowed out and were once used as spouts for collecting maple sap.

Besides these features, keep on the lookout for thorns (hawthorn), catkins (birch, hazel, and alder), seed clusters (ash, birch, box elder), or for trees that retain some leaves in the winter (beech and oak). Sometimes your nose can give you a clue. Both black and yellow birch smell and taste like wintergreen when the twigs are broken or the bark is scratched. Black cherry, on the other hand, has an acrid, bitter almond smell and taste, which helps to deter twig-eating animals. Bitternut hickory smells like lemon oil, and balsam poplar has very sticky buds with a spicy odor.

Buds and young twigs may seem dry and unappetizing to our palate, but they are highly nutritious, and to many animals they are a welcome winter staple. Deer and moose subsist on winter twigs, as do rabbits and hares. Porcupines, squirrels, and grouse eat buds as part of their diets as well. Knowing the food preferences of each animal can give us further tips to identify winter trees and their consumers.

The next time you’re out snowshoeing or skiing in the woods, you may want to take a hand lens and winter twig field guide along with you, or bring a few twigs home in your pocket for a closer look. Get to know the trees in your woods, and you’re sure to solve a mystery or two.

Suggested Reading

Holland, Mary, Naturally Curious: A Photographic Field Guide and Month-by-Month Journey through the Fields, Woods, and Marshes of New England, Trafalgar Square Books, 2010.

Petrides, George, A Field Guide to Trees and Shrubs. Peterson Field Guide Series. Houghton Mifflin Company, 1972.

Ruesink, Ana, and Rebecca Merrilees (Illustrator), “Sorting the Saplings: A Quick Guide to Winter Tree and Shrub Identification,” The Place You Call Home: A Guide to Caring for Your Land in Vermont, Northern Woodlands, 2008, 17-19.

Watts, Mary T., and Tom Watts, Winter Tree Finder, Nature Study Guild Publishers, 1970.

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