During summer, the most commonly used features for identifying trees are leaf characteristics.  To the summer-oriented individual the fall of leaves in autumn marks the end of deciduous tree identification.  However, the assumption that trees in winter lack identifying characteristics is entirely false.  An examination of any leafless twig will reveal some traits that are the same all year and some that are reflections of summer growth activities.

During spring and early summer, there is a burst of growth in most temperate deciduous trees that results in the annual vertical increment.  It is during this time that the winter buds for the next year's growth are formed.  Thus by mid-July or early August bud characteristics are sufficiently formed for use in identification, giving them nearly year-round utility for this function.  Features of winter twigs that are remnants of summer growth include bud scale scars, leaf scars and stipule scars.

Features of Winter Twigs

Buds  Buds contain, in the dormant state, the embryonic stems, leaves, and sometimes flowers of next season's growth.  In most deciduous trees the buds are covered with one or more tiny modified leaves called bud scales that protect the embryonic parts.  The number and arrangement of bud scales is usually constant for a given species.  In a few species there is a single cone-shaped scale, recognizable by a smooth unbroken surface.  Some species have two scales that fit together like the sides of a clam shell, an arrangement described as valvate.  Most commonly there are several scales that overlap, shingle-like, a condition referred to as imbricate.  In the spring when the bud resumes growth, the embryonic stem elongates and the bud scales drop off leaving a ring of bud scale scars.  These can be used to identify the beginning of each season of growth.  Two or three years of growth can sometimes be observed on a single twig.

For many species of deciduous trees there is a large bud, the terminal bud, at the tip of the twig, and smaller ones called lateral buds along the sides.  Lateral buds usually occur in the upper angle between the leaf and the stem, the leaf axil, and thus are also called axillary buds.  Ordinarily there is a single axillary bud in deciduous trees, but in some species there may be buds that flank the axillary bud, called collaterals, or that occur above the axillary bud referred to as superposed buds.  In some species the growing tip of the twig continues to advance until it is halted by cold autumn weather.  The growing point then dies back to the first well developed lateral bud which takes over the growth in length for the next year.  These species have no terminal bud.

It is often very important in identification to accurately determine the presence or absence of a terminal bud.  Terminal buds are usually larger than lateral buds.  Since lateral buds form in the axils of leaves, if the uppermost bud is immediately above a leaf scar, it is lateral and not a true terminal bud.  However, when there is no terminal bud there is either a small stub or a branch scar, usually opposite the leaf scar, reminiscent of the true stem tip.

Leaf Scars  The places of attachment of leaves on a stem are called nodes, and the spaces between nodes are internodes.  As autumn advances a layer of cork-like cells develops at the base of the leaf stalk or petiole.  The cells on the leaf side of this layer become thin walled and the leaf eventually breaks off, leaving a scar in the shape of a cross section of the petiole base.  These scars are important features in identifying winter trees.  Depending on whether there are one, two, or more than two per node, they are described as alternate, opposite, or whorled, respectively.  It should be recalled that lateral buds are axillary, thus, there will be a bud on the stem immediately above each leaf scar.

Within the leaf scars will be observed what appear to be small dots and/or raised lines.  These are traces of the vascular bundles that supplied the leaf with water and mineral nutrients from the vascular system of the stem and they are called bundle scars.  The number and arrangement of bundle scars is sometimes very important in the identification of a winter tree.  If the bundle scars are indistinct it may help to make a thin slice across the leaf scar with a razor blade and add a small amount of an alcohol dye solution.

Stipule Scars  Stipules are small leaf-like structures that are attached to the stem on each side of the petiole.  On winter twigs stipule scars may be observed on each side of the leaf scars as narrow lines that extend part-way or completely around the twig.  Since all trees do not have stipules the presence or absence of stipule scars is often helpful in identifying a winter twig.

Pith Characteristics  In the center of the stem is a tissue, made up of large thin walled cells, called the pith.  It can be observed by making a slanting cut with a razor blade.  If the pith is solid throughout it is said to be homogeneous; if it has irregular cavities with the appearance of swiss cheese it is spongy; if it is solid with darker cross partitions it is diaphragmed; if there are cavities separated by cross partitions it is chambered.  The color and shape of the pith are also diagnostic features in the identification of some species.  To determine the shape it may be helpful to apply a small amount of dye.

Classification

Trees of the temperate zone belong to one of two major groups:  Gymnosperms and Angiosperms.  A technical but basic difference between these groups is that Gymnosperm seeds are not enclosed within another organ but are usually produced on the surface of a specialized structure called a scale.  Angiosperm seeds are produced in an ovary which ripens into a fruit as the seeds mature.

The largest group of Gymnosperms is known as the conifers and it includes such trees as the pines, spruces, firs and cedars in which the seed-bearing scales are clustered in cones.  A characteristic feature of these plants is very narrow, needle-like or scale-like leaves.  Since most of the conifers retain their leaves all year they are often referred to as evergreens and the leaf key for their identification is valid for winter or summer.  The conifers are also called softwoods although in some species the wood is very hard.

There are two groups of Angiosperms: monocotyledons and dicotyledons.  The monocotyledenous trees are palms and palmettos which do not occur in northern forests.  The Angiosperms of northern forests are dicotyledons that usually lose their leaves in winter and are frequently referred to as deciduous hardwoods.  The dichotomous keys which accompany the activities were prepared for a variety of northern conifers (Gymnosperms) and/or northern deciduous trees (Angiosperms).  The instructor should choose the one most appropriate for the students in each class.

Plant Names

In 1753 a Swedish botanist named Carolus Linnaeus developed a binomial system for naming plants.  This system is used by botanists today and, as the name suggests, it consists of two parts; a genus name which identifies a general group of plants, and a specific name that is often descriptive, and refers to a single type.  For example Pinus virginiana is the name for Scrub Pine:  Pinus is the name of the group, or the genus, that includes all pines, virginiana means "of Virginia", and it identifies a specific type of pine.  This is the scientific or Latin name for Scrub Pine and it should always be written with the genus name capitalized and the specific name beginning with a small letter.

Many plants have a common or popularized name and some have more than one.  For example in various parts of its range Scrub Pine may be referred to as Jersey Pine, Spruce Pine, or Poverty Pine.  To further complicate the issue Scrub Pine is a name also given to Pincus banksiana (Jack Pine) in some parts of its range.  Thus common names are not always reliable and sometimes they are confusing.  In order for a plant name to be meaningful it must consistently refer to the same plant.  The advantage of binomial system initiated by Linnaeus is that no other plant in the world has that name.  If you mentioned Pinus virginiana to a Russian, Brazilian or Australian botanist, that person would recognize it as a discrete entity and never confuse it with another plant. 

How To Use The Twig Keys

The keys that accompany the student activities are designed to be most useful north of the area between South Carolina and northern California.  They can be used in some regions south of this area, but the farther south one goes the less reliable they will become.  They are dichotomous keys, and like all such plant keys, they are based on the assumption that any collection of plant specimens can be divided into two groups by some observable characteristic, i.e., one group has the characteristic, the other group does not.  Most of the characteristics can be observed with unaided eye but in some instances a small hand lens with a magnification of 10x to 14x may be useful.

A dichotomous key can be best illustrated with a small group of winter twigs.  Consider the following six specimens.

Twig A - leaf scars opposite, bundle scars 2, pith chambered

Twig B - leaf scars alternate, bundle scars 10, pith homogeneous

Twig C - leaf scars alternate, bundle scars 1, pith spongy

Twig D - leaf scars opposite, bundle scars 5, pith chambered

Twig E - leaf scars alternate, bundle scars 7, pith spongy

Twig F - leaf scars opposite, bundle scars 10, pith spongy.

A key for these twigs is given below

1A.  leaf scars opposite......................2

1B.  leaf scars alternate......................4

2A.  bundle scars 1 or 2.................Twig A

2B.  bundle scars more than 2............3

3A.  pith chambered......................Twig D

3B.  pith spongy............................Twig F

4A.  bundle scars 1 or 2.................Twig C

4B.  bundle scars more than 2............5

5A.  pith homogeneous..................Twig B

5B.  pith spongy...........................Twig E

Two characteristics of dichotomous keys are illustrated in the above example.

(1) At each step the specimens are divided into two groups

(2) The two statements for each division usually begin with the same word followed by a statement expressing contrasting conditions, such as:

1A.  leaf scars opposite

1B.  leaf scars alternate

The sample key given above is for a very limited number of specimens and thus is overly simplified.  If a larger number of plants are to be identified there will be more divisions and a longer, more complex key.  Although the keys that accompany the student activities are valid for many of the genera of trees in northern United States and Canada, the instructor may wish to simplify or make them more reflective of local species. 

References

Graves, Arthur H. 1956. Illustrated Guide To Trees and Shrubs. Harper and Row, Publishers, New York.

Harlow, William M. 1959. Fruit Key and Twig Key To Trees and Shrubs. Dover Publications, Inc., New York.

Muenscher, W.C. 1950. Keys To Woody Plants. Cornell University Press, Ithaca, N.Y.

Preston, Richard J. 1967. North American Trees. The Iowa State University Press, Ames, Iowa.

Trelease, William. 1967. Winter Botany. Dover Publications, Inc., N.Y.