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Forest and Shade Trees of Iowa
By Peter J. van der Linden Donald R. Farrar
University of Iowa Press
Copyright © 2011 University of Iowa Press
All right reserved.
Chapter One Identifying Trees
You are walking through a park in your community and discover a tree you haven't seen before. How do you identify it?
One way is to compare your tree with the photos in this book, checking page by page until you find a match. To make your search easier, we have arranged the trees in groups based upon similarities in leaves. To get started, go to page 57 and select the appropriate group. When you think you've found the right species, be sure to read the entire description on that page. Verify that the buds, fruits, and other features of your tree match that species, too.
Some less common species are not illustrated in this book, so you might not find them using the above approach. In that event, we suggest that you use the keys beginning on page 16.
If you haven't used a key before, don't be afraid to try. Think of it as a scavenger hunt, in which a series of clues helps you find the identity of your tree, one step at a time. Each clue consists of two descriptions, one that matches your tree and one that doesn't. When you have decided which description is the right one, the key refers you to the next clue, and the search continues.
If the above methods don't work, consult a local expert. Most colleges, arboretums, and garden centers employ people who are skilled in plant identification. You can also take a sample to your county extension office, which will send it to Iowa State University. Each sample should be a branch 6 to 10 inches long with several leaves, plus fruits or seeds if they are available.
Names of Trees
Trees have both common names and botanical names. Most people prefer the common names because they are easy to spell, pronounce, and remember. Botanical names, which are based on Greek and Latin words, are intimidating to many people. They do have an advantage, though, in being standardized by international convention and thus more precise.
For example, there are two small trees in the birch family that grow together in woodlands across much of Iowa. One is called ironwood or hop hornbeam; the other is variously known as ironwood, hornbeam, bluebeech, or musclewood. If someone uses the name ironwood or hornbeam, which tree do they mean? There is no question if they use the botanical name — Ostrya virginiana for the first and Carpinus caroliniana for the second.
Another advantage of botanical names is that they are based on natural relationships. The common names green ash, mountain-ash, and prickly-ash all contain the word "ash," but these species only superficially resemble one another and actually belong to different botanical families. Only the first is a true ash. Similarly, Russian-olive is not an olive, Douglas-fir is not a fir, and Kentucky coffee tree is unrelated to the coffee of commerce. Conversely, poplars, aspens, and cottonwood share many features and are closely related, but their common names do not indicate any relationship.
Botanical names have two parts, each of which tells us something about the tree. The first part indicates the genus (plural, genera), or group of related species, to which the tree belongs. The second part, called the specific epithet, designates the particular species. For example, Quercus macrocarpa and Quercus rubra are the names of two trees we commonly call bur oak and red oak. Because the first part of their names is identical, we know they belong to the same genus. The second part of their names is different, however, so we know they are distinct species. The specific epithet usually indicates something about the nature of the tree. For example, macrocarpa means "big fruit" and refers to the large acorn typical of the bur oak and its burlike cup; rubra means red and refers to the reddish color of red oak wood compared to that of white oak.
When the botanical names of trees are written, the genus is always capitalized but the specific epithet is not, even when taken from a proper name. Both parts of the name are italicized or underlined. The specific epithet may be followed by the abbreviated surname of the botanist who originally described the species. The "L." following so many botanical names stands for Carl Linnaeus, a Swedish scientist who designed the modern system of biological nomenclature and described many species.
Some species have been subdivided into varieties. The variety name, when indicated, follows the specific epithet. It is underlined or italicized in print but not capitalized. The abbreviation "var." sometimes precedes it. For example, Quercus macrocarpa var. macrocarpa is the typical variety of bur oak, while var. depressa comprises smaller trees with smaller acorns found in drier areas in the northwest part of the range. As with most varieties within species, where these two varieties grow together they freely hybridize to produce a gradient of tree and acorn sizes.
Within a species or variety, it is often desirable to distinguish a tree that has been bred or selected for horticultural purposes. For example, 'Marshall's Seedless' is a special type of green ash, selected because it produces no seeds and has attractive dark green leaves. Such selections are called cultivars to distinguish them from naturally occurring varieties. Note that cultivar names are not italicized but instead are capitalized and enclosed by single quotes. Many but not all cultivars are clones. This means that they originated from a single parent tree and are propagated by cuttings, grafting, or other vegetative means to ensure that each tree is a genetic replica of the original.
Features Used for Identification
Most people make a leaf collection in school when they are young and through this learn that trees can be identified by differences in their foliage. Other parts of trees are useful for identification, too, including twigs, flowers, fruits, bark, and silhouette. Indeed, it is quite possible to identify trees in winter when leaves are absent. Characteristics of these various tree parts are described below.
During most of the growing season, leaves are the most conspicuous features of trees. Figure 1 illustrates leaf characteristics. The leaves of most species are remarkably consistent, but variation can occur between individual trees or even on a single tree. The oaks are a notorious example, and species within this genus sometimes cannot be identified without examining buds and fruits in addition to leaves.
Leaves for study should be selected from healthy, typical twigs when possible. Leaves from stump or root sprouts (suckers) should be avoided because they can be unusually large and atypically shaped.
A leaf consists of two parts: a broad, flattened portion called the blade and a narrow stalk or petiole. The leaves of most trees, including such familiar types as the oaks and maples, have a single blade. These leaves are said to be simple. Other species have leaves composed of many separate blades or leaflets and are called compound. Ashes and walnuts are two familiar examples.
If the leaflets of a compound leaf are arranged along two sides of a central axis, like the barbs of a feather, the leaf is once-pinnately compound. If the axis is branched, with leaflets along each branch, the leaf is twice-pinnately compound. A few trees have leaflets radiating from a single point, like the fingers of a hand. These are palmately compound leaves.
The composition of a leaf should be the first characteristic noted when you are identifying a tree. Do not confuse the leaflet of a compound leaf with a simple leaf or vice versa, a mistake easily made in spring when the new twigs are green and succulent. How do you tell the difference? Remember that a bud occurs in the angle formed by the twig and each petiole, but not in the angle formed by the axis of a compound leaf and each leaflet.
The spot on a twig where a leaf is attached is called a node. Most trees have only one leaf per node, an arrangement called alternate. Others are opposite, with two leaves at each node, one on either side of the twig. One Iowa tree, the catalpa, is whorled with three leaves at most nodes.
The outline or shape of leaves varies among species but is usually consistent within a species. Some of the more common shapes are illustrated in figure 1. Leaves with large indentations are said to be lobed. The projecting portions of the blades are called lobes and the spaces between the lobes are known as sinuses.
The margins or edges of leaves often have tiny points called teeth. When the teeth are close together, the margin is closely toothed; when they are widely spaced, the margin is distantly toothed. An unbroken leaf margin is said to be entire.
The upper surfaces of some leaves are noticeably glossy or shiny, while the leaves of other species are quite dull. A leaf surface may also be more or less covered with tiny hairlike projections, in which case it is called hairy or pubescent. A leaf with no hairs is glabrous.
LEAF APICES AND BASES
The tip or apex (plural, apices) of a leaf may be blunt, rounded, pointed (acute), or tapered to a slender tip (acuminate). Its base may be rounded, heart-shaped, wedge-shaped, or squared-off (truncate).
Identifying trees might seem hopeless in winter, when no leaves are present. Fortunately, the winter twigs of most trees are as distinctive as their leaves, so no one need give up identification when the branches are bare. Figure 2 illustrates twig characteristics.
When the leaf of a deciduous tree falls in autumn, its petiole leaves a mark on the twig called a leaf scar. The leaf scars of most trees are small and somewhat semicircular, but in some species they are quite large and distinctively shaped. Each leaf scar encloses one or more small dots called bundle scars that mark the spots where the vascular bundles or veins of the leaf entered the twig. Figure 2 illustrates leaf scar shapes.
Because leaf scars mark the positions of leaves, they are likewise arranged in one of three ways: alternate if solitary at the nodes, opposite if in pairs, and whorled if in threes. Compound leaves often have large leaf scars with many bundle scars, while simple leaves have small leaf scars with one to three bundle scars, but there are exceptions.
A bud is a tiny new twig, waiting for the right conditions to grow. Buds begin to develop early in the growing season and are fully formed by late summer, then remain dormant through the fall and winter. When spring comes, leaves and flowers may come from the same or different buds, depending on the species. Figure 2 illustrates bud shapes.
Most buds are covered with one or more scales that protect the delicate tissues within from damage and drying out during winter. The buds of poison ivy and a few small, uncommon trees lack scales and are said to be naked. Naked buds are often protected by a dense covering of hairs.
In some tree species, a bud called the terminal bud forms at the tip of each twig. This bud, which limits elongation for the remainder of the growing season, is usually much larger than the lateral buds growing farther down the twig at each node.
Other species do not produce terminal buds, although their twig tips usually die back to a lateral bud sometime during the growing season, leaving it in a terminal position. This bud, which is called a false terminal bud, can be recognized as a lateral bud by its size and shape. Look also for a stub or twig scar at its base or side, marking the place where the twig tip died back. (Like other lateral buds, a false terminal bud will have a leaf scar at its base. The twig scar may resemble this in size and shape but will be on the opposite side of the twig and will not have bundle scars.)
Stipules are small leaflike structures that occur in pairs on the petioles or twigs of many trees. Those attached to the twig leave two small horizontal lines when they fall, one on either side of the leaf scar. Sometimes one of these will be more prominent than the other. In a few species, they form a line that encircles the twig at each node.
The center or core of the twig is called the pith. Often it is narrow and pale in color, resembling the wood, but in some species it is very thick and colorful. When the twig is viewed in cross section, the pith varies from round to angled to star-shaped, depending on the species.
A cut made lengthwise through the center of the twig exposes the pith in longitudinal section. When viewed in this way, the pith of most trees is continuous, forming a solid strand of tissue. If the solid tissue is interrupted by cross walls, the pith is called diaphragmed. If there are cross walls separated by empty space, the pith is called chambered.
OTHER TWIG FEATURES
The word "thorn" is often used for any sharp, woody structure, but in botany it has a special meaning. True thorns are modified twigs with smooth, often glossy surfaces and no buds or leaf scars. A few trees have short, pointed twigs resembling thorns, but these have buds and/or leaf scars like ordinary twigs.
Spines are stipules that are sharply pointed and woody. They occur in pairs, one on either side of each leaf scar.
A spur shoot is a short, stubby twig that grows only a fraction of an inch each year. Its outer surface is covered with closely packed leaf scars, and there is a terminal bud at its tip. Spur shoots can appear in many species but are characteristic of only a few, including the apples, crabapples, birches, and ginkgo.
The catkin is a special type of flower cluster found in many trees (see the following section). In a few species, the catkins for next year are visible on the twigs during fall and winter, as buds are. They resemble tiny sausages covered with minute, overlapping scales.
The color of twigs is often distinctive. The colors noted in the keys and descriptions of this book are for typical twigs during winter. There is natural variation within species and even from season to season in the same tree, as reflected in the photographs.
All broadleaf trees produce flowers. Most of our larger trees have small greenish blooms that appear before the leaves in spring and are thus easily overlooked. Such flowers are pollinated by wind and have no need for the colorful petals that characterize apples, redbuds, and other small trees. Pines and similar trees are also pollinated by wind, but their reproductive parts occur in cones, not flowers.
Tree flowers are distinctive when present, but their usefulness as a means of identification is limited by their short longevity. One reason that willows, haw thorns, and serviceberries are so difficult to identify is that flowers are often needed to differentiate species, and these trees bloom for only a week or two in spring!
Tree flowers are not described in detail in this book unless they are necessary to distinguish otherwise similar species. Observing them is an interesting pastime in spring, however, and there are many books that describe their features in more detail.
There are four principal flower parts. These, in order from the outside to the center, are the sepals, petals, stamens, and carpels. One or more of these is often absent from small, wind-pollinated flowers.
Sepals and petals are leaflike in structure. Sepals are usually green and petals some other color, but sometimes they are colored alike. The sepals are collectively known as the calyx and the petals as the corolla. Flowers without petals are said to be apetalous.
Stamens produce pollen and the carpels eventually become fruit. Stamens consist of a stalk or filament and a pollen-bearing sac called the anther. The carpels can be separate from one another or united into a flask-shaped structure called the pistil. Most pistils have a bulbous basal part called the ovary, a neck or style, and a swollen tip called the stigma. If the other flower parts appear to arise from the top of the ovary, the ovary is called inferior. If they arise from the base, the ovary is superior.
Excerpted from Forest and Shade Trees of Iowa by Peter J. van der Linden Donald R. Farrar Copyright © 2011 by University of Iowa Press. Excerpted by permission of University of Iowa Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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