THE HERBARIUM PROJECT:
OBSERVING THE DIVERSITY OF TREES
Introduction The Herbarium Project Assignment
Tree Identification Recommended Field Guides and References
The name Pennsylvania, "Penns Woods", indicates a land lush with a wide variety of trees. In Pennsylvania the geographic ranges of many species typical of the northern temperate zone intersect with those of the south, producing a unique diversity of trees. Nearly all species of trees native to temperate eastern North America grow in the state. By studying the trees of Pennsylvania, you will be introduced to some of the most prominent flora of the region.
Trees are the largest and some of the most impressive plants in the world. Their size, age, and beauty inspire wonder and excitement in us all. The importance of trees to man and the rest of the biota as sources of food, shelter, shade, fuel, wood products, oxygen, and erosion control, expand our appreciation of trees. To learn their identity and natural history adds to the understanding of these giants of the plant kingdom. In addition, learning to identify trees will not only add to your appreciation of the American countryside, but it also will serve as an introduction to concepts in biodiversity, taxonomy, and plant anatomy and physiology. Furthermore, a knowledge of the names of trees, their botanical features and their natural history are fundamental to all studies in forestry, environmental science, and botany.
Dendrology is the study of botanical trees. However, the definition of botanical trees is not clear-cut; the line between trees and shrubs is not absolute. A forester may define a tree based on the presence of a substantial trunk, as a source of merchandisable timber. An ecologist is likely to define a tree in terms of the plants competitive ability, as a unit of casting shade on other plants, and as a producer of photosynthetic compounds. A physiologist would be concerned with the rigidity of the plant, as a plant able to produce lignin and vascular cambium. For the sake of convenience, a tree typically is defined as a woody plant that reaches 13 to 20 feet or more in height at maturity, with a single trunk, unbranched for several feet above the above the ground, and having more or less a definite crown (Harlow et al., 1991).
Taxonomy is concerned with nomenclature, the assignment of names to organisms. Historically there have been two nomenclatural systems in botany: scientific names and common names. The forester and environmental scientist learn both in order to communicate effectively with both their colleagues and the general public. Common names have been assigned by people based on characteristics like habitat (Mountain Hemlock), distinctive features (Black Cherry), or use to man (Sugar Maple). These names are used in everyday speech and writing, but they are not precise enough to be used in scientific communication. Science uses a standardized, universal nomenclature using latinized names. Scientific names have followed standard usage since 1753 when Linnaeus published his monumental book entitled Species Plantarum. Since that time it has been convention to write the genus (capitalized) followed by the species (lower case). In scientific writing, both names are either underlined or italicized. The genus may be abbreviated with its initial only if the content is clear, and the genus name has been spelled out previously. For example, Tsuga canadensis, Prunus serotina, and A. saccharum are all correct usages of scientific names for the common-named trees, Mountain Hemlock, Black Cherry, and Sugar Maple.
In taxonomic classifications, the species is the focal point. It is the basic group of potentially interbreeding populations of individuals that share common characteristics, and are morphologically and often ecologically distinct from related species. A species is a concept that is variously defined, so taxonomists may disagree about the level of recognition of a group; a species to one taxonomist may be a "variety" or "subspecies" to another. Such flexibility in the concept is necessary to reflect the complexity of nature. Above the species level are categories of classification that serve to show other relationships. A genus groups together similar species, distinguishing the group from the other genera. Similarly, a family is composed of genera; and an order is composed of families. The top level in plant classification is the phylum. Table 1 shows twelve families of trees native to northeastern United States. These families are not exclusive to this region, nor are they the only families you will find in a Pennsylvania woodland. They are, however, among the most common and well-known tree families of the region.
There are many characters used to identify trees, and it is important that you become acquainted with the following three: leaf, fruit, and silhouette.
Leaves are the primary photosynthetic organs of most plants. Although they are often the most variable part, they are an important means of discriminating among species because of their many distinctive features, and their convenience. Leaves are formed on stems at regular intervals, and the point on the stem at which one or more leaves arise is called a node. The portion of the stem between the nodes is the internode. The expanded portion of the leaf is the blade, and the supporting stalk is the petiole.
There are two fundamental types of leaves. A simple leaf has only a single blade and is joined by its petiole to a woody twig. A compound leaf is divided into three to several dozen leaflets. The leaflet of a compound leaf is attached by its petiole to the rachis, which is attached to the twig. Whether simple or compound, leaves are usually arranged in one of three ways: opposite, when they are paired at the same node, one on each side of the twig; whorled, when more than two are found at the same node; and alternate, when only on leaf is attached at each node and leaves are arranged in spirals around the twig. Types and arrangements of leaves are shown in Figure 1. In addition to leaf type and arrangement, taxonomists use features like leaf size, shape, color, texture, and venation to identify and classify trees.
A seed is a ripened ovule, usually with an embryo. The seed-bearing structure of the plant, termed fruit in angiosperms and cones in conifers, are extremely important in identifying tree species. Fruits and cones are more accessible than flowers because they stay on the tree for a longer time. Some fruits may persist even after the leaves have fallen.
If the fruit develops from a single ovary in a flower, it is a simple fruit. Simple fruits can be fleshy like the pome, berry, drupe, and hesperidium, or they can be dry like the achene, samara, nut, legume, follicle, and capsule. If the fruit develops from several separate ovaries that stay together when matured, it is a compound fruit. Compound fruits are termed either aggregate if they develop from a single flower with many pistils or multiple fruits if they developed from many flowers. Figure 2 shows examples of simple and compound fruits.
Recognizing a trees overall silhouette is often a means to identification because many trees have a characteristic shape. Caution must be taken, however, because trees can assume a variety of shapes. A young tree might look entirely unlike a grizzled veteran of the same species. A forest grown tree reaching for the light, might grow tall, slender, and restricted in its branching, compared to a field grown tree where plenty of sun, soil, and moisture have enabled it to develop a maximum crown. With practice one can recognize the general shape and branching pattern of quite a few trees. Figure 3 provides some examples of silhouettes of trees you can find on campus.
THE HERBARIUM PROJECT ASSIGNMENT:
A herbarium is a collection of dried, pressed, mounted and identified specimens, carefully arranged in a particular sequence. Most colleges and universities maintain herbaria, often with several hundred thousand specimens. The largest herbaria in the United States are at museums or botanical gardens such as the U.S. National Herbarium at the Smithsonian Institution (with over 4 million specimens) and the New York Botanical Garden (over 4.5 million specimens). By comparison, your herbarium will be quite small. You will construct a herbarium including at least one representative species from five of the twelve families in Table 1. Details are explained in the "Procedure".
Field guides and hand lenses
Dichotomous key exercises
Display herbarium sheets
* The Plant Press: A plant press is made of "felt" blotting paper, corrugated cardboards, end boards, and tightening ropes or straps. The "pressing unit" itself consists of corrugated cardboard sandwiched between two blotters. A single, folded sheet of newspaper containing the specimen to be pressed is carefully placed within the blotter paper, and another unit is added. Your plant press will consist of five "pressing units."
* Useful hints:
Hint #1: Collect your specimens in duplicate. Press one specimen for your herbarium and use the second for dissection to key out and identify your specimen. Refrigeration keeps plants fresh for days if stored in air-tight bags.
Hint #2: Your 'plant press' should be placed in front of a fan to speed dry, caused by air moving through the corrugates of the press. You can also place your press in a heated area. Generally, the quicker a plant is dried; more color will be preserved.
See Figure 4 for a model written entry. Remember to include one representative species from five of the twelve families listed in Table 1.
Recommended Field Guides:
Lawrence, D. L., and M. K. Goddard (eds.) 1959. Common Trees of Pennsylvania. Commonwealth of Pennsylvania, Harrisburg, PA.
Petrides, G. A. 1988. The Peterson Field Guide Series: A Field Guide to Eastern Trees. Houghton Mifflin Co., New York, NY.
Whitman, A. H. (ed.) 1986. National Audubon Society Pocket Guide: Familiar Trees of North America (East). Alfred A. Knopf, Inc., New York, NY.
Barnes, B. V. and W. H. Wagner. 1981. Michigan Trees: A Guide to the Trees of Michigan and the Great Lakes Region. The University of Michigan Press, Ann Arbor, MI.
Harlow, W. M., E. S. Harrar, J. W. Hardin, and F. M. White. 1991. Textbook of Dendrology. McGraw-Hill, Inc., New York, NY.
Li, H. 1972. Trees of Pennsylvania. University of Pennsylvania Press, Inc., Philadelphia, PA.
Peattie, D. C. 1966. A Natural History of Trees of Eastern and Central North America. Houghton Mifflin Co., New York, NY.
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Copyright © This lab was initially written by John F. Dunski , was edited over the years by Dianne Burpee (Barlow) and was lastly modified by Dr. Jacqueline S. McLaughlin. This material can not be reproduced without expressed written permission from Dr. Jacqueline S. McLaughlin.