Trees, environment, and genes: in the evolutionary battle to survive and thrive, a species' parentage is just the beginning. (Heartwood).
It's a question that's been debated for decades. Some insist that a tree's genetic material is a recipe for superior trees, that harvesting genetic material and then producing offspring creates a "cloned tree, an exact replica of' the parent. Others insist a tree is as much a product of its environment as its parentage. Who's right? The answer begins with genetics.
The classification system devised for the plant kingdom gives each tree a unique name that provides a road map to its genetic structure. Within the genus or family of maple trees, for instance, a red maple is Acer rubrum and a sugar maple is Acer saccharum.
Knowing the difference between species is essential for environmental, as well as economic, decisionmaking. Sugar maple, for example, makes good syrup and hardwood flooring for basketball courts and bowling alleys; not so with red maple. Red maples boast a larger natural growing range and can more easily adapt to wet and dry sites. And the red maple produces a wide genetic diversity from its seed, which means a red maple in your backyard might produce very different-looking offspring. Those who think maples are basically all the same make a fundamental mistake about species with uniquely different genetic makeups.
Another fundamental mistake would be to assume that so-called "champion" trees--the largest known of their species in the U.S. and therefore on AMERICAN FORESTS' National Register of Big Trees- are somehow genetically superior to their counterparts. In some cases, in fact, just the opposite is true. While many of those trees are true beauties, some are downright ugly, bug-chewed, or dying. One--the national champion American elm, in Grand Traverse County, Michigan--has recently been declared dead from Dutch elm disease.
In the end, the trees that wind up on the national Register are a product of their environment. Although we wish there were some secret gene that made them supertrees, in fact they are no more likely to be genetically superior or longlived than any other tree in the forest or along a city street.
In fact, it's very possible that a tree growing old on one spot would not live to a ripe old age somewhere else. Tree selection is a science best left to experts with a detailed understanding of the process of selection and the production of improved plant material. The experts, too, know the best way to gather genetic material without causing damage to trees. Tree climbing spikes, for example, should be used only for tree removals or for climbing dead trees or telephone poles--never on living trees, especially national treasures.
THE NAMING OF TREES
Trees are grouped by their biological characteristics into order, family, genus, species, and variety. The process of naming plants and classifying them into those groupings is called nomenclature; the study of trees themselves is called dendrology. The "order" is the most easily recognized--a softwood, like pine, as compared to a hardwood. As you move toward "variety" the distinctions become more difficult. Understanding varieties, or differences within species, requires considerable expertise.
A tree geneticist trained in the science of tree breeding understands more than just the principles of population genetics established by Gregor Mendel 150 years ago (remember those peas from biology class). A tree geneticist has learned techniques for selecting desirable genetic material for tree breeding and conducting progeny tests (growing out trees in experimental plots) where the environmental conditions are kept the same and the plants genetic make-up is altered.
Scientists who work in tree genetics specialize. A forest geneticist working for a wood-products company might focus on genetic improvements to change the length of tree cells used for making paper or the volume of wood in a saw log. A tree geneticist working to improve urban trees might focus on the vigor of a tree for wound closure or disease-resistance.
Of course, nature sorted trees (that survival of the fittest exercise) for millions of years before humans came along with their nomenclature classifications. Trees have been growing under natural conditions with flowers pollinated and new seeds formed for more than 200 million years.
When seeds germinate, if the conditions are right they grow into mature trees, although the vast majority of seeds cant make it in the environmental conditions in which they attempt to grow. As a result of this long evolutionary process, only the best survive. Over millions of years they've developed the unique characteristics we know today, allowing botanists Carolus Linnaeus to develop the nomenclature system and Andre Michaux to articulate the place in the nomenclature for North American trees. (The continental U.S. has 850 distinct species of trees and more than 10,000 tree varieties.)
Humans have never been content to just let nature "take its course, and many attempts to improve trees by non-geneticists did just the opposite. Commercial tree nurseries once tried to produce trees that would have popular appeal for characteristics such as an unusual shape--like lollypop-shaped maples planted at shopping malls-rather than the ability to survive in an urban environment.
The story of the American chestnut is one fraught with mistakes but a hopeful ending. The chestnut blight introduced into the United States in 1904 robbed the northeastern forest of 75 percent of its value. Early efforts to propagate a disease-resistant tree were less than successful because the necessary scientific rigor was neglected. The resulting trees, which seemed resistant, later died, leaving present generations of Americans familiar with the great chestnut mainly through song and poetry. However, the American Chestnut Foundation's work (www.chestnut.acf.org) has brought new hope to the search for a tree that combines the beauty and hardiness of the American chestnut with the blight-resistance of its Chinese counterpart.
Its interesting to note that while the process of natural selection has produced genetically improved trees over hundreds of millions of years, humans have wrought their changes--some good, some bad--in the equivalent of an eyeblink on the evolutionary timeline. Imagine the natural selection process compressed into a 24-hour period, with minute one representing 200 million years ago. Darwin's 1859 work The Origin of Species would fall at one second before midnight. A letter that urged AMERICAN FORESTS to help preserve the largest known of individual species, and which launched the National Register of Big Trees, would have been written 1/30th of a second before the final tick.
That letter, written by Maryland forester Joseph Stearns in 1940, decried the wholesale cutting of large trees and urged AMERICAN FORESTS to help save large trees of each species for future generations to enjoy. The resulting national Register marked the first effort by a conservation group to do something about the impact people were having on the forest environment. It was a conservation action, a way to preserve a piece of the ecological system from the tidal wave of environmental change.
The trees in the Register tell a great story about the environment, but it is not a tale of genetically superior trees. These trees represent the saving of an ecological niche in an already limited gene pool, with size not necessarily being a defining characteristic of their superiority.
TRY THIS AT HOME
You can spot signs of genetic variation in trees just by looking at their shape. Remember, though: Physical differences often will show variations between species but not genetic superiority. Pick a couple of trees near your home or workplace and keep an eye on them over the course of a year. Jot notes to yourself about what you see. To compare the shape of two species, look for trees that have the space for their natural forms to develop; avoid trees restricted by wires, buildings, or other trees. Try to isolate the tree as an individual, imagining it with nothing behind it but blue sky.
To give you an example, I picked an American elm near AMERICAN FORESTS' Washington, DC, office and a white oak near my home in Maryland. The most obvious difference between the two was the shape. American elms look like a vase, with arching branches that grow out from the trunk at a sharp angle. White oaks are large and round, with branches that grow perpendicular to the trunk. Each species changed appearances with the change in seasons. In the spring I watched the elm put out seeds, greening the tree with a mass of flat pods before the leaves emerged, while the oak stayed dormant. About three weeks after the elm turned green, the oak turned yellow/green with catkins and small new leaves. In summer the elm's leaves had a jagged edge and what botanists call a simple shape while the oak had multiple lobes and a smooth edge. The oak's acorns were noticeable in the fall about a month before they fell to the ground.
The trees you initially chose are probably big and appealing, trees that have fared well in the two factors, genetic makeup and environment, that determine success in the life of a tree. You're already familiar with one of those, the environment, which can have two causes: human and natural.
Human-caused impacts are physical things like buildings and walkways. Natural environmental conditions are things like soil moisture or the effects of wind (for example, wind can have a dramatic effect on the branch structure of a tree growing near the ocean or on a mountain).
Genetics can cause two trees of the same species growing under the same environmental conditions to have very different shapes. Geneticists call that a tree's phenotype. Look for two trees in the same species that look different. A tree's shape and branch structure are visible displays of genetic differences, although most of the effects of genetics are visible only to a trained geneticist.
Genetic engineering is a familiar topic due to controversy over the merits of genetically engineered foods, but scientists have also looked at the benefits of genetically modified trees. This emerging science could significantly change our landscape. In the Autumn issue of American Forests, we'll talk with geneticists about the potential impact of genetic engineering on trees and where they see this scientific endeavor taking us.
RELATED ARTICLE: BACKCROSSING FOR BRAWN AND BEAUTY
In 1904, chestnut blight come to North America, attacking American chestnuts, which--unlike their Chinese cousins-- proved unable to withstood the fungus. Since the species can be cross-bred easily, The American Chestnut Foundation (TACF) set a goal of breeding a tree that retained the strong, vase-shaped beauty of the American chestnut but included the Chinese tree's blight-resistance. It describes the process on its website, www.chestnut.acf.org.
Decades of breeding research by the US. Department of Agriculture, the Connecticut Agricultural Experiment Station nod recently at TACF show that high levels of resistance to the blight will be present only if all the genes defending against the blight are from the Chinese tree.
TACF's backcrossing method used Chinese-American hybrids produced at its research farms and backcrossed those to an entirely American parent. The resulting tree is 3/4 American and 1/4 Chinese. After two more backcrosses, TACF had a tree which was 15/16 American and 1/16 Chinese, which it hopes will uphold the beauty and adaptability of the American chestnut.
To confirm each backcross, TACF tested the created seedlings by inoculating them with blight. Seedlings showing the greatest resistance were then used to carry the research forward. TACF estimates the first set of blight-resistant American chestnuts will be ready for planting in less than five years. For mare information visit The American Chestnut Foundation at www.chestnut.acf.org. -- Amanda Mclean
Gary Moll is AMERICAN FORESTS' senior up for urban forestry and IT.
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|Date:||Jun 22, 2003|
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