|A unit of Lasting Forests
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A Total Forest Management Plan
The leaves of most plants are green due to the presence of chlorophyll, a green pigment which in addition to its coloration has the unique capability of absorbing energy from the sun. Chlorophyll combines the energy with water and carbon dioxide from the air to manufacture glucose or a plant sugar, the basic component of the world's food chain. This is a process which people have been unable to duplicate.
Autumn leaf coloration is brought about by cooler temperatures and shorter periods of daylight resulting in a breakdown of chlorophyll. Autumn coloration begins when nights become long (but probably when sunlight provides inadequate energy) and trees are signaled to drop leaves in preparation for ice, snow, and the conditions of occasional very severe winter periods. Within Virginia, Autumn colors become evident and a resource from about October 1 to November 15. Peak "peeping" times: Mid-October in the western mountains, late October for eastern part of the state. Color change is affected by temperatures as well as moisture and varies with species.
When the chlorophyll breaks down, other pigments are allowed to be seen. These were masked by chlorophyll during the summer. The varying shades of yellow in flowers and fruits as well as fall leaves are the result of the presence of carotene and xanthophyll. Anthocyanin results in blues and reds. The combinations of these pigments, in varying degrees, bring about the wide variety of color evident in the autumn woods.
The coloration and its timing are the result of a complex combination, thus almost unique.
The change in color which may also be influenced by temperature, insects, rainfall, and soil conditions, starts when the tree grows special cells that shut off water and food to the leaves. Differences may occur year to year and even in portions of the same tree. As the leaves die, the green color fades and other colors which were there become visible. Eventually the leaves die and fall to the ground.
The combination of factors produces "the autumn season"
An autumn-color forest with striking color combinations may be enhanced through selective cutting or retention of trees having desired colors. Thinning, block cutting, removal of species with dull or unwanted colors, retaining those with contrasting colors and keeping pines or other evergreens to break the stark grays and browns of winter ar techniques you may use to tailor the autumn coloration of your woodlands to satisfy personal esthetic tastes or to present specific scenes. (Special work may be provided by the Viewscapes Group.)
The "esthetic forest" has been widely discussed. Discussions typically concern tree type, stem density, and care od shrubs and logging debris. Here we are discussing mainly autumn tree-top coloration, as seen from a viewing point, a vista. Managing forests for such coloration may be an artistic expression for few people. It can be expensive and carried out only in areas of high visibility (e.g., highway frontages, a view from a special window). Where appropriate, the following colors are typically dominant in the listed species. (Some species are listed under two colors since there are site differences and several colors on the same tree.) Combinations for beauty, wood supplies, and wildlife benefits can be challenging.
Since trees are fairly site specific, color aerial photography has been used to map sites as well as stands.
Geobotany experts use tree colorations to map species occurrences indicating soil, moisture, and geological conditions of interest.
Amber lens sunglasses enhance visual differences among shades of green and related hues in foliage. This is especially true in late autumn. Guides might consider supplying such glasses for visitors and guests. Medium strength yellow lenses are best; brown gives some discrimination ability.Thus it is easier to differentiate among species in mixed hardwood stands, see stressed white pines, see chronic moisture stress in azaleas and other plants, see diseased shrubs (dogwood anthracnose) (from Gary L. Wade, NE Forest Exp. Sta, Burlington, VT (NE-INF-131-97)
Why Do Fall Leaves Change Color?
modified from an article by Brian Handwerk for National Geographic News, October 5, 2004
How does the change in color benefit trees? As scientists explain, there is a reason for the season.
John Shane, chair of the University of Vermont's forestry program, notes that the increasing darkness in the Northern Hemisphere this time of year "indicates to the plant that fall is coming on. So it starts recouping materials from the leaves before they drop off."
Evergreens protect their needle-like foliage from freezing with waxy coatings and natural "antifreezes." But broadleaf plants, like sugar maples, birches, and sumacs, have no such protections. As a result, they shed their leaves. But before they do, the plants first try to salvage important nutrients such as nitrogen and phosphorus. The process treats leaf-peepers to displays of autumn color, as green leaves turn into brilliant or muted shades of gold, orange, yellow, and red.
Inside the Leaf Chlorophyll gives leaves their green color throughout the growing season. The compound is essential for photosynthesis, a chemical reaction that converts sunlight into carbohydrates. Leaves also contain carotenoids. These natural pigments, which produce yellow, orange, and brown hues in plants, from buttercups to carrots, are always present. The colors of carotenoids are easily masked by green chlorophyll, at least until shrinking daylight and a nip in the air signal fall's arrival. At that time broadleaf plants slow and eventually stop their chlorophyll production, thus revealing the distinctive golden, orange, and yellow hues of carotenoid pigments.
The recipe for an especially spectacular season? Since daylight wanes at a constant rate each fall, other factors, like soil moisture and weather, ensure that no two autumns are alike. Warm, sunny days mixed with cool, but above-freezing nights appear to produce the brilliant red hues associated with peak fall foliage. Shane believes leaf-peepers judge whether it has been a "good" or "bad" fall based on the proportion of red leaves—the more, the better. "Other things being equal, that [ratio] changes more than anything else," he said. "The real question is: What's going on with these reds?"
Unlike carotenoids, which are always present in leaves, some species of trees produce red-hued anthocyanins, naturally occurring pigments that turn raspberries red and blueberries blue. What's more, trees produce the anthocyanin compounds found in leaves during the autumn season. Exactly why this happens is uncertain. But scientists know that even nature's most arresting displays usually have a purpose.
Paul Schaberg is a research plant physiologist with the USDA Forest Service in Burlington,Vermont. He notes that when trees produce anthocyanins "it ties up a lot of sugar and nutrients like nitrogen that you'd think [trees] would want to conserve." "Why do that when you're about to lose the leaf anyway? And why do some species do this and not others?" Ideas abound. Researchers have variously suggested that the antioxidant acts as natural antifreeze, insect repellent, or sunscreen. "The hypothesis is that if you can protect the leaf so that it stays on the plant a little longer, then you allow the tree a bit more time to recover important nutrients," Schaberg said.
Plant physiologist William Hoch has done extensive research into the sunscreen function of the red-hued pigment. Using single-gene mutations, the University of Wisconsin-Madison researcher created test species from plants that would normally produce anthocyanins. The test species could not produce the compounds. The test plants were compared with natural relatives that could produce the red-hued pigment, as well as with plant species that do not develop red leaves.Pigments are being produced to protect the leaves from excess light during [the fall] period. The mutants suffered more light damage to their photosynthetic systems, so that they weren't able to recover important nutrients in the leaves.For these plants, sunscreen may be more necessary during the fall months than in the blistering days of summer. While sunlight is brighter in June, plants that shed their leaves become more susceptible to sun damage in fall, when their leaves' natural systems are breaking down. Leaves can deal with high light levels when they are intact but as they are breaking down, they become unstable.
Yet many trees never develop red leaves. Aspens turn golden yellow. While some species, such as elms, change little before their leaves simply wither, die, and fall to the ground. Hoch found that when he tested species like paper birch, which remains brilliant yellow, the tree recovered nutrients just as well as its red-leaved cousins. "Nature's figured out multiple ways of doing things," Hoch said. "An analogy I like to use is the pollination of flowers: It happens by bees, birds, wind. Nature has evolved multiple ways of doing the same thing."
Schaberg's extensive research focuses on how red pigments signal a tree under stress. Any number of factors can create the strain. Schaberg has even produced red leaves on a branch-by-branch basis by selectively applying tourniquets to stress certain areas within a single tree. "These antioxidants might be a response that's important for sun, cold, insects, fungal attacks, or anything," he said. Schaberg believes that if science can understand trees' red-flag message, we might acquire a better understanding of overall forest health."We want to confirm a connection between a plant that's stressed and one that's producing red," he said. "If [red pigment] is produced for protection, does it make sense that a tree stressed by drought, pollution, or something else would have extra red? If so, maybe looking at the red of some trees could be a sort of litmus test of tree conditions."
Call for Virginia Fall Foliage/Events: Hotline: 1-800-434-5323
See the Virginia Tech dendrology site for tree identification
Major site: http://ublib.buffalo.edu/libraries/asl/guides/bio/fall_foliage.html
It now seems unlikely that chlorophyll content per plant can be used as a productivity index (Sanger 1971).
Sanger, J.E. 1971. Quantitative investigations of leaf pigments from their inception in buds through autumn coloration to decomposition in falling leaves. Ecology 52(6): 1075-1089
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Last revision April 13, 2004.