FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FALL COLOR STARTS WITH POISON OAK By Toni Corelli
Poison oak is one of our most common plants at Edgewood. It cannot and should not be overlooked along each trail. The scientific name says it all Toxicodendron “toxic tree” diversilobum “diversely-lobed” which translates to “toxic tree with diversely lobed leaves.” The common name western poison oak means “poison” referring to the oils that can cause skin dermatitis, and “oak” to the leaves that look somewhat like oak leaves, but oak leaves are not compound as are poison oak leaves which have compound leaves with three leaflets. “Leaves of three let it be.” Western poison oak is deciduous, Latin for “to fall off,” referring to the leaves. It is one of our most spectacular “fall color” plants. The red color is a result of pigments called anthocyanins. Anthocyanins occur naturally in leaves in the form of dissolved cell sap. As summer turns to autumn and the days are sunny and the nights are cooler, chlorophyll production, which gives the leaves their green color, decreases. Anthocyanin production in the leaves increases in response to surging sugar concentrations, changing the western poison oak leaves to a brilliant red color in the fall. The production of anthocyanin pigments can change the leaves of other species as well to tints of red, purple, and crimson. Anthocyanins are also responsible for the color you see in cranberries, red apples, concord grapes, blueberries, cherries, strawberries, and plums. What causes the leaves to actually fall? Each leaf has a specialized layer of cells called an abscission layer that is located between the leaf and/or petiole and the plant. This layer is composed of specialized cells that transport water to the leaf and carry carbohydrates back into the plant. Again as climatic conditions change in the fall, cells in the abscission layer are triggered by plant hormones to secrete a waxy substance that begins to swell. Eventually all water and carbohydrates are cut off, the abscission layer begins to disintegrate and the leaf falls from its own weight or because of wind and weather. However, not is all lost and the leaves that fall are not wasted. They decompose and restock the soil with nutrients and make up the mulch adding nutrients and keeping the soil moist. Fallen leaves also become food for numerous soil organisms that are so important to the ecosystem as a whole. Some who come to Edgewood might call our poison oak, “poison ivy.” Poison ivy is related to poison oak, and is in the same family (Anacardiaceae, Sumac Family) and genus (Toxicodendron), but poison ivy does not occur naturally in California. The Sumac Family is a large family of over 600 species in 70 genera worldwide most of which occur in the subtropical and tropical regions. Many species produce an oily substance, which causes contact dermatitis (a blistery rash). However, not all members of this family cause dermatitis, some that occur in the tropics are important timber producing trees. Some members of this family have edible fruit such as, mangos, cashews, and pistachios. Handling a green unripened mango fruit can cause dermatitis and contact with unprocessed pistachio and cashews can cause dermatitis. Another familiar ornamental tree in this family is the Peruvian pepper tree (Schinus molle). The fruit has been used as a substitute for black pepper. The Docent Training Manual has much more information about Western Poison Oak written by Kathy Korbholz. References Friends of Edgewood Natural Preserve Docent Training Manual. 1999-2000. Web Sites http://bodd.cf.ac.uk/BotDermFolder/BotDermA/ANAC.html http://scifun.chem.wisc.edu/chemweek/fallcolr/fallcolr.html http://www.parks.ca.gov/?page_id=23487 (where to see fall color in California) http://chemistry.about.com/library/weekly/aa082602a.htm http://www.microscopyu.com/galleries/confocal/abscission.html |
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