| New Site | Old Home | Search | Communities | Counties | Southeast | Southwest | |
 |
| |
| ||||||||||||
Cottonwood
Last updated on Wednesday, January 01, 2003
Autumn slips across the desert quietly. Although nights grow chill, summer's heat lingers in the afternoons, and the greenery brought on by summer rains simply fades to dusty olive, bleached straw, and weathered brown. As the soil dries out, mesquites, desert willows, and ocotillo drop their leaves without any fanfare. But here and there where water flows - a spring, stream, an irrigation ditch, or a river - autumn shows in the rich yellows and golds of cottonwood trees.
Cottonwood tree in New Mexico's Mimbres Valley Photo by Carla DeMarco. It is hard to find a cottonwood in the desert nowadays. Where tall cottonwood overstories once marked the high-water lines of desert rivers and traced the sputtering lines of streams and springs, only an occasional twisted survivor remains. The disappearance of the cottonwoods and the associated bosques, Spanish for "woodlands," and cienegas, "marshes," resulted from several causes. Woodland clearcutting and livestock overgrazing in the late 1800s denuded whole watersheds, stripping the plant cover so that rainwater roared off, carrying the precious soil and causing once-permanent streams to alternate between brown floods and dusty drought. Steep-walled arroyos now gouge the landscape where streams and rivers once meandered, lined by cottonwoods and bosques. Dams controlled river flooding, but without the scouring action and rich sediment deposited by high water, cottonwoods could not reproduce. Farmers cleared the bosques and cottonwood overstory to create fields in the fertile soils of the river floodplains.
But where cottonwoods remain, the gradually shortening days of autumn bring color to their broad, triangular leaves. As the number of hours of daylight decrease, the trees stop producing chlorophyll, the green pigment that both colors their leaves and helps them produce food from the sun's energy. The chlorophyll slowly breaks down and disappears from the leaves, revealing underlying pigments which color the waxy leaves golden yellow.
Many Southwest place names - such as Alamogordo, New Mexico, for instance, named "fat cottonwood" for a long-vanished giant - commemorate the cottonwood of the Southwest's deserts and grasslands. This species grows as tall as 80 to 90 feet, supporting a broad, open crown with large, spreading branches. Grey, deeply furrowed bark clothes a fat trunk that can swell to more than five feet in diameter. The largest-known example of this cottonwood grows on a ranch in western New Mexico and boasts an enormous trunk nearly 12 feet across - 38 feet around - and a crown spread of 102 feet!
The yellow and gold tinging cottonwood leaves in the fall signals the tree's preparations for the oncoming winter. Despite the waxy coating that gives the leaves their slight shine, cottonwoods (and all deciduous trees and shrubs) shed their leaves so that the leaves will not respire more water than the tree's roots can suck from the dry soil over the winter, dehydrating the tree. And by preparing for winter, the cottonwood brings a blaze of autumn color to the desert.
|
||||||||||||
