As the meandering of a river cuts its bank, depositing new land on a point-bar downstream, cottonwood readily seeds in to produce dense stands. Flood waters deposit additional coarse sediments near the river's bank, building high, well-drained ridges or natural levees. Inundation then covers the cottonwood seedlings with water or, at least, covers the soil. Roots die when soil is soaked for over a month, but extensive adventitious roots quickly develop from dormant buds on the main stem. Terminal and diameter growth resume after flooding water subsides. Trees have survived when as much as 3 feet of silt has been deposited, a new root system developing just under the new surface of the ground.34Growth of cottonwood trees depends upon the texture of the soil and its internal drainage. These true poplars grow best where internal drainage is rapid, the soils inherently moist, and the soil texture fine silt or clay. Growth is least on dry sands and on soils with poor internal drainage.35
The life cycle of the cottonwood tree begins in the spring when male and female flowers appear on separate trees as young as 10 years of age. This dioecious (from the Greek, meaning two houses) flower arrangement encourages cross-pollination, in contrast to the monoecious (one house) physiology of many tree species. Swiftly the seeds mature, most of them falling between April and July. Apart from an occasional freeze, following the breaking of dormancy of flower buds, that destroy the crop, abundant seed crops occur every year. Wind and water carry the seeds. Short periods of flooding seem beneficial to germination: annual floods deposit a fresh layer of silt on which the white masses of the cottony seeds settle out to germinate. Favorable soil moisture must prevail, for seeds remain viable under dry conditions for but a few days. Many seeds will not be viable, after even a few hours, if dry. These trees are relatively drought-resistant once the seedlings are established. Many thousands of seedlings per acre will appear. Hard, blowing rain takes its toll of many; as does the hot sun. Few other trees or shrubs will encroach on new land to compete with the cottonwoods. The exception is black willow, but even that hydric tree is crowded out by the
cottonwoods. Both are highly intolerant of shade, seldom persisting under even sparse stands of trees.
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