Rio Grande Cottonwoods

Riparian forest of the Middle Rio Grande Valley are typically composed of Rio Grande cottonwoods (Populous deltoides subsp. wislizenii), a subspecies of the plains (prairie) cottonwood (Populous deltoides). There has been some confusion in the scientific literature as to the specific identity of Rio Grande cottonwoods, but shoot, bud and capsule characters align this subspecies with the eastern Populous deltoides rather than with Freemont cottonwoods (Populous fremontii) as often stated. The center of the distribution of Rio Grande cottonwoods lies along the upper Rio Grande drainage, particularly through the Middle Rio Grande Valley. The species occurs along the Rio Grande from about El Paso northward and probably into the Colorado River drainage of northeastern Arizona, eastern Utah and western Colorado, though Fremont cottonwoods are present in the Colorado River drainage as well. Generally in New Mexico, cottonwoods west of the Rio Grande are Fremont cottonwoods (such as in the Gila and San Francisco watersheds) while those to the east are plains cottonwoods. However, there is considerable hybridization among cottonwoods, and there remains some question as to definite boundaries among species and subspecies. Fortunately, ecology of these floodplain species is similar, so that knowing whether a tree belongs to P. deltoides or P. fremontii is relatively unimportant in the context of this guide.

Cottonwoods grow well only where their roots can reach moisture provided by underground water and where their seeds can germinate on bare, moist soil. Thus, they are limited to areas with permanent water supplies. Although once widespread throughout low-elevation floodplains, many cottonwood forests have been cleared for farming, river-control projects and urban development. The only extensive tracts of Rio Grande cottonwoods remaining are found along the Rio Grande in central New Mexico, from approximately Espanola south to the Belen area. Unfortunately, these areas also face intensive impact by humans, severely threatening these unique and valuable habitats.

Rio Grande cottonwoods have thick and fissured gray bark and heart-or triangular-shaped leaves. They can reach approximately three feet (one meter) in diameter and 90 feet (30 meters) tall, but their size depends largely on water availability and so it is difficult to age a tree based on size alone. Male and female flowers occur on separate individuals, and small cottonwoods (whether young or just water-stressed) typically do not produce any flowers at all. In the spring, male trees bear flowers in long red clusters called catkins. The pollen produced by male flowers is carried by the wind to female flowers on adjacent trees, which are borne in long, greenish clusters. If pollinated, the female flowers develop into capsules containing tiny seeds with cottony plumes. Female trees can produce thousands of these tiny seeds, which are easily dispersed long distances by wind or water. Seeds are produced during a brief period in late spring/early summer, which, prior to river regulation, corresponded to the peak of river flows when the river often flooded over its banks. To germinate, cottonwood seeds require scoured sand bars, which provide bare soil, moisture and plenty of sunlight. Many of the seeds land in unsuitable places, and will fail to develop. Those lucky enough to land on moist, open soil quickly develop roots, and if the post-flood soil moisture does not decrease too rapidly, the roots will soon reach down to the capillary bed above the water table. The water table is the upper surface of saturated ground water, while the capillary bed is a zone of soil above the saturated zone that consists of small spaces containing moisture held by capillary action. The ability of plants to use underground water is important in a region of irregular surface water, and cottonwoods can not survive where the water table is too deep. Plants such as cottonwoods that can tap their roots directly into the ground water are called phreatophytes.