From a raven’s perspective, it’s easy to spot the lush, verdant growth that lines the edges of rivers, streams and canyons. Known as the riparian zone, this ribbon of vegetation is made up of a mix of Rio Grande cottonwood (formerly known as Fremont’s cottonwood), coyote willow, river birch, phragmites, tamarisk, Gambel’s oak and a host of other plants. Tied to the presence of water or a high water table, this zone does not extend far beyond the edges of these waterways into the desert uplands.
Each year, generally coinciding with the peak of spring runoff, the female cottonwood trees release their seeds. Adorned with numerous fine white hairs, the tiny seeds are blown by the wind and parachute to earth. If they hit the water, the hairs give them some buoyancy, hopefully enough to land on an emerging sandbank or an over-bank flooding spot along the river’s edge. Here the seeds settle out, germinate on the newly deposited sediments and grow rapidly to keep pace with the lowering water table.
Timing is everything to a cottonwood. The seeds do not have a great longevity - maybe two weeks - which is in stark contrast to other desert plants whose seeds may survive for years, awaiting proper soil conditions before they germinate. But since the cottonwoods grow along waterways, the possibility of moist soils is quite high.
As the seedlings become established on the sand bars and river banks, the roots grow quickly to keep pace with the descending water table. After one year, the roots may be fairly long. Even though the seedlings may survive that first year, there is no guarantee that next year won’t have a higher spring runoff, tearing out the sandbars laid the previous year. The odds of survival resemble my chance of winning the lottery - pretty slim.
The fast growth of the young seedlings enables the plants to increase their chances of survival season to season. A three-year-old tree may stand fifteen feet tall, with a deep tap root acting like an anchor. Growing in full-sun, cottonwoods don’t really like to grow in the shade of other trees as these trees compete for available nutrients, moisture and sunlight.
Mature cottonwood galleries may contain trees over 125 years old. Easily identified by their tall stature, gnarled appearance and deeply furrowed bark, these trees have withstood the ravages of time. They’ve survived gnawing beavers, fire, insects and the settlers axe. In their crowns, great blue herons or black-crowned night herons nest. Screech owls and flickers nest in cavities excavated in dead limbs. More than 70 different bird species build their nests in cottonwood trees, including summer nesters like Bullock’s oriole, black-headed grosbeaks and warbling vireos.
Porcupines climb these trees to forage in and beavers gnaw away at the trunks. Though the beavers girdled the trees, some cottonwoods continue to stand, resilient but lifeless on some bench above the river. Their presence attracts bald eagles or red-tailed hawks that nest high in the crown of the tree, snapping off twigs to construct their nests.
But under pressure from invasive plants, altered river flows and wildfires, some of these old cottonwood galleries are disappearing. The removal of tamarisk, an invasive plant from Eurasia, has decreased the threat from fire and encouraged growth of native willows and cottonwoods.
So the next time you’re down by the river, enjoy the shade cast by these massive trees and appreciate the life-giving structure of these gnarled cottonwoods.