A few years ago, I was invited on a walk along the Willamette River to look at a parcel of land that contained some remnant backchannels and a riparian gallery forest near Salem. The riparian forest was remarkable because it contained a number of enormous black cottonwood trees that towered over the ashes, alders, maples and conifers. Cottonwoods are one of my most favored trees. They thrive on floodplains. Arno and Hammerly in their wonderful book, Northwest Trees, mentioned that cottonwoods in floodplains “grow tall and develop great, arching branch-trunks that form a light-filtering canopy high overhead.” Some of the largest cottonwood trees soar over 200 feet and have trunks that exceed 6 feet in diameter. The triangular ovate leaves of black cottonwood are notable because the upper surfaces are dark green while the undersides are pale green to silver-white. In the Natural History of Western Trees, Peattie poetically described the black cottonwood forests in the Columbia Gorge: “And when the great drafts blow up and down the Columbia gorge, the cool indrafts of sea wind or the hot exhaling breath of the interior basin east of the Cascades- they go plowing through the riparian forests. Then the black cottonwood comes into its full beauty, for on one side the leaf blades are darkly glittering, on the other almost silvery white with rusty veins….the trees seems, like the sea, to break into whitecaps before the wind. Then, as the breeze dies down, they regain for the moment their green composure.” In the basin and ranges of the west, rivers are necklaces of brilliant yellow as the cottonwood leaves turn color in the fall.
Aside from their great girth, stature and beauty, black cottonwoods are generous seeders and sprouters. Each tree contains either female or male catkins. The female flowers mature into 4-inch long branches of light green capsules that split open in late spring and early summer and fill the air with white cotton-like plumes bearing seeds. The cotton plumes are like late spring snows blanketing forest floors, clustering on shrubby branches of ocean spray, snowberry and elderberry, and occasionally soaring into my open office window near the Corvallis waterfront. Like most poplars, cottonwoods are avid re-sprouters. Disturbance is a common (and critical) ecological function in floodplains and cottonwoods evolved to take advantage of waters that surge into bottomland hardwood forests and scour forest floors. Cottonwood roots are often shallow, just below the surface of sandy rocky soils found in floodplains. If their roots are exposed or disturbed, they often respond by sprouting numerous aerial shoots that may grow, through time, into towering giants. Like willows, cottonwood branch cuttings stuck into the ground will set root and create trees. Arno and Hammerly described how a railroad car filled with cottonwood logs left to overwinter in a side yard was soon “engulfed in a forest of sprouts.”
Scientists refer to species that have been extensively studied as “model organisms”. Cottonwoods fall into that category because they grow fast, are easily propagated and hybridize readily giving them great potential for many human uses such as windrows, flood buffers, and paper pulp. Apparently the entire complex genome of black cottonwood is understood for it was the first tree species to have its genes fully sequenced in 2006. Cottonwoods and humans have a long and productive relationship. Native Americans used cottonwood resin to treat sore throats, coughs, and many other ailments. Nancy Turner in the Ethnobotany of Bella Coola Indians of British Columbia described how an infusion of cottonwood buds mixed with sockeye salmon oil was used as a remedy for baldness. The same infusion mixed with goat fat was an ointment for sunburn and a skin cream. Native Americans fed the inner bark and leaves to their horses. They harvested the inner cambium of cottonwoods, and sundried and ate this sweet bark with grease. Sap was dried or eaten fresh and the trunk and branch wood used to build canoes and lodges. Cottonwood fibers were used to make baskets, fishing nets, ropes (mixed with dog hair), mats, rugs and bedding, and the buds boiled to make yellow dyes and paints. Jeff Hart in Montana Native Plants and Early Peoples said that the sap was also used to conceal human scent when stealing horses. The members of the Lewis and Clark Expedition carved most of their dugout canoes from cottonwood trunks and used the trees to make wheels and axels for portaging equipment.
Cottonwood forests provide great benefits to aquatic and wildlife species. Bald eagles, ospreys and numerous song birds use their broad branches to build nests or roost. Live and dead trees are often pockmarked with holes used by cavity nesting birds and some mammals like western tree squirrels. Branches and tops of cottonwoods frequently split and break, and the cavity left behind becomes the home of some wild creature. We often paused on our walks through Bald Hill Farm to watch an industrious colony of acorn woodpeckers fly in and out of cavities in cottonwood snag. Unfortunately, the snag broke in half in 2011 and the woodpeckers found another home. I once pounded on the large trunk of a cottonwood snag with a stick hoping to hear resonating sounds of a 60 foot hallow drum when a large red-headed pileated woodpecker stuck her head out of a cavity in the tree and peered angrily down at my drumming antics. Occasionally black bears will hibernate in cavities at ground level or higher in up in the trunk of large cottonwoods that have some heart rot. Beavers love cottonwood bark (to eat) and they harvest the branches and trunks to construct dams. During my Salem site visit, one of the largest cottonwoods was nearly girdled by a beaver that ate through the tough outer layers to reach the sweeter inner cambium. Canopies of cottonwoods are permeable and allow for filtered sunlight to reach forest floors. The thick, rich layers of cottonwood leaf litter, compounded through many generations, produce fertile substrates for many deciduous trees and shrubs that in turn provide abundant food and shelter for wildlife.
Cottonwoods are great colonizers. They enter into disturb sites and grow rapidly outpacing competitors such as conifers and other hardwoods. However cottonwood seeds need moist soils for germination and saplings require wet substrates to thrive. The ecology of the species is deeply intertwined with periodic inundations and alluvial soils. Sediments pushed into floodplains by floods create the best germination seedbeds for cottonwoods. Roots need seasonal exposure to waters from high water events. The construction of dams has resulted in the reduction of flood events and also created poor conditions for cottonwood regeneration and survival.
Walking among forests of giant cottonwoods or any gigantic trees is a humbling and profound experience. Paddling into quiet Willamette River off-channel alcoves that are ringed with stately cottonwoods is like entering into an aquatic cathedral with towering columns and soaring arches. Sometimes, sitting motionless in my kayak staring up at the serene canopies of silver and green leaves, I forget that these magnificent trees depend on often violent flood events for their creation and subsistence.
