Cottonwoods: Where Wildlife Take Refuge In Winter

May 11, 2016

Thinking about plants in winter recently, I remembered a particular good-sized cottonwood I saw while walking along a riverbank. What was its story?

From James Halfpenny’s fascinating book “Winter:  An Ecological Handbook,” I learned that cottonwoods, like many northern trees, have very special adaptations to survive the long, cold winters. They begin their “hardening” process in the fall, as temperatures begin to drop and the amount of daylight decreases.  Leaves typically fall during this stage of hardening, but the process continues as winter settles in. 

Inside the tree, the chemistry changes so that water is removed from cells to the intercellular spaces where it can freeze without damaging the living cells.  Meanwhile, inside the cells, less water and more sugars makes freezing far less likely. In very deep cold winters there is a third stage.  Ice inside the tree becomes smooth with a molten quality, instead of having tiny, sharp spicules that could pierce cell walls.

So the tree prepares itself for the cold, non-growing season, but what else happens? How does a cottonwood fit into the winter survival strategies of other creatures? So I explored some more and here’s a rundown of what I found.

First, I saw birds. A pair of eagles preferred the high branches of a cottonwood tree to spot fish in the river.  I saw them perform some kind of ritual, with calling and displaying from the tree. I noted several woodpeckers combing the cottonwoods for insects.  There were two pileateds, a flicker, and two smaller woodpeckers that I couldn’t identify without a spotting scope.  I saw many abandoned woodpecker holes in dead trees, where chickadees could gather in large groups to huddle against the nighttime cold.  They, in turn, might be joined by kinglets, making a mixed group, all shivering together to stay warm.

The broken tops of dead cottonwoods make good homes for owls, too. And squirrels would find a warm home there for the winter. Raccoons use numerous winter denning sites, including holes in standing trees or downed hollow logs. My dog found a vole nest in one of these. I am sorry to say that she ate the vole before I could intervene, but it did suggest that coyotes or foxes might do just as she did — tearing away at the log to get at a meal is part of the winter web of life.

Looking for smaller creatures, I pulled a few pieces of bark away from dead trees and found thousands of small, grayish moths, as well as some small, black flies or wasps. Both species were about ¼ inch long.  Perhaps their bodies had manufactured some sort of antifreeze, like glycerol, since they did move around some when exposed to the cold.  I put the bark back as best I could to protect them.

Finally, on hands and knees, I dug down into the dry, warmer ground protected by an old cottonwood log.  I found numerous little webbed cocoon-like homes. I carefully teased one apart and a small black spider began to crawl about slowly. I put him (or her) back without destroying its home. 

At the end of this exercise I am impressed by the intricate ways in which a cottonwood tree survives the winter. I have new respect. And after several days exploring I have a deeper understanding of how cottonwoods fit into the winter scene in general.

I also found a new natural history exploration technique:  observing one plant species, in one season, to learn its larger wildlife function. I’ll be feeding my own new curiosity by continuing to observe my local cottonwoods throughout the year. I’ll be using this technique with other plants or habitats, too, in the future.  Maybe you would like to try it, it’s easy and fun!

(Broadcast: "Fieldnotes," 3/15/16. You can hear the program on the radio Sundays at 12:55 p.m., Tuesdays at 4:54 p.m., and Fridays at 4:54 p.m., or listen via podcast.)