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Speaking of Nature

Speaking of Nature: The Cottonwood

Sunday was a magnificent day and I made the most of it by parking myself on my porch. In the cool of the morning, I was able to sit back with a cup of coffee and listen to the tail end of the dawn chorus. At this point, however, some of the birds in my yard already have chicks, so the dawn chorus is a little less energetic that it was a couple weeks ago.

As the morning progressed into afternoon, I found my generally blissful experience occasionally interrupted by the appearance of an odd little irritant. The cottonwood tree (Populus deltoides) that has grown next to my porch has apparently hit maturity and had selected Sunday morning as a wonderful time to release her (more in a minute) first crop of offspring. Thousands upon thousands of seeds were sent forth into the world and they managed to get just about everywhere.

Cottonwood trees actually get their name from the cottony filaments that are attached to their seeds. On breezy days with low humidity, these filaments fluff out, which is more than sufficient to provide lift for their tiny cargo. They can actually add a rather magical, whimsical flavor to a spring day, but then there are those other times when they can become a little annoying.

More than once I had to choke back a groan when a sip of coffee included an extra special treat. The cottony filaments are great when they are dry, but when soaked with coffee they become unpleasantly clingy. It is only with much sputtering and “ptewing” that one is eventually able to eject such a seed and it only takes one or two such experiences to force one to carefully scrutinize the surface of one’s coffee before taking a sip.

The only thing more terrible than coffee-soaked cottonwood seeds are those that accidentally find themselves sucked up a nostril. No matter how quickly such a seed is dislodged, there is no remedy for the lingering tickling sensation in said nostril. Don’t ask me how I know this for I shall not speak of it willingly.

Now this is by no means the fault of the tree. The cottonwood, like most trees, is what is referred to as an “r-selected” species. This means that the reproductive strategy of the cottonwood depends on the sheer volume of offspring produced. Using a shotgun approach, the cottonwood releases enormous numbers of seeds and “hopes” that a few will end up in the right kind of place. As a result, the seeds are very small, which helps in their maximum production.

A member of the poplar family, the eastern cottonwood is a close relative of the aspens, and all of these species are among the great “pioneer species” of our area. This is a term that is reserved for plants that specialize in moving into areas that have experienced a disturbance of some kind. The soft parachutes of fuzzy “cotton” can carry the seeds great distances and deposit them in all manner of places. If the seeds find a patch of bare soil, they will put in roots and start to grow quickly. I wonder how many little cottonwood trees will set up shop in one of my flowerpots only to be weeded out a few weeks from now?

Cottonwoods are not particularly picky about where they land. The seeds will make a go of if it wherever they land and they can actually grow in soils of deep sands and clays. The only things they seem to require are bare soils and lots of sunlight. These sorts of conditions are prevalent along the banks of rivers, so cottonwood trees are often associated with water. However, humans produce so much disturbance that the trees have been able to move away from the rivers that they were originally so dependent on.

One of the really interesting things about cottonwood trees is the fact that individual trees are either all male or all female. Such plants are described as being “dioecious” (pronounced dye-EE-shus). Most trees are “monoecious,” which means that an individual tree can produce both male and female gametes. To be single-sexed is quite normal for mammals, but is unusual for plants.

Both males and females produce long, slender flowers known as catkins. The male catkins are much smaller and will quickly turn yellow as they release their pollen. Female catkins are larger and are actually rather subdued in their appearance as flowers go. Since cottonwoods rely on wind for pollination, the flowers don’t have to be attractive for any particular reason.

Once fertilized, flowers quickly change in appearance into green pods that kind of resemble individual soybeans (for anyone out there who enjoys edamame). Once the pods have ripened, and through some decision-making process that must measure the favorability of the weather on a particular day, the pods will pop open and disgorge their contents.

I found a collection of the fluffy seeds that had accumulated at the side of my driveway and as I took photographs of them, I found it interesting how closely they resembled a cluster of frog eggs in a pond. Frogs are also r-selected species, but they bunch their eggs together on purpose. The trees would actually rather have their seeds go in all directions for great distances, but the thin filaments can easily become tangled and produce great rolling tumbleweeds of seeds. The first day it rains it’s all over. The seeds will be stuck wherever they happen to be.

So how do you identify a cottonwood tree? Young trees are the easiest to identify because some characteristics of the twigs make them impossible to mistake for anything else. At the base of each leaf there are three raised ridges that run down the stem. This characteristic is present in trees of all ages, but as twigs thicken into branches and limbs, the raised ridges disappear. The only parts of the oldest trees to show the ridges would be the younger branches on the outer “surface” of the crown.

Another valuable field mark is the large heart-shaped leaf. Growing up to 6” in length, they are they have coarse, rounded teeth along their margins. Sometimes the central veins of the leaves are reddish in color, but leaves on the same tree may also have whitish veins. I don’t know why they are different.

What I do know, however, is that cottonwood leaves have their own little stems (known as petioles) and these stems are rather unusual because they are compressed laterally. Imagine one of those wooden coffee stirrers and you’ve got the right idea. The leaves spread out horizontally, but the stems are flattened in the opposite plane with the thin edges of the coffee stirrer pointing up and down. The aspens, which are close relatives of the cottonwood, do not have this characteristic.

With the rain that arrived on Monday, I suspect that the bulk of the cottonwood seeds have been knocked down for the year, but there may be another surge when the weather improves again. If this happens, I hope you can sit outside and enjoy the sight of so many baby trees floating off on great adventures.

Bill Danielson has worked as a naturalist for 16 years. In that time, he has been a national park ranger, a wildlife biologist and a field researcher. He currently works as a high school chemistry and biology teacher. His Speaking of Nature column runs weekly in The Recorder, except for the first Thursday of each month, which is when his Kids and Critters column for young readers appears. To contact Bill, or to learn more about his writing, visit
www.speakingofnature.com

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