Saturday, November 20, 2021

Live Fast and Die Young: Another Message from Fluff the Cottonwood Tree

This is Fluff the cottonwood tree (she/her/hers). I think this is my fifth message that Stan has agreed to post for me on his blog. I live about a half block away from his house in Oklahoma.

I know Stan is the one with the botany Ph.D., but I know a thing or two myself. And today I am going to tell you about the evolution of life cycles.

Cottonwoods like me are the James Deans of the tree world. We live fast and die young. In contrast, many of the oak trees that grow here in Oklahoma, like the post oaks up on Turkey Mountain, live for several centuries. They grow slowly and invest in the future: they have strong wood, for example. They produce large seeds (acorns). They grow in stable forests where long life and big seeds are important in winning the game of competition. But us cottonwoods live along rivers, creeks, and lakes. I live near Joe Creek, a quintessentially Okie name, although it is a rip-rap lined drainage ditch. We cottonwoods do not have a future to invest in.

The margins of rivers, creeks, and lakes are very unstable habitats. Every few years, a major flood will come along and sweep many of us away into the oblivion of death. I am a little luckier than most; I live far up on the bank. But the flood waters almost got me—and Stan’s house—in 2019. Still, the cumulative risk of death from flooding is very great, and it is a rare cottonwood that is able to enjoy his or her hundredth birthday. Why build strong wood, when we do not live very long? And why invest in big seeds to compete against trees, most of which will get swept away in a flood anyway? We live fast and die young. We reach our full height after maybe fifty years. After that, our trunks keep growing but our roots are not very deep. We are just asking to get knocked over in a flood. But even if we do not, we fall over as our weak wood rots away. From where I stand, I can see a house with a new roof. The old one was damaged by one of my fellow cottonwoods falling on it during an unremarkable storm. If you want to plant a tree near your house, you should probably choose an oak instead of a cottonwood.

We reproduce like crazy as soon as we are about fifteen years old, which is young for a tree. We need to get as much reproduction finished as possible, in case we die even sooner than we expect. Stan took this photo of some of my seeds in 2019.

All these characteristics fit together. In order to grow fast, we have large xylem vessels in our wood, which conduct water rapidly to the leaves at the top of the tree. The very fact that we have lots of big, hollow vessels in our wood is what makes the wood weak. Oak trees have stronger wood, but the price they pay for it is slower growth.

I do not recommend the James Dean strategy in general, but my species lives in a habitat in which trees get destroyed quite often. For us, the James Dean approach works very well. The same is true for all species. If a disaster happens frequently, it is better to grow up fast and reproduce early. One such disaster is predators. Research has shown that fishes that live where there are predators will grow fast and reproduce young. But in the absence of predators, they grow larger and more slowly, reproducing later. The same pattern has been observed in opossums. It sounds like a law of nature, a spectrum of life cycles, and I am at the James Dean end of it.

Despite this, trees almost always live longer than animals, even most tortoises. I will probably outlive Stan.

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