Fresh Ideas: The uncanny parallel between trees and man

By the time my parents began planting their favorite birch trees, the Colorado spruce, the weeping willow, all the pine trees, I already thought about trees as if they were humans: How many years would it take for a tree to grow up? How big would my willow be when I was 15? If we ever moved, how could we abandon the two oak trees the egg lady gave us as tiny seedlings when we moved into our house on Bower Street?

I had reconciled myself to the fact that trees could outlive humans, though when I was a child it didn't seem quite fair since I had gotten the impression that humans were more important than any other living thing. When Dutch elm disease struck our town (and the entire country), however, and we had to cut down the big shady elm in the front yard, I wept. Trees didn't lead any better lives than we did. It wasn't a competition.

Dutch elm disease seemed like an anomaly at the time, but since then all kinds of trees throughout the world have been ailing from one thing or another. We know the ferocious pine beetle out here in the West is also devouring boreal forests in Canada and Russia, and we know that the Bristlecone pine - a tree with a tremendous immune system - is showing signs of stress.

Warmer temperatures play a significant role. Forests in New Mexico in the 13th and 16th centuries, were decimated by severe droughts, but were able to eventually rebound because temperatures and precipitation returned back to "normal." But if we suffer from severe drought now, Dr. Nate McDowell, a plant physiologist at the Los Alamos National Laboratory tells us in an article in a recent New York Times by science writer Sandra Blakeslee, there is no expectation that forests will rebound as they did in the past. The warming of the climate is happening "faster than at any time in the geologic record" and it's having a systemic effect on trees.

What McDowell and other scientists have discovered is that "this warmer air acts like a gigantic sponge or wick." Even if we continue to get the same amount of snow or rain, "the atmosphere will inexorably pull that moisture away from the soil and trees."

In order to track exactly how trees die, McDowell is conducting "the biggest [experiment] of its kind in the world" by isolating 63 pinon and juniper trees to monitor how "they breathe, make food, take up or release water, fight off insects and cope with air that is warmer than usual." As if the trees were patients in an intensive care unit, each one is "hooked up to a variety of sensors, probes and lines that monitor vital signs."

The parallel between a tree's and a human's metabolism strikes me as uncanny. For instance, trees "drink" water which moves from the soil and roots up through the trunk in narrow little channels called xylem. When the air is warm, the tension in the xylem increases, the "rope" of water being pulled up the trunk breaks, air is introduced, and as Blakeslee, the writer of the article on McDowell, points out "like an embolism that can kill a person, air bubbles can block the flow of water" and a tree can dry out and die.

If there is a drought, trees can starve to death because many close off the pores in their leaves or needles to prevent water loss. This, Blakeslee reports, shuts down the photosynthesis, forcing the tree to "consume its carbohydrate stores." When the tree no longer has any reserves left, it dies.

Even a tree's sap, which functions to "fight pathogens, as well as predators like bark beetles" is reminiscent of how our own bodies fight off infections.

Blakeslee quotes McDowell's assessment of the experiment thus far: "We have insights into how trees die, but we are far from capturing just how big the problem is going to get. Only by understanding the cascade of steps that lead to tree mortality can we make accurate predictions into the future."

One of the predictions that scientists are already making however, is that by the end of this century, trees in the American Southwest may well be gone.

• Ursula Carlson, Ph.D., is professor emerita at Western Nevada College.


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