One Tree, One Year, One Book

kw: book reviews, nonfiction, natural history, trees, forests, phenology

A woman once wrote of a talk with her daughter, who was being affected by the licentious, free-love, promiscuous atmosphere of the late 1970's. The daughter had asked, "How can you be satisfied to be with just one person for a lifetime?" The woman reminded her daughter of her youthful experience living near the sea shore: "Every day that you could, you went to the little cove. You explored it over and over. You never tired of it. If we still lived there, don't you think you would still enjoy it, just that one little cove, so full of things to see, that changed a bit every day, but was always the same?"

These are wise words. They explain how a couple can, with a bit of imagination, remain fascinated with one another for 40 or 50 years or more (my parents were married 58 years; so far for me, 42 years). I thought of this story when I began to read Witness Tree: Seasons of Change with a Century-Old Oak by Lynda V. Mapes. She spent a year living in visitors' quarters at Harvard Forest near Petersham, Massachusetts, lending much of her attention to a single tree, a red oak about 100 years old and more than 80 feet tall.

The Harvard Forest is home to dozens, perhaps hundreds of experiments in forestry, botany, climatology and a number of other disciplines. Many have been going on for decades, though it is unlikely any have continued since the founding of the Forest in 1907. The map pin in this image is the approximate location of the Witness Tree.

One of the author's mentors has been walking the same route at least weekly, sometimes twice weekly, recording his observations of selected plants—trees and shrubs, mostly—and has compiled a record of more than 25 years of the phenology of those plants and that bit of the forest.

Phenology! I had to look it up (I'd seen the word before and could guess its meaning, but…):

Phenology is the study of periodic plant and animal life cycle events and how these are influenced by seasonal and interannual variations in climate, as well as habitat factors. (from Wikipedia)

A phenological record for a simple annual plant such as a sunflower might include when the seed first sprouted, the first true leaf emerged, the height and spread of the plant on various dates, when each flower appeared, when the seeds ripened, when goldfinches began to eat the seeds and when they were all finished off, the date of the first killing frost, and when the stem fell over.

Such a record for a perennial plant, particularly a shrub or tree, would include numerous events throughout the year, for year after year, not only of what the plant is doing but what significant weather and other environmental events occurred, and the plant's response to them. Disease or locust invasion or hailstorm? It all goes in the record.

The author makes clear throughout the book her interest in climate change and how this tree and those around it are responding, and have responded over the past half century or so. A core sample taken at the beginning of "her" year showed that for the past decade the tree has been doing very well, adding thicker rings than at any similar period in its past. In a sense, the changing climate has been good for this tree. Also, the tree is doing what it can to absorb carbon dioxide and make wood out of it, which mitigates the rapidity of climate change.

I want to deal with a quibble before going on: In Chapter 9 the author does her best to explain the greenhouse effect that carbon dioxide participates in, to warm the earth. A good biologist is not necessarily a good physicist, and the following statement needs amendment:

"As it enters our atmosphere, the radiant shortwave energy of the sun is transformed to long-wave radiation – heat. Molecules of carbon dioxide in the atmosphere absorb this heat and vibrate as they warm, creating even more heat." (emphasis mine)

It takes a moment to understand what she is saying here, because heat-induced warming of any gas does not create more heat. What is happening is that the molecules absorb radiation of medium wavelengths (near infrared), which induces vibration in the molecules so that they re-radiate longer wave energy, a broad spectrum of medium-to-far infrared. No "heat" is created. Infrared radiation is not specifically "heat" radiation, because all radiation heats up anything that absorbs it, in equal measure. A beam consisting of one watt of green, blue, ultraviolet, or whatever radiation, will cause just as much heating as a beam consisting of one watt of infrared radiation. So, more accurately, and specifically related to the greenhouse mechanism:

…the shortwave energy of the sun is absorbed by the earth's surface—dirt, plants, pavement, water—which warms them so that they emit long-wavelength infrared. Some gases in the atmosphere, primarily water vapor, carbon dioxide, and methane, absorb a lot of infrared, which warms them so their molecules vibrate and re-radiate infrared. Half of it is directed generally back down, and half generally outward into space. This redirection is a barrier to the infrared being radiated directly outward, so the earth's surface must get a little warmer and radiate more infrared, bringing about a balance between all the light that was originally absorbed and what is radiated back outward.

Nowhere does she mention that the primary greenhouse gas is water vapor. Though she does say that without the greenhouse effect the earth would be 33 degrees C cooler, and thus mostly frozen, nearly all of that warming to temperatures we consider "comfortable" is because of water vapor. The 280 ppm (0.028%) of carbon dioxide in the pre-industrial atmosphere added about 2°C. Now its level is about 400 ppm, and this has added another degree C.

About a century ago Svante Arrhenius determined, using calculations so simple that I have done them myself, that if atmospheric carbon dioxide were doubled from 300 ppm to 600 ppm, global average temperature would rise by about 4°C (7°F). However, the conclusion that doubling it again to 1,200 ppm (0.12%) would cause an 8°C rise is false. It is not a linear relationship. Better calculations show that carbon dioxide cannot drive warming beyond a level of about 5.5°C (10°F), even with several percent of the atmosphere being composed of carbon dioxide. At that point we would find our breathing affected! Also, The Arrhenius calculations don't take weather into account. When energy is added to the system, some of it goes into stronger winds and more frequent extreme weather events. These can reduce the extra warming by about half. This is a good-news-bad-news situation: global temperature rise is limited to about 3°C, but insurance companies are going to be paying out more claims related to floods, tornadoes and hurricanes.

Now, back to the wonderful tree in Harvard Forest. You can follow it day by day here. It is currently the last in a list of 13 "phenology cameras" (this image was captured an hour or two before I began writing this post).

Ms Mapes, with the help of several colleagues, measured the tree, studied the animals and plants that lived nearby, in, or on it, and climbed it a few times. Such a tree is no easy climb. The lowest branches begin about 40 feet above the ground. Just throwing a bean bag on a string over a branch to start hauling up a climbing rope is no easy feat. Once she learned to do that, her tree-climbing mentor pulled out a large slingshot that can rather accurately place a bean bag over a limb of choice!

Dozens of insects and other small animals depend on forest trees. A "trail camera" also showed her the various animals, from badgers and skunks to deer and coyotes, that passed by the tree, usually without paying attention to it. Trees are remarkable, having coping mechanisms of many kinds because they don't have the option of going indoors when it snows, or of packing up and moving elsewhere when threatened. They must just sit and take it. Two chapters on the way trees "talk", including the way they send signals to one another about new insect depredations, and how trees that receive such signals change their leaf chemistry to discourage the attackers, show that they are far from passive receivers of whatever nature dishes up.

We have, most of us, a certain affinity for trees. We like them in our yards: A house on a quarter-acre lot that hosts 20-30 trees sells for 20% more than one with a tree-free lawn. Eight of the ten most-visited National Parks are forested (see here). While I love the desert, even there I most enjoy areas with "large vegetation", such as at Joshua Tree in the Mojave or the Saguaro-studded areas around Tucson.

In pre-electronic days, a Witness Tree was a landmark used by surveyors, from which the survey of a neighborhood-sized area was conducted. Even in an age of GPS and ubiquitous cell phone towers (which are sometimes camouflaged as rather odd pine trees), it is no waste to give time to observe the changes of a single tree, or a small stand, as they respond to the seasons of the year, and the changes from year to year. When we slow down to not just "smell the roses" but to truly see what is going on, we are all natural-born phenologists.