Lessons Of Leaves

As I write this, spring is well under way in Washington, D.C., where the American Chemical Society headquarters is located. The daffodils, forsythia, and cherry blossoms are already faded; crab apple, redbud, and the showy kwanzan cherry trees are in full bloom.

In what is, to me, an annual miracle, the deciduous trees are unfurling their leaves this week. A month ago, the trees were still barren; two weeks ago, they were tinged with red as the leaf buds swelled; a week ago, there was a hint of green scattered through the woods; this week, forests are painted with broad brushstrokes of green, broader and deeper with each passing day.

I love trees, especially deciduous trees and their emblematic relationship to the changing of seasons. I spent 14 very happy years as C&EN’s West Coast bureau head based in the San Francisco Bay Area. One of the few things I didn’t like about Northern California was the absence of four clearly defined seasons.

My conception of leaves has evolved over the years. Through all my early years of wandering through forests, I never really thought much about why deciduous trees lose their leaves every year. It just happened. Then, many years ago, there was a very early snowfall in late October in the middle-Atlantic states, with 2 to 3 inches accumulating in the Washington area. Six to 8 inches of heavy, wet snow fell in the Shenandoah Mountains to the west of the city.

A week or two later, my wife and I planned to take a long day hike in the Shenandoahs on a trail we particularly liked. When we reached the trailhead, we faced a forest that had been devastated—thousands of trees were down across the mountain. We hiked about a mile, struggling through numerous downed trees still fully leafed, before giving up the hike. Leaves are a surface on which snow can collect. In places where it snows, leaves are an impossible burden for a hardwood tree to bear in winter. And I realized that deciduous trees lose their leaves because it snows.

As a homeowner whose yard has several large oak trees and which borders a forest, I’ve developed another perspective on leaves—they’re substantial. One dead brown leaf doesn’t represent much in terms of mass, but tens of thousands of them certainly do. Leaves are a massive commitment of resources on the part of the trees that produce them each year.

Which has led me to yet another perspective: The leaves unfurling this week are, in fact, a vast, biologically based solar array, exquisitely designed to capture a portion of the energy in sunlight and convert it into useful chemicals—glucose and other sugars, cellulose, lignin, and the vast array of organic compounds that make up a tree. The greening of temperate zones throughout the Northern Hemisphere as forests leaf out in spring is the unfurling of one part of nature’s great energy conversion machine.

Over the years as a reporter for C&EN, I had occasion to write quite a few stories on research on photosynthesis. It is a field that engenders fierce disputes. I remember long conversations with a variety of chemists about the structures of the photosynthetic reaction center and photosystems I and II, the positioning of this chlorophyll or that ubiquinone, how electrons move through the array of molecules to effect charge separation. This is the remarkable process carried out in leaves, the process that powers the world.

My wife rolls her eyes at me when I start in on this riff on photosynthesis and leaves and trees. Perhaps some of you are doing the same thing right now. I think of leaves as a metaphor not only of the changing of seasons, but of the connection between life and the sun.

A few weeks ago, I wrote two editorials on energy issues. In them, I argued that the greatest challenge facing humanity today is to learn how to live off the energy provided by the sun. On these spring days, we watch another noble life-form that has learned to do just that. Another way of looking at the energy challenge is to think of it as learning to do on an industrial scale what trees do so effortlessly.

Thanks for reading.