By Lawrence J. Winship Gazette Contributing Writer
Even in our modern, instant-messaging, planned-obsolescence culture, something about really old trees still captures our imagination. If that tree could talk, we wonder, what might it tell us?
When we imagine a tree of great age we usually picture a stately, spreading oak, sugar maple or sycamore—but if we read stumps and tree rings they reveal a different image. Old trees are often the smaller, more gnarled specimens, challenged by time and storms, yet still intact. Those of us who seek out old-growth forests learn to look for “broccoli tops”—trees with clusters of very thick upper branches, trees that have ceased to put on height growth and are now, like so many of us older humans, growing out instead!
Great age for trees is a matter of geometry and luck, and rapid growth may not always be the best approach for individual survival. The tallest trees face the stiffest wind. As trees get bigger, the proportion of light-gathering leaves to non-photosynthetic tissue—like roots and bark—gets smaller and smaller.
Eventually there is precious little net carbon left each year for making new xylem, the water-conducting tissue. A tree out in the open shades itself, so each branch vies with others for essential sun. In the same way, a tree in a forest can be overtopped and lose the battle to a taller neighbor. Some trees solve the puzzle by waiting out bad times, adding very little wood to their trunk and branches, until water and warmth and sun return. Or they may wait for bad fortune to claim a nearby competitor before investing much energy in growth.
In many ways, old trees do talk to us, telling stories about past climate, ecology and human activity. The broad crown of a huge old tree suggests an open field while a tall and narrow form with few side branches indicates a once-crowded neighborhood, perhaps in an even-aged stand growing in an old field. To create proxy climate records, dendroclimatologists read the patterns in the annual growth rings of cross-dated ancient trees and sub-fossil wood, marking years of drought and old age. Ecologists look for the rapid decline in the width of annual rings that may signal the peak year in a cyclical insect infestation, while a return to faster growth records a setback for the defoliator. Until we found a way to use the carbon preserved in tree rings to calibrate the carbon-14 dating method, our estimates of the age of prehistoric human habitations were often hundreds of years in error—trees set us straight. And chemical analysis of wood from individual tree rings can tell us about air pollution and the effects of acid rain. Though they’re silent witnesses, trees can be great history teachers.
For some trees, it seems that environmental stress may actually help prolong life—an effect called longevity through adversity. Bristlecone pines, high in the dry cold mountains of the western United States, often have but a thin strip of bark left behind by scouring winter wind and ice, a narrow lifeline that supports a single branch full of needles and cones. Unlike trees with full crowns, self-shading does not appear to limit their growth potential. Perhaps there is no upper bound to the life span of these strip-bark trees. Even in extreme age, bristlecone pines produce prolific pollen, cones and seeds, a continuing promise of a new generation.
I’ll never tire of hearing the delighted gasps from students who have just finished counting out the 300-plus rings of a hemlock from the slopes above the Cold River in Charlemont and along the Dunbar Brook watershed in Monroe, or the 950 years on a ponderosa pine section from the Oregon lava beds. How can a tree so small be so old?! Whence comes such awe, reverence and delight? We know that the living part of the tree, the cambium, is far younger than any of us—but a few years old! It is the dead wood at the core of a bristlecone pine that was once alive thousands of years ago. Perhaps we are gripped by the concrete presence of a living lineage extending back though abstract time.
I like to think we, as a species, instinctively respond to another organism that has persisted and prevailed, literally rooted to the spot, for so long. We can feel the hundreds of winters in our own bones, and marvel at an organism so well adapted, and so lucky, that it has seen the ages of humankind passing around it, and yet remains standing.
Lawrence J. Winship, a member of the Hitchcock Center board, is a professor of botany in the School of Natural Science and director of the Southwest Studies Program at Hampshire College.
Earth Matters, written by staff and associates of the Hitchcock Center for the Environment at 525 South Pleasant St., Amherst, appears every other week. For more information, call 413-256-6006, or write to us.Click here to return to full list of Earth Matters articles.