Wild Side : Circles of time
I like trees as much as anyone. But in a way, trees are most interesting when they're gone: I simply can't pass a tree stump without looking at its rings.
That the rings of a tree tell its age is basic elementary school biology. Each year, rapid growth in the spring and summer produces large cells and less dense wood; as the growing season winds down, smaller cells produce darker wood, and the diameter of each of these dark rings is the diameter of the tree (technically, its diameter just inside its bark, where growth occurs) at the end of each growing season. So on a freshly cut tree stump, you can count inward from the present, finding the rings that correspond to any year during the life of the tree.
Since the width of each annual ring reflects how much the tree grew during that year, a stump reveals the ebb and flow of the tree's fortunes. The study of this record has been formalized into a scientific discipline, dendochronology, which has in turn spawned sub-disciplines like dendroecology and dendroclimatology. Studying large numbers of stumps (or thin cores taken from living trees), sometimes using distinctive patterns of rings to establish dates in lumber cut long ago, scientists can assemble regional records of temperature, rainfall, and vegetation going back many centuries, even in the complete absence of human records.
I'm content to leave such studies to the experts. And I confess that I rarely have the patience to count back through all the rings of a large tree (with the Island's current abundance of large, dead, oaks, it's easy to find stumps of trees well over a century old). For me, the appeal of tree rings is the snapshots they provide of each tree as an individual. Trees generally may reflect regional patterns of drought; but every stump also reveals scars, asymmetries, and patterns of ring width adding up to a unique history.
The circles on a stump aren't just statistics or records; they're a tangible result of things that happened decades or even centuries ago. Chemicals were taken up by the tree, synthesized into the raw materials of tree cells, locked into structures that were hidden for years and revealed only when the tree was cut. When you touch the wood near the center of a tree, you're touching the past.
Part of the appeal of tree rings is the fact that they only offer partial answers, hinting at rather than spelling out events in the remote past. Sophisticated chemical analyses of the wood may be able to reveal specific causes, but basically, the width of a ring tells just one thing: was the tree happy that year, or not?
But thoughtful examination of the stump and its surroundings may provide explanations. For instance, many stumps on the Vineyard (and indeed elsewhere) show fairly rapid growth during the first few decades of life, followed by a slowing pace of growth. Partly this may reflect structural limitations that trees, like all other living things, must face: the bigger you get, the more challenging it is to keep all of your cells adequately nourished. But also, much of the Vineyard was nearly treeless 150 years ago, cleared to create pasture for sheep. With nothing tall around it, an oak sprouting in an abandoned pasture would have had years of unimpeded growth before its root system came in contact with a competitor's roots, and before overlapping branches in a closing canopy limited its ability to soak up sunlight.
Sometimes, rings unexpectedly widen as a tree ages. If the change is temporary, involving just a few rings, it probably reflects a period of good growing conditions - a felicitous mix of rain, sunlight, and temperatures. But if the wider rings persist, something permanent must have happened. Logging may have removed competitors, or perhaps some change in drainage provided a bounty of water.
Other stumps show sudden decreases in ring width, an abrupt change easily distinguished from the gradual squeeze of a maturing woodland. If it's temporary, the explanation might be a drought or an outbreak of leaf-eating caterpillars. But if the change persists, something must have constrained the tree's ability to grow. Perhaps some excavation damaged the root system; perhaps a storm broke the trunk. If you're lucky and count back carefully, you might be able to assign a particular cause: maybe damage from Carol or Edna, the twin hurricanes of 1954, or Bob in 1991.
Living trees provide grace, color, shelter, and habitat - important gifts. When cut, a dead or ailing tree provides another service by linking us to the past. A tree's rings are a book assembled from the air and water of vanished decades, bringing back to life long forgotten weather, events, and landscapes.
