Trees of Life

by Andrew Kerkhoff, Associate Professor of Biology and Mathematics

An associate professor of biology and mathematics ponders how trees make the world, and give it a memory.

A tree is not just a thing; it is also an action, a memory. The arching and divergence of its branches tell you its history. Trees lengthen only at the tip of a branch, so as you move your eyes from the trunk outwards, you are actually tracing the movement of leaves slowly chasing sunlight through the years. As the branch extends, it also dilates, adding layers of vasculature along its length to pull long strands of water up from the roots and to gird itself against the torque of wind and gravity. The variety of forms taken on by trees, from the crenelated hemisphere of the meadow-grown oak to the dogwood's modest umbrella in the forest understory, from the tapering fletched shaft of a towering spruce to the bedraggled bristlecone pine hunched on a lonely windswept peak, each is a story detailing this dance of leaves as they are pulled and pushed by sunlight and shade, wind, water, and gravity.

And if you know how to read it, a tree tells more than just its own story. Walking through the forest on the Bishop's Backbone trail, you will come across an ancient white oak. Its formerly stout lower branches, now leafless and broken, are a record of the pasture that used to surround it. Where nineteenth century cattle used to lounge beneath the oak's broad crown, black cherries, hop hornbeams, and tulip trees have grown up, shading out its lower branches. Now the oak bears leaves only on its uppermost limbs, its lower trunk a knot of stumps, made redundant by the changes in the world around it.

Just as in humans, when many trees join their voices and their memories, their stories can become the stuff of epics. The width and chemical composition of tree-rings, those annual layers of growth cut in cross-section, can tell us about past climates, reaching back as far as 9,000 years. No single tree lives that long, but because wood is so durable, dead trees can still tell tales. Their rings can be cross-dated and overlapped to lengthen the record, like an old story handed down and embellished through the generations. The tree-ring epic, deciphered from tens of thousands of trees and timbers, even contains our own history, recording the collapse of the Anasazi culture and the slowing of Europe's economy during the Black Death.

The stories embodied in trees don't just record history; they have changed its course as well. The earliest known trees, related to ferns, arose in the late Devonian, about 385 million years ago, well before the first vertebrates managed to colonize the continents. Like all plants, trees grow by photosynthesis, using the energy of light to spin carbon dioxide and water into sugar and oxygen. The growth of these first forests decreased the concentration of carbon dioxide in the atmosphere, leading to global cooling, the flipside of our current climate crisis. In fact, it is the carbon captured by these early plants and their carboniferous-period descendants, long buried and compressed, that we are returning to the atmosphere when we burn coal.

With the foresting of the world, the continents became not just cooler and shadier, but also more architecturally complex, driving the evolution of animals adapted to diverse habitats. Hundreds of millions of years after those first trees, after the evolution of flowering plants and the extinction of the dinosaurs, a lineage of small mammals took to the trees. Becoming adapted to arboreal life, they evolved opposable thumbs and sensitive fingertips for grasping branches; their eyes shifted forward and their brains expanded to better discern distances and guide their movements through the canopy. Eventually, one lineage of those primates descended from the trees. In these apes, those same fingers, eyes, and brains would be used to fashion stone axes and steel plows, sitars and symphonies and science, constitutions and computers. They would even come to discover that they are but one tiny twig-tip on a vast tree of life, and that the world as they knew it, their own brains and bodies, was made by trees.

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