Around midsummer when the cottonwood trees start to produce clouds of snowy cotton that carry their seeds away on the wind, my dad talks about cutting down the venerable old trees in my childhood back yard. He hates how the cotton clumps into the gutters and sticks in the screen door. He would never actually fell the two giants that shade the yard, no matter how much we tease him about cutting down the apple trees and junipers years ago. But sometimes I do wonder what they look like on the inside. One is full of carpenter ants; we watch them come and go bringing sawdust up and out to the surface in little clumps, always busy. The other has a perpetually bleeding wound where a threatening limb had to be removed when I was young. I wonder how old they are; how many winters and summers they’ve seen; how deep their roots go. 

As it happens, my mystical musings about the life of trees are not unlike the questions that Valerie Trouet, a dendrochronologist, has made a career of understanding. In her new book, Tree Story: The History of the World Written in Rings, Trouet explains how the stories of trees are deeply, inextricably, intertwined with stories both personal and global. Part memoir, part field diary, part lucid and engaging science communication, Tree Story moves from the finest micron-level of individual tree rings to the deep geological time of the planet and the world-wrapping forces of its climate. Trouet’s account of her career, far from what we might imagine to be the dull work of counting tree rings, reads like an exciting adventure story, complete with far-flung locales, the ridiculous macho pretensions of male coworkers, and the ever present thrum of the ancient magic that seems to emanate from the trees themselves.

Trouet began her scientific career as an environmental engineer in Ghent, Belgium. Faced with a dwindling number of thesis projects to choose from she took on a tree ring study in Tanzania at the suggestion of a professor. From a somewhat serendipitous beginning, Trouet built a career traveling the world to understand the secrets hidden inside trees. Trouet chronicles her experiences as a woman scientist working in a field science, where macho ideas about hard work and extreme conditions sometimes took over for practicality. In one episode in the Pyrenees, while on a core-sampling mission with a group of men, it was Trouet who finally insisted they break for lunch (after a three-hour hike up the side of a mountain and a marathon coring session) because none of the others wanted to admit they were hungry. Trouet later learned that the men slept with the windows open at night because they refused to concede that the near-freezing temperatures at their mountain field site were uncomfortable. Trouet’s bafflement at these moments is hilariously palpable. 

Focusing on the traces that past climate change leave within the rings of trees, Trouet and her colleagues explore droughts, forest fires, and insect outbreaks in long-live forests but also ancient weapons and structures made from wood, such as musical instruments and pieces of furniture. The best thing about Tree Story is the feeling of vast interconnection Trouet creates in describing the far-reaching implications of dendrochronology. From astronomy to archaeology to climate science and history, there are trees and forests and orchards and beams and spears and violins everywhere that dendrochronologists can read and draw conclusions from.

Trouet moves through these vignettes in thematic chapters, bouncing lightly from personal stories from the field to historical narrative and back to lucid explanations of the technical aspects of her work. But her focus throughout, and the subject to which all others always return, is climate change. Tree rings create some of the most reliable and largest data sets about changes in global climate, something Trouet and her colleagues showed in their study of the warm medieval period (which was followed by the Little Ice Age). Their discovery of the mechanism behind this climate anomaly was made possible because their data went far enough back in time, into the long lives of trees (and stalagmites), to show a pattern that developed over millennia. Trouet closes with an urgent exhortation for more and better climate science that uses the powerful tools of dendrochronology, to better understand past changes in climate, and to devise new ways to mitigate the unprecedented anthropogenic climate change we now face.

As a historian, there is something very satisfying in the precision of the stories tree rings can tell. A tree produces one ring per year, marked by the bounties and deprivations it faced in a code legible to the dendrochronologist. It feels like a well-organized archive box, with a folder for each year, filled with detail. But the more arcane aspects of this science appeal to me as well. To peer back in time, and not just back into human history but into the archive of the planet itself, seems to me a special privilege. And, as Trouet knows, this knowledge of deep time and the history of nature comes with enormous responsibilities. 

Image Credit: Cottonwood Tree by 77krc on Flickr | CC BY-NC-ND 2.0