Guest Writer Mary Hrovat Reviews Tree Story by Valerie Trouet

The historical sciences—geology, for example, and evolutionary biology—are the most interesting to me. I enjoy learning about how people piece together chains of evidence to get a better view of the past using information based on what we see now. Tree Story: The History of the World Written in Rings, by Valerie Trouet (Johns Hopkins University Press, 2020) brings together tremendous amounts of information about another historical science, dendrochronology. It gives a good sense of both the details of how the science is done and the broad sweep of its results.

The book begins with some background about tree rings and tree-ring research. I’m embarrassed to admit that I had never thought about why trees have rings. Tree Story answers this question (it has to do with different kinds of wood growth early and late in the season) and describes the variation in ring formation among tree species, which is much greater than I had realized. Not all trees form clear rings, and the sizes and patterns of the cells of different tree species can be so distinctive that a specialist can identify a tree from only its wood. In addition, particular years in a specific location can exhibit distinctive patterns called tree-ring signatures, which become familiar to people who look at many tree samples. Later in the book we learn finer details, for example, how tree growth is affected by fire, frost during the growing season, strong seismic events such as severe earthquakes, and other hazards. Each tree carries its story in its rings.

I have a background in astronomy (another historical science). The process of identifying features in the rings in addition to counting them to determine ages reminded me of the way you can use the dark lines in the spectra of stars not only to tell which elements are present but also to determine things like temperature and how fast a star is rotating using, for example, the intensity and shape of the lines. There’s a wealth of information in both spectra and tree cores.

As it happens, an astronomer, Andrew Ellicott Douglass, is central to the history of dendrochronology. Douglass came to the Arizona Territory in 1894 in search of an observatory site for Percival Lowell. He selected a site near Flagstaff, supervised the construction of Lowell Observatory, and worked there until 1901.

While he was in Flagstaff, he became curious about a possible relationship between sunspot cycles and climate. He was the first to propose that tree-ring data can provide historical information about climate. He began working on the assumption that wider rings indicated years with more tree growth. Because he was looking at Ponderosa pines in the American Southwest, where water is generally the most important factor in a tree’s annual growth, he guessed (correctly) that wider rings indicate wetter years. He was fortunate to be working with the Ponderosa pine, because it’s an abundant, long-lived tree that forms clear rings and is sensitive to precipitation.

Ultimately his studies of tree rings resulted in a collaboration with archaeologists of the American Southwest, who were using a relative chronology to date Ancestral Puebloan ruins in the Four Corners area. They could place each site on a timeline with respect to the others, but they didn’t have calendar dates for any of them. By examining timbers from the ruins, Douglass was able to link their floating chronology to his chronology based on the rings of living trees, and thus to provide absolute dates for the archaeological sites.

Douglass made significant contributions to both dendrochronology and archaeology, in addition to pursuing his astronomical research. In 1906 he began teaching astronomy and physics at the University of Arizona. In 1937, he established the Laboratory of Tree-Ring Research, which was the first lab in the world dedicated to this kind of work. Until 2013, the LTRR was located under the west end of the university’s football stadium. Interestingly, a laboratory for research on methods of making telescope mirrors was built under the east end of the stadium in 1985. It’s now known as the Richard F. Caris Mirror Laboratory, but it was originally called the Steward Observatory Mirror Laboratory. (Note that Douglass also founded the Steward Observatory, in 1916.)

After this introduction to the field, the book follows Trouet’s career, beginning with the challenges of her first coring expedition, to Tanzania. Readers learn about coring equipment and how samples are handled and analyzed. What I enjoyed the most, though, were the more personal details about field work, things involving food and gear and silly jokes. I really appreciated the vicarious experience of travel and working in different environments. I also appreciated the insights into how past climate trends and events emerge slowly as data are analyzed. Here’s where the puzzle-solving aspects of the work come into play, and it was intensely interesting to follow various research projects from their uncertain beginnings to a published paper. I got the feeling that Trouet loves her work and wants to share it. Her enthusiasm makes for an enjoyable reading experience.

Each chapter focuses on a particular project or problem. Certain themes tie the chapters together—anthropogenic climate change, for example, and the ways that climate has affected history. I really had no idea how much of history, including uprisings, the collapse of states, and possibly even piracy in the Caribbean, is linked to climate. It was fascinating to learn about how tree rings and other climate proxies have revealed these stories.

These other climate proxies include ice cores, which have a longer timeline than tree-ring data. The original hockey-stick curve that demonstrates global warming was based on both tree-ring and ice-core data. Other proxies include sediments in oceans and lakes and growth bands in the hard parts of various marine organisms—or, as Trouet puts it in a lovely phrase, “corals, clams, and the ear bones of fish.” (The study of these growth bands is called sclerochronology.)

I was surprised to learn that under certain circumstances, stalagmites also form growth rings. One of Trouet’s research projects began as an attempt to see whether a drought hockey-stick curve, analogous to the temperature curve, could be obtained for the Northern Hemisphere. It turned out to be too difficult, because rainfall exhibits much more local variation than temperature. However, work on a drought curve for Europe led her to consider the well-known North Atlantic Oscillation, a large-scale atmospheric phenomenon that affects Europe’s climate. Her research, which was based on a comparison of tree-ring data from Atlas cedar trees in Morocco and growth layers from a stalagmite in Scotland, ultimately provided new information about two atmospheric phenomena that influence the NAO. A stalagmite! I was bowled over by this story.

In addition to revealing new connections between human history and climate, some of these findings cast new light on previously known facts. For example, time has shown that the Colorado River Compact of 1922, which allocates the waters of the Colorado River to seven western states and Mexico, is overly optimistic about how much water is available, in part because it was written during a relatively wet period in the area. In fact, tree-ring data have revealed that it was written during the wettest period in 500 years. Similarly, the 16th century Lost Colony of Roanoke and an early 17th century settlement at Jamestown had, in retrospect, spectacularly bad timing, coming as they did during the greatest drought in 800 years. I wonder how the people who were already living there fared during this time period.

Tree Story is full of examples of how human societies have been disrupted by natural climate variation, although climate is often only one of several factors in societal collapse. The main thing I took from these stories is that living with natural fluctuations in climate would be sufficiently challenging, even if we didn’t also face human-induced global warming. As I read, I thought about the inflexibility of our infrastructure, not to mention our political and economic systems, and I wondered if we can learn to be more adaptable in our response to climate change. The final chapter of the book presents an overview of human activities that have affected trees, mainly deforestation and the use of fossil fuels, with a focus on how tree-ring research can contribute to mitigation efforts.

The sheer scope of the book is exhilarating, if sometimes a bit overwhelming. The book offers various tools to help readers keep things straight. One of the first things I noticed were the end papers, which orient readers in space and time. The map at the front of the book shows the locations of various trees mentioned in the text (with a key to the chapters in which they’re discussed), and the timeline at the back shows historical events and the relevant tree-ring chronologies. The book also provides a list of all the tree species mentioned (again, keyed to the chapters in which they appear), a glossary, and a list of recommended reading, as well as notes about the sources and an index. The illustrations and black-and-white photographs throughout the book are also helpful.

I found Tree Story to be informative and inviting. It was a pleasure to read, and my mental timelines and maps are richer for the experience.