In the Circle of Ancient Trees Our Oldest Trees and the Stories They Tell

Six years ago, Belgian dendrochronologist Valerie Trouet blew me away with Tree Story. For her latest book, she has taken on the role of editor to let her colleagues tell you first-hand of their research. In a nicely balanced collection of essays that features long-lived trees from around the globe, ten senior dendrochronologists provide ten different and sometimes personal answers to the question: "And what else can you learn from tree rings?"

Trouet limits her visible contribution to the introduction, where she lays down some of the basic concepts so the others do not have to. What are tree rings? How do you study them without cutting down a tree? And how do you combine tree-ring records from different trees to create chronologies that can stretch back millennia, allowing you to date trees and the wooden objects we construct from them? After this, she steps back to let her colleagues speak. Several aspects, though, betray the invisible hand of a careful editor: there is very little repetition of basic concepts, the essays are of near-uniform length at 18–22 pages, and she features trees from all continents (minus Antarctica).

The essays kick off with the Giant Sequoia, and there are two good reasons to start here. First, many people will be familiar with its tree rings as several natural history museums have large slabs on display that highlight all the key historical events that happened during its lifetime. Second, it was the species that the founder of modern dendrochronology, Andrew Ellicott Douglass, first studied. His work was fundamental in formulating the guiding principles of the discipline, and in 1937, he established the world's first Laboratory of Tree-Ring Research in Tucson, Arizona, where Trouet works.

Following on from this, each essay offers a mix of several elements. There is some natural history, describing the trees' appearance, often exceptional size, and extreme longevity measured in the hundreds to thousands of years. There is discussion of their evolutionary history and biogeography, and how that changes with time. There is research history with authors tying in some personal history recounting how they got into this field. A recurrent theme, given that all these long-lived trees make hard wood that is often decay-resistant, is the long history of deforestation. Though it is true that early humans already impacted forests, exploitation really ramped up with European colonialism. Many of the species featured here have been left teetering on the brink of extinction.

The last element in each essay is the exciting research, and it is this that I want to highlight here. We all learn from a young age that you can determine a tree's age by counting its rings. However, this book is a showcase of possible answers to the question: "But what else can you learn from tree rings?" As it turns out, an awful lot, and some of it quite unexpected!

One of the foremost applications is the use of tree rings as a proxy in climate reconstructions. Our instrumental weather records extend back no more than a few centuries, and in most places far less. Scientists can, however, turn to proxies: indirect evidence of past climate that has accumulated in our environment. Given that the width of a single ring is influenced by the weather during the growing season, a tree is effectively a living weather recorder over its entire lifetime. For one, this gives you a record of extreme weather events such as drought or cold snaps. However, once you start comparing tree-ring patterns between multiple trees and between different regions, larger patterns emerge, e.g. showing when regional climates were warmer and wetter or cooler and drier. Independent lines of evidence from other proxies, historical documents, or archaeological evidence can back up such findings. And this does not just rely on counting or characterising rings, but also zooming in and examining cell wall density or measuring stable isotope ratios—the level of technical sophistication has become high.

Another obvious application of dendrochronology is the dating of archaeological objects. By examining tree-ring patterns visible in a wooden building or artefact and comparing these with properly dated reference chronologies, you can determine its age. Long-lived tree species yield chronologies going back centuries to millennia, but even that is often not enough. Fortunately, the tree rings of dead wood are just as informative. Several tree species grow in environments where dead wood barely decays because it is either too dry or completely waterlogged. The study of such subfossil wood is extending chronologies back in time by millennia, benefiting archaeologists.

The last example I want to highlight is dendroprovenance studies. In other words, where did the wood for the construction of buildings or artefacts come from? Not only is the question of origins itself interesting, but you can also study how this changes over time. Pedunculate Oak was a popular wood of choice in artworks in the Low Countries from the 15th to the 18th century. In response to trade wars in the region, there was a clear shift in where the wood was sourced from.

In the Circle of Ancient Trees is livened up by woodcut illustrations of British landscape artist Blaze Cyan. Each essay opens with a portrait of a tree, while tree-ring engravings are scattered throughout the book. A note from the publisher clarifies that these do not illustrate the species in each essay and are purely decorative. You will indeed soon notice that there are about five or six different ones that have been repeated and mirrored or rotated to mix it up a bit.

To conclude, how does this book compare to Trouet's earlier Tree Story? To be honest, there is a fair amount of overlap in the types of research and applications discussed, though here you get ten first-hand accounts. The illustrations are actually helpful in characterising the difference between the two books. In my review of Tree Story, I praised the infographics of Oliver Uberti that really helped you visualise what scientists are doing. How, for instance, do you compare tree ring patterns to create chronologies, or how do you resolve floating chronologies? Tree Story is the more academic of the two books that goes deeper into the technical nitty-gritty, though I reiterate it is still very accessible. In the Circle of Ancient Trees, in contrast, only has decorative illustrations, but no photos or infographics (calling the illustrated timeline in each chapter an infographic is a bit of a stretch). The personal stories and the numerous subheadings that divide each essay into bite-sized sections make this book accessible to a far wider audience, and make it a suitable book to gift to anyone interested in trees.