Running rings around world history

Tree story: The history of the world written in rings, by Valerie Trouet 2020. Johns Hopkins University Press

What links the following: That (in)famous ‘hockey stick graph’ which showed that Earth’s temperature, relatively stable for 500 years, had spiked upward during the 20th century thereby providing strong evidence of anthropogenic global warming from burning of fossil fuels; the volcanic eruption that contributed to the decline of the Minoans, a major Bronze Age Mediterranean civilisation; and the chlorosulphonic acid cloud used to hide the Second World War German warship Tirpitz in its Norwegian fjord anchorage? Tree rings! Either wholly, or in part, dendrochronology – “the technique of dating events, environmental change, and archaeological artifacts by using the characteristic patterns of annual growth rings in timber and tree trunks” – contributed to the data behind the scientific papers of the hockey stick graph (Michael Mann et al., Nature 392: 779–787, 1998; *, the eruption of Thera (Charlotte L. Pearson et al., Science Advances 15 Aug 2018: Vol. 4, no. 8, eaar8241; doi: 10.1126/sciadv.aar8241) **, and the science of warfare dendrochronology (Claudia Hartl et al., Anthropocene 27 (2019) 100212; **.

If you’re now intrigued by the connection between tree-rings and those historical phenomena mentioned above, you should read Tree story by Valerie Trouet. If you’re not – yet! – intrigued by those events, you should still read Trouet’s marvellous book; it’s a gripping read, and provides many examples of how tree-ring data can provide much-needed context – and evidence – for historical events. As the book’s sub-title states, this tome really does show “the history of the world written in rings”. But, Trouet is careful to point out that the connections between tree-rings and environmental or historical events are not always straightforward: Dendrochronology is very much a forensic science that requires careful analysis and consideration – and sometimes enlightened leaps of imagination – to solve puzzles. And that important element of detective work that combines science, history, the environment, politics, etc. is what makes Tree Story so compelling – and a great addition to the literature on plants-and-people.

Trouet states that her intention is: “to write stories of excitement of scientific discovery, about our long and complex human history, and how it has been intertwined with our natural environment and ingrained in the story of trees”, and to “write the tree-ring stories that I find fascinating”. Tree story delivers on both of those goals admirably, and the stories told are as fascinating to this reader as they clearly were to the author. And the fact that Trouet’s own research contributed to many of those stories in the book’s 16 chapters is a great bonus; it allows her to write from personal experience about the process of scientific discovery, the intellectual thrill of making new connections between seemingly unconnected phenomena, and to report on the sheer physicality involved in securing the tree-ring samples in the first place.

Not only that, but Tree Story is great story-telling: Trouet has an engaging style that draws you in and you almost feel part of the action that’s being described. Tree Story is well-written, often with great phrasing and much humour, and gives a welcome insight into the people and personalities behind the science. In places, in terms of style, Tree Story is reminiscent of David Beerling’s Emerald Planet and Making Eden – which is great company to be in ***. Tree Story is another example of great science communication, often dealing with quite technical matters, but in an accessible way for both the non-specialist scientist and the intelligent lay person. For an insight into Valerie’s narrative style – which may help you decide whether to buy the book, here’s a freely-available read she penned explaining how tree-rings and shipwrecks can help us predict where hurricanes will hit in the future, and an extract from the book here.

But it’s not just about great story-telling, Tree Story has much of pedagogic value as well, and would make a great text for a plants-and-people course. As is becoming a feature of more populist science books these days, there are no in-text references, but there is a Bibliography. Extensive and listed at the end of the book by chapter, the Bibliography is mainly comprised of scientific articles, at least 35 of which are dated post-2014 (and features 15 of Trouet’s own tree-ring papers – she is a very productive researcher!). There is an additional Recommended Reading list, which contains a high % of post-2014-dated books, a Playlist (you’ll have to read the book to see where this features!), and a Glossary (featuring such great entries as: cat face, dzud, imposter syndrome, otolith, ridiculously resilient ridge, and Smokey Bear effect). Occasional within-text footnotes provide extra information. Tree Story has lots of images, but I found the choice of illustrations often a little dry. I would have welcomed some – additional – images of the people mentioned, maybe in full tree-ring fieldwork, chainsaw-wielding action (because you can have too many diagrams of chronologies…); the personalities don’t detract from the stories, they add to it in showing the human face of science.

You should by now have concluded – correctly! – that I’m a fan of Tree Story and Valerie Trouet’s work. However, no book appraisal would be sufficiently thorough if it was completely uncritical. I therefore comment on a couple of matters that need to be challenged. And, lest these ‘negatives’ mar one’s appreciation of an otherwise great book, let’s get them out of the way here, before resuming our consideration of the book’s – much-more numerous! – positive aspects.

First, Trouet writes: “Of the entire trunk of the tree, this wafer-thin cambium layer just inside the bark is the only part that is actually alive. Everything else – the wood and the bark – is dead material…” Although in other places Trouet gives good – and accurate – anatomical background to tree physiology, wood formation, and and tree-ring development that is important and necessary to appreciate the dendrochronology considered in Tree Story, the quoted statement on p. 26 is incorrect. Yes, cambium is a living tissue, but it’s not the only living part of the tree trunk. There is plenty of living material also in the phloem – to the outside of the cambial ring containing such structures as companion cells and their accompanying sieve tubes that transport sugars, etc. within the inner bark. Additionally, there are long-lived parenchyma cells – ray cells in particular – that extend from the cambium towards the bark, and inwards into the wood (Nigel Chaffey & Peter Barlow, Planta 213: 811–823, 2001;; Martyna Kotowska et al. (2020), Frontiers in Plant Science 11:86; doi: 10.3389/fpls.2020.00086). These living cells are essentially to the proper growth and development of the tree; without them very little wood would be produced and there would probably be few tree-rings for the dendrochronologists to work with.

Second, Trouet writes about the Industrial Revolution in England and its use of coal as a fuel source, but refers explicitly to it as mineral [my emphasis] coal (p. 204) ****. She then goes on to say that this fossil fuel was supplanted by two other fossil fuels – oil and gas – which, because they “originate in organic material – plants and plankton – they contain a lot of carbon”. Although coal is considered to be a fossil fuel, one is left wondering what Trouet considers to be the origin and nature of coal – the carbonrich product of the partial decomposition of prehistoric plant material. This appears to be an uncharacteristic lapse in the otherwise textually-precise writing that are hallmarks of Tree Story since it can be interpreted as meaning that coal is not a carbon source and therefore not a contributor to excess atmospheric amounts of CO2 – unlike, the explicitly organic material-derived, gas and oil. Whilst Trouet seems to correct this view a few sentences further on – on p. 204 – when stating that “some dead plant (and plankton) material also gets incorporated into the deeper layers of the earth as coal and natural gas”, this seeming contradiction might just add to the reader’s confusion: Is coal organic – carbon-containing – or not? Does this really matter that much? Since the book is primarily about dendrochronology its main message is not unduly affected by this potential misinformation. However, in Tree Story Trouet writes quite a lot – and often passionately – about global climate change and atmospheric addition of anthropogenic carbon dioxide. Whilst usage of coal may have decreased in England, it continues to be a major fuel and energy source globally – accounting for a 38.5% share of the global power mix in 2018 – and therefore an important contributor of CO2 to the atmosphere. Getting this information about coal correct – or at least clearing up any potential confusion or ambiguity in the text – is therefore important. Furthermore, in a book that is otherwise very good on science communication, communicating correct science is important. Two things to consider for a reprinted version with corrections? Back to the appraisal now…

As a scientific discipline, dendrochronology emerged about 100 years ago – in the American Southwest largely due to the efforts of astronomer Andrew Douglass who decided to study Earth-bound tree rings to help him understand a question about the Sun. Ever since his ground-breaking work dating pueblo ruins in Chaco Canyon (New Mexico, USA), which “pushed back the horizons of history in the United States for nearly eight centuries before Columbus reached the shores of the new World” (Andrew Douglass, National Geographic Magazine 56: 736-770, 1929), the range of phenomena and global events whose understanding has been advanced by dendrochronology has expanded greatly.

Accordingly, Tree Story includes dendrochronological dimensions relating to the Chernobyl nuclear incident, meteoroid-damage in Tunguska (in Siberia) (Timothy Mousseau et al., Trees 27: 1443–1453, 2013;, tropical cyclone activity and wrecks of Spain’s Silver Fleet in the Caribbean (Valerie Trouet et al., PNAS 113(12): 3169-3174, 2016;, and volcano-induced Nile flood-failure in Ptolemaic Egypt (Joseph Manning et al., Nat Commun 8, 900 (2017).

Tree Story features the in-depth tale about the discovery of driving mechanisms behind the Mediaeval Climate Anomaly in Europe – a warm period from c. 900 – 1250 CE that preceded the Little Ice Age (c. 1500 – 1850 CE) – that brings together data from a Scottish stalagmite with tree-rings of Moroccan cedars and a link to the North Atlantic Oscillation [NAO]. This is a particularly satisfying story of scientific sleuthing because it’s one that Trouet was intimately involved in solving and that helps to make it a very engaging and involving read (Valerie Trouet et al., Science 324: 78–80, 2009; doi: 10.1126/science.1166349).

And there is a whole chapter devoted to the “Fall of Rome”, an event for which “no socioecological web is as labyrinthine” – as Trouet so convincingly demonstrates. That section stands out as being a great example of how tree-ring data can contribute evidence, that’s not available from other sources, to interpreting a major event in world history, the collapse of the Western Roman Empire (Ulf Büntgen et al., Science 331(6017): 578-582, 2011; doi: 10.1126/science.1197175).

Many of these insights and connections are possible because, in Trouet’s words, “like people, many trees do enjoy talking about the weather”. And that, as they say, has the ring of truth about it…


Reading Tree Story by Valerie Trouet will teach you a lot about the science of dendrochronology and its impressive analytical power. Although you won’t be an expert in the technique by the end of the book, Trouet’s writing will probably make you want to be!

* Trouet devotes an entire chapter to the hockey stick graph – and considers at some length the politics surrounding this discovery, the debate about climate change, and the reputational consequences for the Nature paper’s authors. This rather sorry saga can be summed up in Trouet’s words: “Scientific facts are not decided by a majority vote“.

** Sadly, the Minoan demise is not featured in Tree Story, neither is the Tirpitz tale (but that’s probably because the latter wasn’t published until September 2019). I think this goes to show that there are more fascinating tree-ring detective stories out there to discover for yourself. Or, put another way, there’s probably plenty of material for a follow-up book, Tree Story 2..?

*** Tree Story is also reminiscent of John Perlin’s Forest Journey in terms of its praise for and appreciation of wood, particularly its great homage to wood use amongst various human societies on pp. 200/1.

**** It is possible that Trouet uses the term ‘mineral coal’ to contrast with charcoal [my emphasis] – also on p. 204, but that still leaves ambiguity in the text…