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From ancient timbers to mountaintop forests, trees hold important climate clues

Tree Story: The History of the World Written in Rings

Valerie Trouet
Johns Hopkins University Press
2020
256 pp.
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Many of us have counted the rings of a tree to reveal its age. But did you know that evidence of epic fires, volcanoes, hurricanes, floods, drought, famine, and the rise and fall of ancient empires is also embedded in a tree’s circumference? Valerie Trouet’s Tree Story is an informative introduction to the science of tree rings. An accomplished and globally recognized dendroclimatologist, Trouet is knowledgeable across diverse fields of science and is a talented writer and engaging storyteller.

Long chronologies are formed by crossdating the rings of living trees with those of dead trees. The oldest living dendrochonologically dated tree is a bristlecone pine in California that is ~5000 years old. The longest tree-ring chronology is composed of living and archaeological oak-pines from Germany and spans an astounding 12,650 years.

The quest for long-lived trees leads dendrochronologists deep into the wilderness and to the tops of mountains. Here, they extract increment cores using hand-operated borers, a nondestructive way to collect rings from living trees. Dendrochronologists can also be found analyzing archaeological ruins and shipwrecks at the bottom of the ocean, where tree rings of past centuries are preserved in ancient timbers. Trouet entertains readers with adventurous tales of accessing remote field sites and the inevitable mishaps that occur when one is navigating an unfamiliar culture using a foreign language.

Climate history is often embedded in long tree-ring chronologies. In some cases, the relations are intuitive—wide rings in trees growing on mountaintops indicate warm years, whereas narrow rings in trees of semiarid climates indicate hot, dry summers. Other relations are more challenging to decipher and require multiple proxies and other climate-sensitive biotic and abiotic time series. Drawing from a diversity of tree-ring research and interdisciplinary collaborations, Trouet chronicles fascinating examples of how dendrochronology helps to answer questions about past environments and human history.

Drought-sensitive tree rings from the Mediterranean cross-referenced with the annual increments of a stalagmite deep in a Scottish cave, for example, have revealed the seesaw climate of the North Atlantic Ocean, a driver of contemporary global climate and the key to understanding the onset of the Little Ice Age. Meanwhile, the tree rings of blue oaks in the Central Valley reflect regional drought and snow accumulation in the nearby Sierra Nevada, showing California’s recent megadrought to be a 500-year record.

Cross-referencing tree rings with marine records from corals, fish otoliths, and bivalve shells has enriched our understanding of the relationship between the ocean and the atmosphere, shedding light on, for example, the El Niño–Southern Oscillation and other variations in the Pacific climate system. Cross-referencing tree rings with ice cores from the remote ice sheets of Greenland and Antarctica, meanwhile, has allowed us to chronicle volcanic eruptions that had global effects on temperature, and river flows that drove agricultural collapse and famines in countries ranging from Ireland to Egypt. Trouet even demonstrates how tree rings helped decipher the environmental context of the bubonic plague, revealing how climate change amplified past epidemics.

In Tree Story, Trouet tackles some of the pressing environmental challenges of this millennium. She clearly explains the science underpinning the now-famous “hockey-stick” curve and deftly debunks the arguments of climate-change deniers. She shows how tree-ring science provides distinctive context to the record-breaking droughts, hurricanes, and wildfires that have plagued the world in the past decade. In the case of wildfires, using crossdated fire-scar records and tree-ring climate proxies, she shows how the combination of displacement of indigenous peoples by Europeans, ongoing land-use change, and fire suppression has transformed landscapes, making them more flammable, especially as the climate warms.

Using tree rings to disentangle complex interactions, Trouet shows how past societies have dealt with unexpected climatic changes. She concludes with a challenge to scientists and our society: For the first time in human history, thanks to centuries of scientific discoveries and research, we have foresight into the changes that lie ahead. “We have our work cut out for us,” she warns.

About the author

The reviewer is at the Department of Forest and Conservation Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada.