In ‘Witness Tree,’ a writer spends a year exploring a red oak’s changes

“Witness Tree: Seasons of Change with a Century-Old Oak”

by Lynda V. Mapes

Bloomsbury, $27, 224 pages

“What do trees do all day long?” asks Seattle Times environmental writer Lynda V. Mapes. Much more than meets the eye, she explains in her fourth book, “Witness Tree,” researched during 2013-15 as a Knight Fellow in Science Journalism at MIT and then as a Bullard Fellow in Forest Research at the Harvard Forest near Petersham, Massachusetts.

Although trees may seem inactive, they are busy photosynthesizing — using carbon dioxide to convert sunlight energy into sugar and other organic compounds necessary for their survival — and releasing oxygen. They send nourishment and water through their roots, trunks, leaves and branches. Below their bark and cambium layer, cells divide, laying down growth rings. Based on seasonal clocks, leaves appear, their sizes and shapes determined by position. Pores called stomata on the undersides of leaves open, releasing water, which evaporates. Trees produce fruit, offer shelter and provide nesting places.

So trees aren’t passive. In fact, Mapes writes, they “manipulate their environment, exuding chemicals to deter pests and call in predators. They make soil, alter the hydrologic cycle, climate, atmosphere, and habitat. Trees move, breathe, operate a whole-body circulatory system, eat, have sex, communicate, expel waste, socialize, wage war, compete, cooperate, and create.”

To tell their story, Mapes wanted to spend a year with a special tree. In colonial times, she notes, big trees served as landmarks for surveyors. Called “witness trees,” they were sometimes solitary but always huge old trees that had experienced many years of local history. Mapes’ witness tree would be a “living marker, from which to understand our past, interpret our perplexing present, and regard our future.”

She chose a red oak that began life when an acorn took root. Acorns are such prized food, only one in 500 produces a year-old seedling. Nevertheless, this tree, now more than 80 feet tall, with a magnificent crown, flourished. It is part of a long-term survey that studies timing of annual events such as when its buds open or autumn leaves fall; weekly data collected over many years helps reveal how climate change “reset the seasonal clock.”

To learn more about its life, Mapes accompanied an expert taking a core sample, which provided information on rainfall, disease, fire and age; this oak was about 115. To learn more about life in and on it, she noted insects, mammals, birds and lichens. In the duff at its feet were fungi and tiny ferns, amphibians, roots and reptiles. And although its lowest branches were 40 feet above ground, she arranged to scale to its upper stories with a professional climber. There, everything looked different: the far view, the near trees that didn’t encroach on the oak’s space, dead branches sporting mushrooms, “a whole garden up in the sky I was never aware even existed.”

To learn about the century of life around her oak, Mapes accompanied researchers on fieldwork, read Harvard Forest Archives, consulted maps and old photos, newspaper articles, government reports and, especially vivid, describes Sarah Luce Mann’s journal from 1861-62, “when these woods were pastures and farms.” Even earlier, old-growth trees had been cleared for agriculture, then white pine took over before mixed hardwoods appeared. Her oak even survived the great hurricane of 1938.

She learns that if “dead” wood is left instead of salvaged, it lives on as food and a water source. She explains the greenhouse effect and climate change, how trees help “to sustain our world.” Mapes’ findings are beautifully detailed and lovingly written, her witness tree and the Northeast’s woodlands “a great green wellspring of hope for the world.”