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A forest may be made up of many trees, but some trees can provide more insight than others. While trees can be revered for their beauty, scientists have also studied unique, old trees, or “witness” trees, to learn how a forest has evolved throughout history.
Writer and researcher Lynda V. Mapes spent a year at the Harvard Forest in central Massachusetts, investigating how a hundred-year-old red oak tree might shed light on climate change. Working alongside ecologists, she incorporated scientific research and observation to study the oak’s place in the 4,000-acre forest.

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Mapes is not the first person to study a witness tree. Settlers used notable trees—large or otherwise memorable—to mark boundaries and areas on a landscape. The term “witness” can therefore refer to bearing witness to historical events.

Through observation, and phenology and carbon-storage research studies, Mapes constructed a history of the red oak.

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Phenology is the study of seasonal natural phenomena like leaf senescence, whose timing can illuminate how a tree or a forest is being affected by climate change. Phenology used to come only from visual observation of trees, marking events such as when leaves change color, fall off, and bud in the spring. But today, scientists incorporate technology to examine how the entire tree canopy is changing.

Using a camera inserted above the canopy of the witness tree and throughout the forest, the team of scientists took pictures to determine when the forest experienced these seasonal changes each year. This combination of research and observation led to a eureka moment. Previous models showed that warm temperatures in spring would lead to an early fall, but the cameras showed something different: the timing of autumn correlated more strongly to the onset of spring rather than temperature.

The team also analyzed carbon storage in the forest to see how climate could be affecting sequestration, the storage of carbon over time. Studying red oaks like the witness tree showed that how a forest stores carbon can be species-specific.

As Mapes writes, “Red oak is putting on more mass than any other tree species in the Forest, and faster. True, that is partly just red oak’s nature…But the red oak’s surge is also the result of climate change, manifest in warmer temperatures on average in winter, increased rainfall, and growing seasons lasting longer than at any point in the last two decades.”

Overall, incorporating observations from a witness tree can strengthen our understanding of the whole forest. “You could see all this [change] even within one tree,” Mapes concludes. “The big oak’s witness was clear: Our world is already changing.”

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Arnoldia, Vol. 74, No. 4 (2017), pp. 23-31
Arnold Arboretum of Harvard University