A new study finds that trees that lived through many wet years struggle to cope with dry spells, while young trees that have never known plentiful rainfall may be better equipped for ongoing drought.
The findings come from a 20-year study of trees in the Rhône River Valley in the Swiss Alps. For the study, scientists irrigated Scots pines growing in a mature forest, comparing them with plots nourished by rainfall alone. Researchers found that in irrigated plots, Scots pines grew faster.
Eleven years into the study, scientists stopped delivering water to half of each irrigated plot. The formerly irrigated pines, they noted, appeared deeply stressed by drought, even more so than the trees that had never been irrigated.
Trees that live through drought change in ways that would allow them to better cope with future dry spells. They develop smaller, hardier cells and grow deeper roots to soak up scarce water, while their leaves shift away from harvesting sunlight and toward storing water.
The formerly irrigated trees, it seems, overreacted to the decline in water. Compared to the trees that had never been irrigated, the formerly irrigated trees were more adapted to drought, and those adaptations significantly slowed their growth. The findings, published in the American Journal of Botany, suggest that trees with a “memory” of wet periods may be coping poorly with a warmer world.
Writing in The Conversation, authors offer a silver lining, noting that in most temperate woods, young trees have grown up in a time of chronic drought. “Those young trees, which have survived an endless dry period, will form the forests of the future,” they say. Young trees today “may be better prepared to cope with the world as humans have shaped it.”
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