Global Warming: Old trees may handle climate extremes better

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New Delhi: Old-growth trees are more drought tolerant than younger ones in the forest canopy and may be better able to withstand future climate extremes, researchers report.

An analysis of over 20,000 trees on five continents highlights the importance of preserving the world’s remaining old-growth forests, which are biodiversity strongholds that store vast amounts of planet-warming carbon, says Tsun Fung (Tom) Au, a postdoctoral fellow at the Institute for Global Change Biology at the University of Michigan.

“The number of old-growth forests on the planet is declining, while drought is predicted to be more frequent and more intense in the future,” says Au, lead author of the study published in Nature Climate Change. “Given their hi­gh resistance to drought and exceptional carbon storage ca­p­acity, conservation of ol­der trees in the upper cano­py sho­uld be the priority from a climate mitigation perspective.”

The experts found younger trees in the upper canopy — if they survive dro­u­ght — showed greater resi­l­i­ence, defined as the ability to return to pre-drought growth rates. Reforestation, deforest­a­tion, selective logging, and other threats have led to the global decline of old-growth forests, subsequent reforestation – either through natural succession or through tree pla­nting – has led to forests do­minated by rising younger trees.

For example, the area covered by younger trees (<140 years old) in the upper canopy layer of temperate forests worldwide already far exceeds the area covered by older trees. As forest demographics continue to shift, younger trees are expected to play an increasingly important role in carbon sequestration and ecosystem functioning.

“Our findings – that older trees in the upper canopy are more drought tolerant, while younger trees in the upper canopy are more drought resilient – have important implications for future carbon storage in forests,” Au says.

“The results imply in the short term, drought’s impact on forests may be severe due to the prevalence of younger trees and their greater sensitivity to drought. But in the long run, those younger trees have a greater ability to recover from drought, which could be beneficial to the carbon stock.” The implications will require study, according to Au and colleagues, given reforestation has been identified by the Intergovernmental Panel on Climate Change as a potential nature-based solution to help mitigate climate change.

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