A new analysis of more than 20,000 trees on five continents shows that old-growth trees are more drought tolerant than younger trees in the forest canopy and may be better able to withstand future climate extremes.
The findings highlight the importance of preserving the world's remaining old-growth forests, which are biodiversity strongholds that store vast amounts of planet-warming carbon, according to University of Michigan forest ecologist Tsun Fung (Tom) Au, a postdoctoral fellow at the Institute for Global Change Biology.
"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," said Au, lead author of the study published online Dec. 1 in the journal Nature Climate Change."Given their high resistance to drought and their exceptional carbon storage capacity, conservation of older trees in the upper canopy should be the top priority from a climate mitigation perspective."
The researchers also found that younger trees in the upper canopy -- if they manage to survive drought -- showed greater resilience, defined as the ability to return to pre-drought growth rates.
While 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 increasingly 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 said.
"These results imply that 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."
Those implications will require further study, according to Au and colleagues, given that reforestation has been identified by the Intergovernmental Panel on Climate Change as a potential nature-based solution to help mitigate climate change.
