A study of over 20,000 trees spanning five continents has found that old growth trees are more drought resistant, meaning they’re priorities for conservation.
As deforestation and reforestation continues apace, trees around the world are getting younger.
“The number of old-growth forests on the planet is declining, while drought is predicted to be more frequent and more intense,” says Dr Tsun Fung Au, a forest ecologist at the University of Michigan, US, and lead author of the study, which is published in 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.”
Au and colleagues used the International Tree-Ring Data Bank, which collects information on trees from over 5,000 sites around the world, to do their analysis.
They looked at the growth of 21,964 trees from 119 drought-sensitive species in North and South America, Eurasia, Africa and Oceania, during and after droughts in the past century.
They focussed specifically on hardwoods and conifers in the upper canopy layer, where younger trees are beginning to dominate.
While they found that younger trees in this layer were more sensitive to drought when it happened, they were also more resilient when it ended.
This means that younger trees had more significant growth reduction during droughts: their canopy cover shrank and died off more.
But younger trees which did survive drought were more likely to return to pre-drought growth rates once a drought was over.
“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,” says Au.
“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.”
Senior author Justin Maxwell, an associate professor at Indiana University, US, says that their findings are important for forest managers.
“Historically, we have managed forests to promote tree species that have the best wood quality,” says Maxwell.
“Our findings suggest that managing forests for their ability to store carbon and to be resilient to drought could be an important tool in responding to climate change, and thinking about the age of the forest is an important aspect of how the forest will respond to drought.”