Tsun Fung Au et al., 2022

https://doi.org/10.1038/s41558-022-01528-w

Reforestation is recognized as a nature-based solution to mitigate climate change, but a new study led by IGCB’s Tsun Fung Au with an international team of global change scientists suggested that a top priority should be given to conservation of existing old trees for their exceptional resistance to drought and long-term residence time of atmospheric carbon dioxide captured several hundreds to thousands of years ago.

Global forests are dominated by younger trees due to ongoing widespread deforestation, compounded with natural regeneration and human reforestation effort. While heat-induced droughts have become more frequent and intense, it poses a daunting threat on these younger trees that have increasingly important roles in carbon cycling and ecosystem

functions. Therefore, it is imperative to understand how the younger trees respond to drought comparing to the older one.

The study analyzed over 20,000 trees across five continents, with a primary focus on trees from the upper canopy as these trees have stronger carbon sequestration ability and provide fundamental ecosystem services such as microclimate buffering and habitat provision to other organisms than forest components.

During mild droughts, young hardwoods from the upper canopy layer experienced a 28% growth reduction, compared to a 21% reduction for their oldest relatives. The difference could amplify to 17% during extreme drought. Young conifers were also less resistant to drought (27% growth reduction) than the eldest and the difference between young and old conifers was 2.5%. On the other hand, young trees were more resilient to drought with greater ability to recover from drought than older trees, which could contribute to better carbon sequestration in the long run. The findings suggests that it is important to consider forest age as an aspect of climate change mitigation, such that a mixture of old and young trees in a forest is essential to cope with the short-term and long-term impacts of drought.