Old-growth trees highlight the importance of conservation

7 December 2022

A recent study suggests that top conservation priority should be given to the older trees in the upper canopy due to their exceptional drought tolerance and carbon storage capacity. Furthermore, a diverse forest structure and composition could boost their ability to withstand future droughts under climate change.

Reforestation has been identified as a key nature-based solution in efforts to mitigate climate change. Led by geographers from the University of Hong Kong and Indiana University, the study analysed nearly 22,000 trees across five continents and reveals that old-growth trees are more tolerant to drought stress than younger canopy trees and could better buffer extreme climate.

A unique aspect of this study is its focus on trees in the upper forest canopy, which reduces confounding effects of tree height and size. Additionally, trees in the upper canopy layer play a more critical role in providing ecosystem functions such as carbon uptake and microclimate buffering than the understory trees.

The results highlight the importance of conserving the existing old-growth forests, not only for harbouring biodiversity in the forests, but also for carbon sequestration and mitigation of climate change. The findings were recently published in Nature Climate Change.

“Past deforestation and selective logging on old-growth forests, as well as recent reforestation endeavours have made the world’s forests dominated by younger trees. However, extreme climate events such as droughts raise serious concerns for these less mature forests,” said Dr Jinbao Li, Associate Professor of Geography at the University of Hong Kong.

Using long-term tree-ring data of 21,964 trees from 119 drought-sensitive species in five continents, Au and Li with an international team of climate scientists and ecologists found that younger hardwood trees in the upper canopy reduce growth by 28% during drought, compared to a 21% growth reduction in older hardwood trees. This disparity in drought response could magnify to 17 percentage points during extreme drought.

The authors also found age-dependent drought response differences in conifers were smaller than in hardwoods, likely because conifers tend to live in more arid environments and have evolutionarily adapted to dry climate.

While the differences in growth reduction may seem trivial, they could have a substantial consequence on the carbon budget when applied on a global scale, with future impacts of drought on forests and carbon sequestration becoming increasingly severe due to the dominance of younger trees and their heightened drought sensitivity.