HK University Study Finds Older Trees Are More Drought Resistant

A joint study by the University of Hong Kong and Indiana University found that older trees have exceptional drought tolerance and carbon storage capacity. (Courtesy of Dr. Jinbao Li)

A joint study by the University of Hong Kong and Indiana University suggests that top conservation priority should be given to older trees in the forest canopy owing to their exceptional drought tolerance and carbon storage capacity. Furthermore, a diverse forest structure and species composition can also improve the ability of forests to resist future droughts under climate change.

The study analyzed the growth-ring data of almost 20,000 canopy trees from 119 tree species on five continents and found that older trees in the forest canopy are more resistant to drought than younger canopy trees and could better buffer the impact of extreme climate conditions. The results highlight the importance of conserving existing mature forests, not only for maintaining forest biodiversity in the forests but also storing carbon dioxide to mitigate climate change.

The research team found that during the drought, the growth of young hardwood trees decreased by 28 percent, while the growth of older hardwood trees decreased by only 21 percent, a difference of 7 percentage points. During extreme drought, this difference increased to 17 percentage points. Compared with broad-leaved trees, conifers of different ages have less difference in response to drought; the team suggests that this may be because conifers have been better adapted to dry environments after long-term evolution.

Differences in the growth of trees of different ages during droughts have a major impact on the global carbon cycle and carbon budget, the study says. Forests are dominated by young trees, which are less capable of withstanding drought, so the impact of drought on forests and carbon sequestration may become severe in the short term. However, young trees also have better resilience and may benefit from carbon storage in the long run. The study focused on canopy trees, which excluded the interference of tree height and size, and compared with forest bottom trees, canopy trees play a more important role in ecosystem functions such as carbon sequestration and the microclimate.

The research was led by Dr. Jinbao Li, Associate Professor of the Department of Geography, University of Hong Kong, and Dr. Tsun Fung Au, Ph.D., Indiana University, who is now a postdoctoral researcher at the Institute of Global Change Biology, University of Michigan. Relevant results have been published in the academic journal Nature Climate Change.