Golden Oldies

The findings have thrown up something of an environmental irony. “Global forests have been gradually shifting to a younger age structure owing to major reasons like deforestation, natural regeneration and human reforestation. As a result, younger trees are playing an important role in carbon storage and being seen as a nature-based solution to mitigate climate change,” said Dr Jinbao Li, a senior author of the study and Associate Professor of Geography.

“However, under global warming drought is occurring more intensely and frequently in many regions of the world, owing to precipitation reduction and more water loss through evapotranspiration due to higher temperatures. We embarked on the study because it had become imperative to understand whether younger trees in comparison with older trees can cope with drought.”

Nearly 22,000 trees across five continents were analysed, focussing on trees in the upper canopy as these are the main components in a forest and take up more carbon from the atmosphere than other forest components. They also provide important ecosystem services such as microclimate buffering – including cooling the temperature – and are habitats for other organisms.

“The most significant finding is that younger hardwoods from the upper forest canopy have a greater growth reduction (28 per cent) compared to the older ones (21 per cent) during drought,” said Dr Li. “During extreme drought, the difference could amplify to 17 per cent. Young conifers are also more sensitive to drought (27 per cent growth reduction) than the eldest but the difference between them is not remarkable (2.5 per cent). We did not expect this finding as some previous studies reported that larger, presumably older, trees are more sensitive to drought with greater growth reduction.”

Dr Li explained that these previous studies may have been confounded by the effects of canopy position – that is, smaller trees tend to live in the lower canopy. “In addition, tree size does not necessarily represent tree age when the trees reach the upper canopy. In a subset of species in the upper canopy with both tree size and tree age data, we observed a substantial variation in tree age (up to 200 years) even when the tree individuals share a similar size.”

The team undertaking the research was a mix of long-term and new colleagues from across the globe. “Some of us are long-time collaborators, such as first author Dr Tsun Fung Au, an HKU graduate whom I supervised (now at the Institute for Global Change Biology, University of Michigan) and co-author, Dr Teng Li, who was formerly my PhD student at HKU and is now at Guangzhou University,” said Dr Li. “We also established new collaborations with ecologists and climatologists from institutions like Indiana University in the US and Centre national de la recherche scientifique (CNRS) in France, which enabled access to more worldwide tree-ring data, as well as brilliant ideas to improve the quality of this research.”

The team used two key methods for the study. First, they considered how to measure tree growth reduction and recovery after drought, and decided to use two metrics – drought sensitivity and drought resilience. Drought sensitivity was defined as how much tree growth is being reduced during drought. Drought resilience was defined to quantify how trees recover after drought and restore to pre-drought-level growth.

Species-specific age

“We also considered at what age trees are considered as young or old, because an individual tree being young or old is subjective and possibly species-specific,” said Dr Li. “For example, a 200-year-old oak tree can be very old but a pine tree of the same age is still quite young, since pine species can live up to a few thousand years (for example, Bristlecone pine). In this work, we looked into the age distribution and longevity of each species and classified individuals from different species as ‘relatively young’ or ‘relatively old’ in a comparable and fair manner.”

For each drought-sensitive species, the team classified the youngest and oldest 25 per cent of the population as the young and old cohorts while the remaining 50 per cent was classified as the intermediate cohort. This relative ranking allowed the team to compare tree individuals from different species that have very distinct longevity.

Their findings have led them to suggest a shift in the approach to the role of forests in mitigating climate change. “Preserving mature trees and planting new ones are both important, as more younger trees can boost carbon storage and younger trees have a higher ability to recover from drought than older ones,” said Dr Li. “However, we often ignore what we already have – the mature trees. These existing old trees contribute to long-term storage of atmospheric carbon captured several hundreds to thousands of years ago while also being more tolerant to climate-induced droughts. Were these old trees to be destroyed, carbon captured long time ago would be released back into the atmosphere and exacerbate climate change.

“We hope future national and international carbon- and climate-related policies will take into account our findings and emphasise the conservation of exiting old forests instead of merely focussing on reforestation efforts.”