Heat and drought slow down tropical tree growth
Stem growth of tropical trees is reduced in years when the dry season is warmer and drier than normal. This is the main finding of a global study on the sensitivity of trees in the tropics to climate change, published in Nature Geoscience. A hundred authors, including researchers from the Royal Museum for Central Africa, supplied tree-ring data of more than 14,000 trees from 350 different locations in 30 tropical and subtropical countries. The researchers found that the effect of drier and hotter years is larger in more arid or warm regions. This suggests that climate change may increase the sensitivity of tropical trees to climatic fluctuations.
Stem growth of tropical trees like this Afrormosia tree in the Yangambi biosphere reserve (DR Congo) is reduced in years when the dry season is warmer and drier than normal. © Axel Fassio/CIFOR
For a long time, ecologists assumed tree rings to be absent in tropical trees because of a lack of cold season. But in recent decades, the formation of growth rings has been proven for hundreds of tropical tree species. First author, Prof Pieter Zuidema from the University of Wageningen: "For the first time, we get a pantropical picture of how tropical tree growth reacts to climate fluctuations."
Co-author Dr Hans Beeckman of the Royal Museum for Central Africa: "Tropical forests store large amounts of carbon and, moreover, continue to capture a significant part of our CO2 emissions every year thanks to the growth of trees. To support climate policy, we need good data on tree growth. Trees are like sensors that store information on fluctuations in the climate: a pattern of tree-rings on a cross-section of the trunk can be read as a sequence of conditions that promote or inhibit tree growth. Such high quality data is highly sought after: it is essential to gain a good understanding of the health of tropical forests." For this international study, co-authors from the Royal Museum for Central Africa in Tervuren provided research material from both rainforests and dry forest types, in the form of series of tree-ring widths.
This study helps to understand the large fluctuations in carbon uptake by tropical vegetation globally. Pieter Zuidema: “Model simulations show that during hotter or drier years, tropical vegetation grows less and therefore takes up less CO2 from the atmosphere. But actual measurements of vegetation growth have been lacking so far. Our results thus provide empirical support of these global models.”
The authors were surprised by the finding that during the dry season climate had a stronger effect on tree growth than the wet season. Co-author Valerie Trouet from the University of Arizona, and former researcher at the Royal Museum for Central Africa: “We know that photosynthesis and wood production of tropical trees generally peak during the wet season. So why do year-to-year fluctuations in stem growth depend on the dry season? That surprised and puzzled us. Our explanation is that water is available for a longer period of time during years with wetter or cooler dry seasons. Put simply, the growing season is longer. This then leads to more stem growth.”
Filling the gap
The study fills an important data gap in tree-ring data. Pieter Zuidema: “World maps showing the locations of tree-ring studies typically have a hole in the middle, in the tropics. Our network fills that tropical data gap.” Mélissa Rousseau of the Royal Museum for Central Africa: “Even in the rainforest, the memory of trees can be unravelled by a thorough analysis of the wood between the pith and the bark. To conduct tree-ring analysis, we have installed a fully equipped laboratory in Yangambi in the equatorial region of Congo. This is used by Congolese and international researchers".
Global warming is expected to increase temperature at the study sites by 0.5 degree per decade in the future. The authors expect warming to aggravate the negative effects on tree growth of hotter and drier dry seasons. If slower growth increases the risk of tree death, tropical vegetation may more frequently become a source of CO2 instead of absorbing this greenhouse gas.
A study published in Nature in 2020 with Dr Wannes Hubau (Royal Museum for Central Africa) as lead author already provided evidence that a feared switch of the world’s undisturbed tropical forests from a carbon sink to a carbon source has already begun.
Scientific article: P.A. Zuidema, F. Babst, P. Groenendijk, V. Trouet, et. al. Tropical tree growth driven by dry-season climate variability (2022). Nature Geoscience. https://doi.org/10.1038/s41561-022-00911-8