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Climate variability drives tropical tree growth during dry season

A new study published in the journal Nature Geoscience has found that tropical tree growth is driven by climate variability during the dry seasons. By analyzing tropical and subtropical tree-ring data – a reliable indicator of tree growth – a team of researchers led the University of Arizona in the US and the Wageningen University & Research (WUR) in the Netherlands has discovered that, when the dry-season season is warmer and drier than normal, the growth of tropical trees is highly reduced, suggesting that climate change may increase the sensitivity of tropical trees to climatic fluctuations.

“These (tropical) tree rings contain a wealth of information on the growth history of trees,” said study lead author Pieter Zuidema, a professor of Forest Ecology and Forest Management at WUR. “In this study, we exploit that potential. For the first time, we get a pantropical picture of how tropical tree growth reacts to climate fluctuations.”

By analyzing over 14,000 tree-ring data series from 350 locations across 30 tropical and sub-tropical countries, Professor Zuidema and his colleagues were surprised to find that, during the dry season, climate appeared to have a much stronger effect on tree growth than during the wet season.

“We know that photosynthesis and wood production of tropical trees generally peak during the wet season,” said study co-author Valerie Trouet, a dendroclimatologist at the University of Arizona. 

“So, why do year-to-year fluctuations in trunk 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 trunk growth.”

These findings could help explain the large fluctuations in carbon uptake by tropical vegetation worldwide, showing that, during hotter or drier years, tropical vegetation grows less and thus takes up less carbon dioxide from the atmosphere. Finally, the study suggests that slower growth may increase the risk of tree death, which would then become sources of carbon dioxide rather than carbon sinks.

By Andrei Ionescu, Staff Writer

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