For a long time, ecologists assumed tree rings to be absent in tropical trees because of a lack of temperature and rain fluctuations in the trees' environment. But in recent decades, the formation of growth rings has been proven for hundreds of tropical tree species, which are sensitive to drought and usually experience at least a month or two of slightly reduced rainfall every year.

When scientists better understand how tropical trees respond to unusually dry and warm conditions, they can better predict how these trees will be affected by climate change.

A new study, co-authored by University of Arizona researchers and published in Nature Geoscience, has found that tropical trees' trunk growth is reduced in years when the dry season is drier and warmer than normal. The study defines the tropics in a way that also includes the subtropics – or anything between 30 degrees north latitude and 30 degrees south latitude.

The researchers also found that the effect of drier and warmer years is more dramatic in more arid or warm regions in the tropics. This suggests that climate change may increase the sensitivity of tropical trees to climatic fluctuations. Temperatures at the study sites are expected to increase by half a degree Celsius per decade in the future.

The results of the study help explain the large fluctuations in carbon uptake by tropical vegetation globally. Model simulations show that during hotter or drier years, tropical vegetation grows less and therefore takes up less carbon dioxide from the atmosphere. But actual measurements of vegetation growth have been lacking until now.

Research shows that slower growth increases the risk of topical tree death, so tropical vegetation may more frequently become a source of carbon dioxide instead of absorbing this greenhouse gas that causes climate change.

"These (tropical) tree rings contain a wealth of information on the growth history of trees," said lead study author Pieter Zuidema of Wageningen University & Research in the Netherlands. "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."

The study was an international collaborative effort that included University of Arizona dendrochronology Valerie Trouet, a professor in the Laboratory of Tree-Ring Research, and Flurin Babst, an assistant research professor in the UArizona School of Natural Resources and the Environment. The findings are based on a new global network, created by the collaborators, of over 14,000 tree-ring data series from 350 locations across 30 tropical and sub-tropical countries.

The authors were surprised to find that during the dry season, climate had a 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," Trouet said. "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."

The study also fills an important gap in tree-ring data.

"World maps showing the locations of tree-ring studies typically have a hole in the middle, in the tropics," Zuidema said. "Our network fills that tropical data gap."

The tree-ring data from more than 100 study locations has been uploaded to the International Tree-ring Databank, the global database for tree-ring data.

"In this way, the tree-ring data we've put together will be freely available for everyone," Zuidema said.