A new study leveraging 20,000 tree-ring records and nearly 150 scientists' contributions from across the globe shows that, while droughts appear to have had a modest impact on tropical tree growth in the past, that may not be the case for long.
The research team calculated that, on average across the tropics, trees grew 2.5 percent less during drought years compared to years with normal or above-average precipitation. Surprisingly, the researchers also found an almost complete recovery in the year following the drought. However, that resilience, they warn, could weaken when droughts occur more frequently and become more intense – particularly within drier, semi-arid regions of the tropics.
Few comprehensive studies on tropical tree rings have been published, largely because year-long warm and moist conditions were thought to prevent regular annual ring formation. But scientists have come to understand that some tropical tree species and their rings can chart water availability, explained Valerie Trouet, co-author on the study and professor of dendrochronology at the University of Arizona's Laboratory of Tree-Ring Research .
"Tropical dendrochronology has long been deemed impossible or at least too complex to use for synthesis science," said Trouet. "In this paper, we use the largest network of tropical tree-ring chronologies yet, to do exactly that, to synthesize the impact of drought on woody tree growth in tropical forests."
"Until now, we didn't know to what extent stem growth in tropical forests decreases during droughts," said Pieter Zuidema, lead author on the paper and professor at the Netherland's Wageningen University & Research. "With our new network of tree-ring records, we were able to calculate the effect of droughts on stem growth across the tropics for the first time."
Clogging the carbon sink
Tropical forests and woodlands are key components of the global carbon cycle, specifically in their role in sequestering CO2 – often for decades. During drought, tree growth usually slows, which reduces the amount of carbon stored.
To understand and predict the risks that droughts pose for the long-term capture of carbon in tropical vegetation, the researchers used the largest collection of tropical tree-ring data to date: more than 20,000 tree-ring series from nearly 500 locations in 36 countries.
The tree-ring data were collected at 483 locations spread across the tropics – from wet and warm Amazonian forests to dry forests in southern Africa and cooler Asian mountain forests.
The researchers determined the driest years since 1930 for all locations, then calculated how much narrower the tree rings were during those years compared to normal years. They also measured the width of tree rings in the two years following a drought.
Focusing their analysis on the top 10 percent of driest years, the research team observed stem growth of tropical trees decreased on average by 2.5 percent. For the top 5 percent of extreme years, that global average of growth decreased even further, down to 3.2 percent, explained Flurin Babst, a co-author on the study who helped design and run the analysis and an assistant professor in the U of A School of Natural Resources and the Environment .
"An average growth reduction of 3.2 percent under drought may seem small," said Babst. "But given the dominant role that tropical forests and woodlands play in the global carbon cycle, the impacts of intensifying drought on the land carbon sink could be substantial. This is relevant for the many initiatives worldwide that use tropical trees as carbon reservoirs to offset anthropogenic CO2 emissions."
In drier, semi-arid tropical regions, the average tree growth slowed down even further, by roughly 10 percent in a quarter of the locations included in the study.
"This happened mainly in hot and dry regions, such as in northeastern Brazil and southern Africa," Zuidema said. "The effects of droughts can be more extreme there because trees shed their leaves quickly and the soil retains less moisture. In wetter areas, such as Amazonian forests, the effects were weaker."
The researchers warn that stem growth resilience could weaken when droughts occur more frequently and become more intense – something they have witnessed within the past few decades in research locations within the study.
"Recent droughts have already caused stronger reductions in stem growth compared to earlier droughts. It gets harder for trees to recover," Zuidema said. "We expect that ongoing climate change will only exacerbate this effect."
Reduced tree growth has widely been associated with higher tree mortality, and while this study did not directly quantify tree mortality at the 483 tree-ring locations, the team used existing research to estimate that drought increased the normal tropical tree mortality rate by 0.1 percent.
"Again, 0.1 percent is not a huge number," said Babst. "But considering the vast area that tropical forests and woodlands occupy, this increase in tree mortality leads to a notable amount of biomass that dies, decays, and releases CO2 back into the atmosphere. More research is clearly needed to better quantify this feedback."
A tropical tree-ring network
The study is the result of a recently formed network of tropical tree-ring studies , which aims to unite data from tree-ring studies across the tropics to better understand how tropical trees respond to current and future climatic conditions.
"Hundreds of tree-ring studies have been conducted on tropical trees over the past few decades," said Peter Groenendijk, co-author on the study and an assistant professor at University of Campinas in Brazil. "However, this is the first time those data have been compiled for large-scale analysis. We are trying to understand the past in order to predict the future."
Zuidema, Groenendijk, Trouet and Babst are the initial founders of the network and continue to serve as the core members responsible for its overall management. Today, the initiative has grown to include over 170 collaborators, contributing nearly 500 ring-width chronologies from more than 30 countries across all tropical continents, representing over 139 tree species.
