Over the past 35 years, tree mortality risk in Australia’s tropical regions has doubled, according to new research. Such mortality would radically reduce biodiversity as well as carbon residency time, a threat faced by tropical rain forests around the world.
“Moist tropical forests are some of the most biodiverse and productive ecosystems on the planet,” said David Bauman, lead author and a postdoctoral researcher at the French National Research Institute for Sustainable Development and the University of Oxford, noting that just 13 hectares (32 acres) can support more than 530 species of trees. “They’re critical allies in mitigating climate change, but they’re also vulnerable to climate change.”
Bauman and his colleagues attributed high tree mortality risk to the increasing vapor pressure deficit (VPD) associated with climate change. VPD describes the difference between the amount of moisture actually in the air and the amount of moisture the air can potentially hold. As the climate warms, VPD, sometimes nicknamed “atmospheric drought,” limits plant growth.
“With increasing temperatures, vapor pressure deficit will rise, and that’s why this paper is relevant,” said Pieter Zuidema, a professor of tropical forest ecology at Wageningen University and Research in the Netherlands. “It shows that these mechanisms will continue to increase tree mortalities over time.”
The results were published in Nature earlier this year.
A Rare Long-Term Monitoring Data Set
Bauman and his coauthors analyzed tree dynamics from 24 old-growth forest plots in northern Queensland, covering 49 years. The data set, made available by Australia’s Commonwealth Scientific and Industrial Research Organisation, encompassed climate gradients and many species of trees. The length of the monitoring study, however, is what sets it apart.
“Long-term monitoring of forest populations is rare,” said Benjamin Poulter, a research scientist at NASA’s Biospheric Sciences Laboratory. Poulter noted the study’s “incredible 49-year-long record of tree growth, establishment, and mortality.”
The broad, long-term analysis revealed a loss in biomass, making Australia’s tropical rain forests unique in recent years. “Observations from forest inventory plots in the Amazon or in African tropical forests generally show an increase in biomass over the last few decades, but this wasn’t the case with the Australian plots, where we saw decreases,” said Lucas Cernusak, a coauthor and an associate professor at James Cook University in Australia.
Scientists said results from the Australian study may help guide research and even policy outside Australia. If similar patterns of tree mortality are documented in other forests, their rate of biomass gain might decrease, and eventually the rate might drop. This means less carbon storage in the forest and more carbon in the atmosphere.
“The first and most important step is to cut emissions and decarbonize our systems,” Bauman said.
Another step is to conserve old-growth forests, he said. Data from long-term monitoring, like those Bauman was able to analyze in Australia, can help in this effort.
Poulter concurred. “Sustained investment in long-term forest monitoring is essential to understand the processes that are causing tropical tree mortality. Expanding the networks to new geographic locations, and to include different disturbance and recovery histories, will help inform conservation and management,” he said.
—Rishika Pardikar (@rishpardikar), Science Writer