In a recently published article in the journal Science, an international team of scientists from the US, Brazil, and Australia found that synchronization of new leaf growth with old leaf loss causes large seasonal increases in photosynthesis and greenness. The scientists made the discovery through the use of high-tech forest photography, field observations, and remotely-sensed satellite observations.
Most tropical forests appear green throughout the year, but scientists have long debated whether and by how much the “greenness” of these forests can increase and decrease seasonally. In particular, researchers have been puzzled by the seeming discrepancy between large seasonal changes in photosynthesis seen from towers on the ground versus the smaller changes in “greenness” seen from satellites in space.
Photosynthesis is the process by which all plants live, using sunlight to manufacture sugars and carbohydrates (their “food” supply) from atmospheric carbon dioxide, giving oxygen back to the atmosphere along the way.
In contrast to the temperate forests of North America or Europe, where new leaves grow in the spring and die in the fall, this study in the Amazon finds that the leaves in tropical forests grow at almost the same time that old leaves are dying, the biological system controlling the seasonality of tropical forests. “The timing of leaf production is surprising, particularly as climatic conditions during the peak of the dry season can be harsh,” says WHRC scientist and co-author Paulo Brando. With the inclusion of this new understanding of the biological system of leaf production and decay, future climate models will become more accurate.
The researchers used special tower-mounted cameras to discover that synchronized growth and loss of leaves in evergreen forest trees across broad areas of the Brazilian Amazon is the cause of strong seasonal increases and decreases in overall tropical forest photosynthesis. The ways in which forests regulate their seasonal uptake of atmospheric carbon dioxide will improve scientists’ understanding of the two-way relationship between forests and climate. “If climate change hurts the forests’ capacity to store carbon and cycle carbon, forests will in turn hurt Earth’s climate by speeding up warming rates,” explains Dr. Brando. “The current climate-vegetation models used to predict the potential trajectories of tropical forests should more accurately represent this delicate relationship.”
“Across the landscape, at large scales seen by satellites, the forests always look green,” says lead author Dr. Jin Wu of Brookhaven National Laboratory. “However, when we used cameras to look carefully every day at the trees one by one, it was very exciting. We saw dramatic leaf loss and rapid growth spurts of new leaves that were not visible by satellite.”
The discovery through photos that old leaf death and new leaf growth happened at the same time as the annual seasonal increase in forest photosynthesis was the key to the whole story, explained University of Arizona co-author Scott Saleska. “Simply put, when it comes to leaves and photosynthesis, it is no different than with us humans. Age matters,” says Dr. Saleska. “When you swap out a bunch of old leaves and exchange them for young new ones instead, overall photosynthesis has to go up, even if the total amount of leaves doesn’t change very much.”
And the amount of leaves being exchanged is surprisingly high. Another collaborator in Brazil was astonished when he first began looking at images of the forest from his site near Manaus. “Fully a third of the trees in this forest lose most or all of their leaves, and within just a month grow them all back again,” he says. “It’s an amazing example of how dynamic individual trees can be, even in a forest that overall is classified simply as ‘evergreen’.”
WHRC is an independent research institute where scientists investigate the causes and effects of climate change to identify and implement opportunities for conservation, restoration and economic development around the world.