New leaves are much more effective photosynthesizers than old ones. A team of scientists are arguing that our failure to take this into account explains why we have struggled to understand seasonal changes in the productivity of tropical rainforests. By looking at the forests, not the leaves, we've ignored the impact of leaf quality.
Tropical forests influence the atmosphere enormously. Given the rate at which they are being destroyed, we can't count on this lasting forever, making understanding them urgent. Although these forests don't lose their leaves in winter, there are seasonal changes, but attempts to track these changes from space have proven frustrating.
Dr. Jin Wu of the Brookhaven National Laboratory described the problem in a statement: "Across the landscape, at large scales seen by satellites, the forests always look evergreen." Seasonal variations are observed, but models based on color changes have produced swings between a tenth and half the actual changes in carbon dioxide absorption that can be measured.
Observations are complicated by differences between regions. “Remote-sensing observations suggest that central Amazon forests seasonally increase their photosynthetic capacity (“green-up”) during dry seasons,” Wu's paper in Science reports, “whereas southern Amazon and African tropical forests show declines.” The confusion has led to suggestions there might be problems with the satellite measuring technology.
"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 couldn't be easily seen by satellites," said Wu. This changeover coincided with the times when the forests were most productive in drawing down carbon dioxide.
Wu revealed that while new and old leaves looked similar from space, their productivity is very different. "Simply put, when it comes to leaves and photosynthesis, it is no different than with us humans: Age matters," said Wu's former supervisor, Dr. Scott Saleska of the University of Arizona. "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."
Seasonal differences between regions appear to be based on which factor is limiting in a particular forest, water or sunlight. In the central Amazon, forests usually have so much water that their most productive time is the dry season, when more sunlight is available. In dryer locations the wet season is growth time.
Surprisingly, it is not the wet season, but the dry, that is most productive in the central Amazon. Swisoor/Shutterstock
The paper notes that leaf changes appear synchronized across sites hundreds of kilometers apart, “despite enormous biodiversity,” in contrast to the wide gaps between the dates when temperate species produce and drop their leaves.
The authors note there is a lot of feedback between rainforests and climate. The increased release of water as leaf transpiration surges in the Amazon “hastens transitions to the wet season” and may even help bring the distant North American monsoon to an end.
Understanding these patterns is essential for predicting how rainforests will respond to climate change.