Trees in the Amazon make their own rain
By Ilima
LoomisAug. 4, 2017 , 2:45 PM
The Amazon
rainforest is home to strange weather. One peculiarity is that rains begin 2 to
3 months before seasonal winds start to bring in moist air from the ocean. Now,
researchers say they have finally figured out where this early moisture comes
from: the trees themselves.
The study
provides concrete data for something scientists had theorized for a long time,
says Michael Keller, a forest ecologist and research scientist for the U.S.
Forest Service based in Pasadena, California, who was not involved with the
work. The evidence the team provides, he says, is “the smoking gun.”
Previous
research showed early accumulation of moisture in the atmosphere over the
Amazon, but scientists weren’t sure why. “All you can see is the water vapor,
but you don’t know where it comes from,” says Rong Fu, a climate scientist at
the University of California, Los Angeles. Satellite data showed that the
increase coincided with a “greening” of the rainforest, or an increase in fresh
leaves, leading researchers to suspect the moisture might be water vapor
released during photosynthesis. In a process called transpiration, plants
release water vapor from small pores on the underside of their leaves.
Fu thought
it was possible that plants were releasing enough moisture to build low-level
clouds over the Amazon. But she needed to explicitly connect the moisture to
the tropical forest.
So Fu and
her colleagues observed water vapor over the Amazon with NASA’s Aura satellite,
a spacecraft dedicated to studying the chemistry of Earth’s atmosphere.
Moisture that evaporates from the ocean tends to be lighter than water vapor
released into the atmosphere by plants. That’s because during evaporation,
water molecules containing deuterium, a heavy isotope of hydrogen made of one
proton and one neutron, get left behind in the ocean. By contrast, in
transpiration, plants simply suck water out of the soil and push it into the
air without changing its isotopic composition.
Aura found
that the early moisture accumulating over the rainforest was high in
deuterium—“too high to be explained by water vapor from the ocean,” Fu says.
What’s more, the deuterium content was highest at the end of the Amazon’s dry
season, during the “greening” period when photosynthesis was strongest.
The
tree-induced rain clouds could have other domino effects on the weather. As
those clouds release rain, they warm the atmosphere, causing air to rise and
triggering circulation. Fu and colleagues believe that this circulation is
large enough that it triggers the shift in wind patterns that will bring in
more moisture from the ocean, they report in the Proceedings of the National
Academy of Sciences.
Scientists
have studied the connection between trees and rain in the Amazon before. A 2012
study found that plants help “seed” the atmosphere for rain by releasing tiny
salt particles. But the new study strongly supports the idea that plants play
an important role in triggering the rainy season, says Scott Saleska, an
ecologist at the University of Arizona in Tucson, who was not involved with the
work. The deuterium provides a clear “fingerprint” for what plants contribute
to the process, he says.
The
findings also address a long-standing debate about the role plants play in
weather, says Saleska, suggesting that they are more than just “passive
recipients,” and that they instead can play an active role in regulating
rainfall. If that’s true in the Amazon, Saleska says, climate scientists will
need to take into account practices like deforestation when predicting regional
changes in weather patterns. And curbing deforestation will be an important
step for people to take in preventing drought.
Next, Fu will be studying rainforests in the Congo, to see whether the same process is happe
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