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 “Plants have to balance water supply and demand. Both are
extremely critical, but people overlook the demand side of the
equation, especially in the U.S. Corn Belt,” says Kaiyu Guan,
principal investigator on two new studies, Blue Waters professor
in the Department of Natural Resources and Environmental
Sciences and the National Center for Supercomputing Applications
at Illinois.
The demand Guan refers to is atmospheric dryness, often
expressed as vapor pressure deficit (VPD). The drier the air,
the more moisture is sucked out of pores, or stomata, in plant
leaves. Plants have to open stomata to take in carbon dioxide as
their food, but if they sense the atmosphere is too dry, they’ll
close pores to avoid drying out. Keeping stomata closed too long
leads to reductions in photosynthesis, plant growth, and grain
yield.
The kicker? Plants shut down stomata due to atmospheric dryness
even when there’s an adequate supply of moisture in the soil.
“If you only consider rainfall and soil moisture, which is how
most people think about drought, that’s mostly describing the
supply side. Of course if you have low soil moisture, plants
will be stressed by how much water they get. But the supply is
often pretty sufficient, especially here in the U.S. Corn Belt,”
Guan says. “However, the demand side from the atmosphere can
also severely stress plants. We need to pay more attention to
that drought signal.”
Guan’s two recent studies used multiple technological
approaches, including field measurements, various sources of
satellite data, hydrological model simulations, and government
crop yield statistics. The first study, published in
Agricultural and Forest Meteorology, used data from seven sites
across the Corn Belt to conclude VPD accounts for nearly 90% of
the changes in crop stomatal conductance, a proxy for drought
stress, and approximately 85% of changes in gross primary
productivity, a measure of productivity.
“By comparison, soil moisture typically accounts for 6-13% of
these measures for corn and soybean, and up to 35% when
considering time lag effects,” says Hyungsuk Kimm, doctoral
student in Guan’s group and the study’s lead author.
In the other study, published in the Journal of
Hydrology, Guan’s team focused on grain yield. Yield depends on
many factors related to water cycles, but the researchers found
that VPD explains the biggest proportion of variability in crop
yield and also provides the earliest warning for yield loss when
comparing with other water cycle metrics and traditional drought
indices.
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“This led us to build a new drought index integrating
VPD, soil moisture, and measures of evapotranspiration, which can
account for more than 70% of yield variation. Our index outperforms
all the existing drought indices,” says Wang Zhou, postdoctoral
researcher in Guan’s group and the study’s lead author.
Guan adds, “In these two studies, we tried to understand the demand
side of drought from two major angles, one using eddy covariance
data which measures landscape water and carbon use very accurately –
the gold standard – and the other leveraging satellite data and
model-simulated hydrological variables correlated with regional
yield,” Guan says. “In both, we demonstrate VPD is more important
than soil moisture to explain the crop drought response in the U.S.
Midwest.”
Adjusting the drought concept for crops will be critical for global
food security under a changing climate.
“When we look at climate change scenarios, the amount of rainfall is
not changing much for the Corn Belt, but we for sure know
temperature and VPD will increase here. That means not much will
change on the supply side, but demand stress will increase
significantly. And that type of stress is so connected to
end-of-season crop yield,” Guan says.
His group is working on follow-up studies evaluating the role of
irrigation in increasing supply and decreasing demand, but for now,
Guan says breeding for improved water-use-efficiency could be an
important part of the solution.
“Redefining droughts for the U.S. Corn Belt: The dominant role of
atmospheric vapor pressure deficit over soil moisture in regulating
stomatal behavior of maize and soybean,” is published in
Agricultural and Forest Meterology [DOI:
10.1016/j.agrformet.2020.107930] and “Connections between
hydrological cycle and crop yield in the rainfed U.S. Corn Belt,” is
published in Journal of Hydrology [DOI: 10.1016/j.jhydrol.2020.
125398]. Support from NASA, USDA NIFA, DOE, NSF, and Illinois NREC
made the research possible.
The Department of Natural Resources and Environmental Sciences is in
the College of Agricultural, Consumer and Environmental Sciences at
the University of Illinois.
[Source: Kaiyu Guan
News writer: Lauren Quinn] |
