Indiana University
Newly published research by Indiana University scientists finds that low relative humidity in the atmosphere is a significant, growing and often under-appreciated cause of plant stress in hot, dry weather conditions.
The finding suggests that models used to gauge the impact of
drought on ecosystems should be refined to more accurately account for the role
of low atmospheric humidity. It will become more important in the future, as
the Earth's climate grows warmer and lower humidity has an increasing impact.
And as these humidity levels fall, plants may become less
effective at removing carbon from the atmosphere, reducing their ability to
offset climate change.
At the same time, agricultural management strategies
like irrigation, which improve soil moisture but have a smaller effect on
humidity, may become less effective in the future.
"There is much uncertainty when it comes to our ability to predict future patterns of carbon uptake by plants," said Kimberly Novick, assistant professor in the IU Bloomington School of Public and Environmental Affairs and the lead author of the study. "A lot of that uncertainty is related to an incomplete understanding of how ecosystems respond to drought.
"Our work suggests that properly specifying how plants
respond to variations in atmospheric humidity is one way to reduce this
uncertainty."
The study, "The increasing importance of atmospheric demand
for ecosystem water and carbon fluxes," was published in the journal Nature
Climate Change. Darren Ficklin, assistant professor in the IU Bloomington
College of Arts and Sciences' Department of Geography, is the second author.
Lixin Wang, assistant professor of earth sciences in the School of Science at
Indiana University-Purdue University Indianapolis, and Richard Phillips, associate
professor in the IU Bloomington College of Arts and Sciences' Department of
Biology, also are among 13 co-authors.
Dry weather affects plants in two basic ways: via changes in
moisture in the soil and relative humidity in the atmosphere. Think of it as a
supply-and-demand situation, Novick said. Soil moisture supplies water to
plants; during drought, it supplies less. Low relative humidity creates a
"demand" that pulls water from plants; during drought, the dry
atmosphere demands more water.
Plants have evolved to close their stomates -- tiny pores on the
surface of their leaves -- during dry weather to prevent the loss of excessive
moisture. But plants also uptake carbon dioxide from the atmosphere through
their stomates. When the stomates close, they capture less carbon.
Before this study, it had been difficult to know how much of
plants' response to drought was because of low soil moisture and how much was
because of low relative humidity.
Climate models tended to rely on soil
moisture, which is easy to measure and to manipulate experimentally -- even
though plant scientists know that low relative humidity could have an
important, independent effect.
Novick, Ficklin and their colleagues were able to separate the
effects of soil moisture and relative humidity by analyzing data collected in
hourly increments from 38 "flux towers" in the AmeriFlux Network, a
collection of sites in various ecosystems that collect important
micro-meteorological data every hour, and share these data with other researchers.
While soil moisture changes little over the course of a day, humidity can vary
significantly, allowing for the effects of soil moisture and humidity to be
independently assessed using these "high-frequency" data.
One of the flux towers, in Morgan-Monroe State Forest north of
Bloomington, is operated by a team led by Novick and Phillips in cooperation
with the Indiana Department of Natural Resources.
The analysis found that soil moisture and relative humidity have
markedly different influences on plant functioning at different times of the
day. Over time, humidity tends to be the dominant influence as weather grows
hotter and drier, especially in temperate forest ecosystems that are especially
important in removing carbon dioxide from the atmosphere.
In addition to having important implications for the management
of agricultural systems, including row crops and forests, the findings may help
scientists anticipate future effects of climate change, Novick said.
Although the study doesn't focus on carbon fluxes, scientists
know that plants remove less carbon from the atmosphere when their stomates are
closed to reduce water loss.
Thus, as the climate gets hotter and dryer, plants
will become less capable of taking in carbon, which will lessen their ability
to counteract climate change. Testing this hypothesis forms the foundation of
ongoing work being conducted by the study's authors.