By Fangzhou Liu
A team of Stanford and Columbia University researchers have found that U.S. grasslands may be more sensitive to atmospheric dryness than rainfall; their study suggests that scientists may have to look more to rising temperatures than precipitation in predicting plants’ response to global warming.
Published on March 6 in Nature Geoscience, the researchers’ study examined 33 years of satellite data to understand grassland productivity in dry conditions. The timescale and quantity of data the team examined allows the study to inform predictive models of how environments will respond to droughts — which are likely to become more prevalent with rising temperatures around the globe.
“Just looking at changes in precipitation isn’t going to tell you the whole story,” lead author Alexandra Konings, an assistant professor of Earth System Science, told Stanford News. “U.S. grasslands are way more sensitive to vapor pressure deficit (VPD), which is important. Because VPD is so tightly linked to temperature, we can predict that it’s going to keep going up in the future.”
VPD measures the amount of water in the atmosphere, which directly affects plant productivity. Plants employ varied strategies in response to dryness, from closing up the openings on their leaves and stopping growth in order to conserve water to remaining open to absorb carbon dioxide despite the risk of drying out.
As the largest land-cover type on earth, grasslands are particularly important because they store vast amounts of carbon from the atmosphere and support a wide range of wildlife and livestock. Understanding how plants respond to changes in the atmosphere is especially important in U.S. grasslands, which are a predominant source of carbon uptake — or storage of carbon from the atmosphere.
One of the researchers’ major tasks was distinguishing the effect of the plants’ behavior from the impact of climate conditions in publicly available remote sensing satellite data. The study’s authors combined statistical methods with expertise in biology and geoscience to obtain usable data on plants’ productivity and identify finer distinctions in their behavior across different regions. The study uncovered regional variation in plant responses to drought, an important addition to current climate models because many models treat all grasslands as the same.
“Carbon uptake is associated with growth, and how that responds under climate is a large source of uncertainty in future climate change predictions,” Konings said. “Under increasing temperatures, we’re going to potentially see a lot less green grasslands – but this study shows that’s going to be more true for some regions than others.”
The study shows that grasslands that continue to absorb carbon dioxide in drought conditions are more easily damaged by drought than plants that close their openings to conserve water. Although the grasslands seek to take in carbon to continue growing amid drought, the dry conditions ultimately inhibit plant growth throughout the season.
Unlike previous efforts, the study was able to obtain data for several decades because it used satellite images of plant greenness, rather than on-the-ground measurements. Researchers hope to apply the novel method to ecosystems and climate patterns around the world.
“I think there’s a lot still to be done with this metric,” Konings said.