A new study shows that dryness of the atmosphere affects U.S. grassland productivity more than rainfall does. The findings could have important implications for predicting how plants will respond to warming climate conditions.
Scientists at Stanford University and Columbia University looked at 33 years of climate and vegetation satellite data to determine how plants regulate water and carbon dioxide under dry conditions. The team concluded in a study published online March 6 in the journal Nature Geoscience that U.S. grasslands are more than three times more sensitive to vapor pressure deficit (VPD), or atmospheric dryness, than they are to precipitation. The study’s large-scale methods to understand plant behavior could be used to improve predictive models of how environments will respond to rising temperatures and droughts, which are expected to intensify in the 21st century.
“Just looking at changes in precipitation isn’t going to tell you the whole story,” said lead author Alexandra Konings, formerly a postdoctoral fellow in Columbia Engineering Professor Pierre Gentine’s lab and now an assistant professor of earth system science in Stanford’s School of Earth, Energy & Environmental Sciences. “U.S. grasslands are way more sensitive to vapor pressure deficit, 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.”
In the study, co-authored by Gentine and Alton Park Williams, an assistant research professor at Columbia’s Lamont-Doherty Earth Observatory, the researchers analyzed the conditions under which grassland plants open and close their stomata—microscopic openings on leaves that enable the transfer of water vapor, oxygen, and carbon dioxide. When the stomata are open, a plant can take up CO2 from the atmosphere to make energy, but it risks losing water in dry conditions. The strategy different plants use—whether to risk drying out in order to keep taking up carbon dioxide, or to close up and stop growing—affects their productivity. A plant’s behavior depends on the amount of water in the atmosphere as measured by atmospheric dryness: higher atmospheric dryness indicates greater potential for dry air to pull moisture out of the plant.
Variability in drought response
Using statistical analysis and remote sensing observations, the researchers separated out the effect of plants’ behavior from the impacts of regional drought conditions, such as differences in precipitation or temperature. Although many climate models treat all grasslands the same, the study revealed large variability in terms of how they respond to drought.
“More than the plant type, it is the plant physiology that seems to regulate their response to drought and heat wave,” said Gentine, an associate professor of earth and environmental engineering at Columbia Engineering.
The researchers carried out their analysis using publicly available remote sensing satellite data from 1981 to 2013 displaying plant greenness, which is an indicator for plant productivity. They then combined that data with climate and observational precipitation datasets to divide American grasslands into different regions, depending on their behavior.
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. Grasslands host a variety of biodiversity and provide an important habitat for livestock in the meat and dairy industries, covering about 26 percent of the United States and nearly 20 percent of the planet’s land surface; they are the largest land cover type on Earth.
“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.”
Analysis shows grasslands that respond to drought by keeping their stomata open (anisohydric behavior) are more sensitive to dryness of the atmosphere than those that regulate and close their stomata and stop growth to save water (isohydric behavior). Both behaviors are present in the U.S. The study shows that plants that keep their stomata open are more damaged by drought in U.S. grasslands because it suppresses their growth over the course of a growing season.
“Grasslands are really interesting because they show such a huge diversity in that isohydricity (plant physiological) behavior,” said Konings, who conducted initial research for the study at Columbia Engineering before joining Stanford Earth. “They have really different strategies in how they respond to drought.”
While previous methods for understanding drought response entailed on-the-ground measurements, the new metric enables researchers to measure these patterns across the globe. Konings said she hopes the method can be used to see if the study’s findings about grasslands can be applied to other ecosystems or to better understand what causes some trees to die in response to warming conditions while others do not.
“I think there’s a lot still to be done with this metric,” Konings said.
Gentine and Konings plan to continue their collaboration. “We’re starting to look at forests now to understand how those ecosystems are responding to either atmospheric dryness or soil moisture drought,” Gentine said. “How they respond will have widespread implications for the continental carbon storage in future climate.”