Study Highlights Importance of Plants in Water Cycle

The view from Guanica Biosphere Preserve, Puerto Rico. (Kevin Krajick/Earth Institute)

(CN) – Anticipating how rising atmospheric carbon dioxide will affect the water cycle, including extreme weather forecasts, is critical to preparing for and predicting the future of the planet.

It is widely thought that changes to the water cycle are sparked by precipitation and radiation shifts produced by climate change and counterbalanced by land surface adjustments that make the planet drier.

But a study published Monday in the journal Proceedings of the National Academy of Sciences finds that vegetation plays a key role in Earth’s water cycle and that plants will regulate and control the mounting stress placed on continental water resources in the future.

“Our finding that vegetation plays a key role future in terrestrial hydrologic response and water stress is of utmost importance to properly predict future dryness and water resources,” said co-author Pierre Gentine, a professor of earth and environmental engineering at Columbia University. “This could be a real game-changer for understanding changes in continental water stress going into the future.”

Gentine’s team is the first to distinguish the response of vegetation from the global warming total complex response, which includes variables for the water cycle like soil moisture, runoff and evapotranspiration. By separating the vegetation reaction to the global rise of CO2 from the atmospheric response, they were able to quantify it and determined that vegetation actually is the dominant factor foreshadowing future water stress.

“Plants are really the thermostat of the world,” said lead author Leo Lemordant, Gentine’s doctoral student. “They’re at the center of the water, energy, and carbon cycles. As they take up carbon from the atmosphere to thrive, they release water that they take from the soils. Doing that, they also cool off the surface, controlling the temperature that we all feel.

“Now we know that mainly plants – not simply precipitation or temperature – will tell us whether we will live a drier or wetter world.”

The team examined Earth system models with decoupled surface and atmospheric CO2 responses and used a multi-model statistical analysis from CMIP5, the most cutting-edge set of coordinate climate model experiments which are part of an international cooperation project for the International Panel on Climate Change.

They incorporated three runs: a control run with CO2 at the leaf level and in the atmosphere, a run where only the atmosphere reacts to the CO2 spike, and one in which only vegetation responds to the rise in CO2.

The team’s findings demonstrate that changes in important water-stress variable are strongly adjusted by vegetation physiological effects in response to elevated CO2 at the leaf level, explaining how significantly the physiological effects produced by rising atmospheric CO2 affect the water cycle.

The CO2 physiological response has a dominating role in evapotranspiration and has a key impact on long-term runoff and soil moisture compared to precipitation or radiative adaptations due to increased atmospheric CO2.

“This work highlights an important need to further study how plants will respond to rising atmospheric carbon dioxide,” said James Randerson, professor of earth system science at the University of California, Irvine, who was not involved in the study.

“Plants can have a big effect on the climate of land, and we need to better understand the ways that they will respond to carbon dioxide, warming, and other forms of global change.”

The report highlights the crucial role of vegetation in directing future terrestrial hydrologic response and underlines that the continental water and carbon cycles are closely coupled over land and must be examined as an interconnected system. The paper also emphasizes that hydrologists should work with climate scientists and ecologists to better anticipate future water resources.

“The biosphere physiological effects and related biosphere-atmosphere interactions are key to predicting future continental water stress as represented by evapotranspiration, long-term runoff, soil moisture, or leaf area index,” said Gentine.

“In turn, vegetation water stress largely regulates land carbon uptake, further emphasizing how tightly the future carbon and water cycles are coupled so that they cannot be evaluated in isolation.”

The team plans to further analyze the various physiological effects.

“The vegetation response is itself indeed complex,” Gentine said, “and we want to decompose the impact of biomass growth vs. stomatal response. There are also implications for extreme heatwave events we are currently working on.”


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