Aridity, rather than nitrogen supply, drives water use efficiency in global forests, new study finds

Plants greatly influence our climate. In Northern Africa around 6,000 years ago, variations in weather patterns led to decreased water availability for vast savannas, which, in turn, led to less water restored to the atmosphere through plant transpiration. Due to this negative cycle, what was once a verdant landscape became the Sahara Desert, the largest hot desert on the planet.

New research published in Nature Communications has found that water availability in soil is more important than nitrogen for plant water use efficiency, deepening our understanding of how various environmental factors affect plant development, especially with rising levels of carbon dioxide. The project is a continuation of prior research, which examined how rainfall and rising CO2 affect water use efficiency in global forests.

“We know that water use efficiency, or carbon gain per unit of water loss, varies dramatically across the world's forests, due in part to aridity,” said Tom Buckley, an associate professor in the Department of Plant Sciences at the University of California, Davis, and a researcher on the project. In total, the team analyzed climate data pulled from tree rings collected in 349 distinct geographical areas.  

Initially, the team expected to quantify the importance of nitrogen for water use efficiency. The nutrient is crucial for the enzymes of photosynthesis. Instead they found that the chemical element was less important than originally thought. This was especially true for more industrialized areas, where there has been significant nitrogen deposition from industrial pollution. "We thought differences in nitrogen deposition might drive global variation in water use efficiency, but our results suggest otherwise."

But the researchers found a surprising quirk in the results: the importance of aridity – the balance between rainfall and evaporation from the landscape – abruptly vanished below a certain level of aridity. Buckley designed a mathematical model to help explain this discovery in terms of how plants adapt to local environmental conditions.

“In arid places, where the soil is dry, leaf pores called stomata will close to conserve water, which increases water use efficiency," Buckley said. "But in places where rain provides more water than the sun can evaporate, the soil tends to be saturated with water, so trees don't benefit from any more rainfall."

These findings, combined with the team's other recent work, will be especially significant for predicting how forests affect global water cycles and climate change, further explained Buckley.

“Hydroclimate models often assume water use efficiency will increase as atmospheric CO2 rises, and decrease as rainfall increases,” Buckley said. "Our results suggest both of those responses may reach a saturation point faster than previously thought."

The research team was led by Mark Adams, a professor of bioscience and innovation at Swinburne University of Technology, Melbourne. In addition to Buckley, Adams was joined by his Swinburne colleague Tarryn Turnbull, as well as Dan Binkley of Northern Arizona University, Flagstaff, and Mathias Neumann of the University of Natural Resources and Life Sciences, Vienna for the research project.