Rising Carbon Dioxide Is Helping Central China’s Ecosystems Squeeze More Growth From Every Drop of Water

Every leaf on Earth performs a delicate balancing act. To pull carbon dioxide out of the air and turn it into plant tissue, vegetation must open tiny pores on its surfaces, and every time those pores open, water escapes. The ratio between the carbon a plant gains and the water it loses is known as ecosystem water use efficiency, and it is one of the most important numbers in climate science, linking the global carbon cycle to the global water cycle. A new study of Henan Province in central China, published in Theoretical and Applied Climatology, has now tracked this efficiency across nine years and found a clear and encouraging trend: the region’s ecosystems have been getting steadily better at producing carbon for every kilogram of water they expend.

The research, led by Jingru Zou of Henan University together with Fuqian Zhou and Jinrui Zhu, focused on Henan, a densely populated agricultural heartland that sits at the crossroads of China’s major farming zones. The team drew on satellite data from the MODIS instruments, which estimate both gross primary production, the total amount of carbon that vegetation fixes through photosynthesis, and evapotranspiration, the combined loss of water from soil evaporation and plant transpiration. Dividing one by the other yields water use efficiency, expressed here in grams of carbon fixed per kilogram of water consumed per year.

Across the study period from 2008 to 2016, the mean annual water use efficiency of the region came out at 1.99 grams of carbon per kilogram of water per year. Perhaps surprisingly, given how varied Henan’s landscapes are, from intensively cultivated croplands to woodlands and grasslands, the differences between vegetation types were not statistically significant. Whatever the plants were, crops or trees or grasses, they operated at broadly similar levels of carbon-per-water performance, suggesting that regional climate conditions exert a strong common influence across the province’s ecosystems.

The more striking finding was the trend over time. Water use efficiency rose significantly during the nine-year window, at a rate of 0.13 grams of carbon per kilogram per year, and the increase was consistent across all the vegetation types examined. In other words, the entire regional mosaic of ecosystems was drifting in the same direction, becoming more productive relative to its water consumption. Because Henan is a major grain-producing province where water resources are already under pressure from irrigation demands and groundwater depletion, an improvement in natural water use efficiency carries real practical significance for regional water planning.

To understand what was driving the change, the researchers turned to pathway analysis, a statistical technique that traces how individual environmental variables influence water use efficiency both directly and indirectly through its two components, gross primary production and evapotranspiration. Their analysis showed that changes in water use efficiency in the region are jointly controlled by both terms of the ratio: shifts in carbon uptake and shifts in water loss each contribute to the overall pattern. That dual control matters, because it means any driver that affects photosynthesis or transpiration can ripple through to the efficiency metric.

When the individual drivers were separated, two emerged as statistically significant. Precipitation had a significant negative effect on water use efficiency, a result that may seem counterintuitive at first but follows from the structure of the ratio. When more rain falls, soils stay wetter and evapotranspiration tends to increase, often outpacing any corresponding gain in carbon uptake, so the carbon-to-water ratio declines. Conversely, in drier conditions plants tend to conserve water by closing their stomata, and the efficiency metric can rise. The finding aligns with a broader body of work showing that water availability shapes ecosystem efficiency in complex, sometimes opposing ways depending on the timescale and the aridity of the environment.

The second significant driver was atmospheric carbon dioxide, which had a clear positive effect on water use efficiency. This reflects one of the best-documented physiological responses in plant science: as the concentration of carbon dioxide in the air rises, plants can achieve the same carbon gain with less water loss, because the gradient driving carbon into the leaf strengthens while the stomata can partially narrow. This so-called carbon dioxide fertilization effect has been observed in forests across the northern hemisphere and in global satellite-based analyses, and the Henan study confirms that it is operating measurably at the ecosystem scale in central China.

Just as revealing were the factors that did not register as significant. Temperature, vapor pressure deficit, solar radiation, and the leaf area index all failed to show a statistically significant effect on water use efficiency in this region during the study period. Vapor pressure deficit, the difference between how much moisture the air holds and how much it could hold, is widely recognized as a powerful driver of transpiration, and recent global work has shown that rising atmospheric dryness can suppress vegetation growth and reduce efficiency. That it did not emerge as a dominant factor here suggests that, over this particular window and in this particular climate, the carbon dioxide signal and the precipitation signal outweighed the influence of atmospheric dryness and energy availability.

Synthesizing these results, the authors conclude that the observed increase in water use efficiency in central China arises from a combination of precipitation and carbon dioxide effects, with the rising concentration of atmospheric carbon dioxide playing the leading role. The study adds a valuable regional data point to a growing global picture in which terrestrial ecosystems, buoyed by carbon dioxide fertilization, are fixing more carbon per unit of water than they did decades ago. Whether this trend can continue is an open question, since the fertilization effect is expected to saturate over time while warming continues to increase the atmosphere’s demand for water.

For Henan Province, the findings carry a note of cautious optimism. A region that must feed a large population while managing strained water resources is, at least for now, seeing its ecosystems quietly become more water-frugal. The study also demonstrates the power of pairing freely available satellite products with pathway analysis to disentangle the competing influences of climate and rising carbon dioxide, an approach that can be extended to other regions where the balance between carbon gain and water loss will shape how ecosystems weather the decades ahead.

Subject of Research: Spatiotemporal variability and drivers of ecosystem water use efficiency in central China

Article Title: Spatiotemporal variability in ecosystem water use efficiency and its driving factors during 2008–2016 in central China

Article References: Zou, J., Zhou, F., & Zhu, J. (2026). Spatiotemporal variability in ecosystem water use efficiency and its driving factors during 2008–2016 in central China. Theoretical and Applied Climatology, 157(10), Article 686. https://doi.org/10.1007/s00704-026-06625-w

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06625-w

Keywords: water use efficiency, gross primary production, evapotranspiration, carbon dioxide fertilization, MODIS, Henan Province, precipitation, vapor pressure deficit, remote sensing, climate change, ecosystems, China