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

The study, published in Nature Plants, brings together results from 19 CO₂-enrichment experiments conducted in glasshouses, growth chambers and field environments. The experiments included plant communities ranging from grass-dominated ecosystems to multispecies assemblages. By comparing plants grown alone with plants grown in mixtures, the researchers examined a question that has often been overlooked in climate-change experiments: does elevated CO₂ help all species equally, or does competition determine which plants are able to use the additional carbon?

The contrast was striking. When plants grew without neighbours, most species responded positively to elevated CO₂, or eCO₂. This response is biologically plausible because CO₂ is the raw material plants use during photosynthesis. Inside leaves, carbon dioxide is fixed into sugars through the Calvin cycle, providing the carbon skeletons needed to build new tissues. Higher atmospheric CO₂ can also reduce the amount of water plants lose while taking up carbon, because leaf pores known as stomata may not need to remain as open. Under controlled conditions, these effects can translate into faster growth and greater biomass.

Yet those benefits often disappeared when plants had to share their environment. More than half of the species examined showed neutral or negative responses to eCO₂ when growing in mixtures. In other words, a plant that grew better under elevated CO₂ by itself was not necessarily able to capitalize on the same conditions when surrounded by other species. The finding challenges the tendency to use single-species experiments as a guide to how natural plant communities will respond to a carbon-rich atmosphere.

Competition can block the conversion of extra carbon into additional growth in several ways. Plants may compete for sunlight, water, nitrogen and phosphorus, all of which are required to turn photosynthetic products into leaves, stems, roots and seeds. A plant may absorb more CO₂ and produce more sugars, but if nutrients are scarce, it may be unable to construct the proteins and tissues needed to use that carbon. Similarly, a taller or faster-growing neighbour may capture the available light before a smaller species can benefit, creating a physical barrier between elevated CO₂ and actual plant growth.

The community-level result was more subtle than a simple collapse in productivity. Although many individual species experienced neutral or negative responses, dominant species often drove modest increases in total mixture biomass under eCO₂. This means a plant community can appear to benefit from elevated CO₂ even while many of its members do not. A rise in total biomass therefore does not necessarily indicate that growth has been shared evenly, or that the community has become more productive in a way that benefits all species.

That distinction could have major implications for biodiversity. If elevated CO₂ consistently favours species that are already dominant, they may capture an even larger share of light, nutrients and space. Subdominant species could then face stronger suppression, reducing their growth and potentially altering the composition of plant communities over time. The result may be a greener-looking ecosystem with fewer species, rather than a uniformly healthier or more productive one.

The researchers’ trait analysis offers a clue about why some plants succeed while others fall behind. Acquisitive strategies, associated broadly with rapid resource capture and fast growth, were better predictors of performance under competition than a species’ intrinsic sensitivity to CO₂ alone. This suggests that the key question is not simply whether a plant can respond physiologically to elevated carbon dioxide. It is whether the plant can acquire enough light, water and nutrients quickly enough to turn that response into a competitive advantage.

The findings also expose a limitation in how climate models and ecological forecasts often treat CO₂ fertilization. If models assume that species respond independently, they may overestimate the benefits that rising CO₂ will deliver to plant communities. Real ecosystems are networks of interactions, and the outcome for one species depends partly on the traits and abundance of its neighbours. The same atmospheric change can therefore stimulate growth in an isolated plant, suppress a subordinate competitor and produce only a small increase in total community biomass.

As atmospheric CO₂ continues to rise, understanding this uneven distribution of benefits will become increasingly important. The new synthesis does not suggest that elevated CO₂ has no effect on vegetation. Instead, it shows that its effects are filtered through competition. Dominant plants may gain enough to lift overall biomass, while many less competitive species receive little benefit or even lose ground. The future of plant communities may therefore be shaped not by a universal CO₂ boost, but by an intensified struggle over which species can claim it.

Subject of Research: The effects of elevated carbon dioxide on plant growth and competition in multispecies communities.

Article Title: Interspecific competition negates CO₂ benefits for most plant species.

Article References: Raubenheimer, S.L., Simpson, K.J., Ripley, B.S. et al. “Interspecific competition negates CO₂ benefits for most plant species.” Nature Plants (2026). https://doi.org/10.1038/s41477-026-02360-2

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41477-026-02360-2

Keywords: Elevated CO₂, climate change, plant competition, biodiversity, plant communities, ecosystem productivity, dominant species, subdominant species, CO₂ fertilization, plant traits