The Stomata Stalemate: A Global Phenomenon in Plant Life

A recent study in Science reveals that as temperatures increase due to climate change, the thousands of microscopic pores known as stomata on the underside of leaves are becoming narrower. This is hindering plants’ ability to absorb carbon dioxide and water vapor.

Gram for gram, a plant loses far more water every day than any terrestrial animal—99% of the water taken in by roots is released into the air as water vapor. While the stomata are open, water vapor travels out, and carbon travels in. The ratio of carbon assimilation per unit of water loss is called water use efficiency, and the new research says that globally, it has stalled.

In the past, scientific experts believed that as emissions continue to rise, water usage efficiency would also improve. This was based on the idea that with higher levels of atmospheric carbon, more of it would enter plant stomata.

According to Jingfeng Xiao, an Earth systems scientist at the University of New Hampshire and coauthor of the study, their findings are distinct from previous research. They have determined that the overall plant water use efficiency has reached a steady state on a global level.

The Narrative Shift Caused by Vapor Pressure

This is due to the fact that carbon emissions occur within a complex system, rather than in isolation. Vivek Arora, a climate scientist at Environment and Climate Change Canada who was not involved in the study, stated that not only is carbon dioxide levels rising, but also temperature and air dryness, which is where vapor pressure deficit plays a role.

The vapor pressure deficit refers to the variation between the present level of water vapor in the atmosphere and the maximum capacity it can hold. While warmer air has the potential to contain more water vapor, it does not guarantee that it will.

According to Fei Li, a coauthor and Earth scientist at the Chinese Academy of Agricultural Sciences, a high vapor pressure deficit indicates a dry atmosphere. This can result in a loss of water, causing the stomata to partially close in order to limit water loss.

However, when plants close or partially close their stomata, they also decrease their capacity to absorb carbon.

The researchers examined the efficiency of global water usage by utilizing 24 machine learning methods to estimate information from FLUXNET sites located worldwide. According to Xiao, FLUXNET instruments track the movement of carbon dioxide and water between ecosystems and the atmosphere. He noted that this trend was observed in all models: an increase in water use efficiency from 1982 to 2000, followed by a plateau, despite the continued rise of carbon emissions in the atmosphere.

According to Arora, the researchers’ methodology in the study is satisfactory, even though no model is flawless. “It is important to thoroughly investigate, and the use of 24 machine learning algorithms by the authors gives assurance in the accuracy of these findings.”

Nowhere to Store Carbon Emissions

According to Arora, while the land and ocean currently absorb approximately 50% of carbon emissions, increasing vapor pressure deficits could cause plants to close their stomata in order to conserve water. This could potentially restrict the amount of carbon that plants are able to absorb. The latest findings suggest that this behavior began more than two decades ago, as a way for the countless plants in the terrestrial biome to adapt to the water loss caused by climate change.

According to Arora, if the absorption of carbon by land decreases, it will lead to an acceleration of climate change. This is because there will be an increase in the amount of carbon dioxide remaining in the atmosphere. The goal is to predict the future concentration of carbon dioxide if we continue emitting at the current rate.

The findings also hold significance in comprehending the patterns observed in Earth’s evolving terrains. “In recent years, there have been numerous studies suggesting an increase in regional and continental runoff,” stated Xiao. Certain scientists have suggested that the rise in global water use efficiency resulted in more water being cycled through, ultimately leading to the increased runoff. However, the latest discoveries indicate that water use efficiency has plateaued, indicating that there must be another explanation for the trend.

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Reference: Shepherd, E. (2023), Plants around the world experience a stalemate in stomata, Eos, 104, https://doi.org/10.1029/2023EO230378. Published on October 5th, 2023.

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