Increase in Ecosystem Water-Use Efficiency as Atmospheric Carbon Dioxide Concentrations Rise

Note: We discussed the response of water use efficiency to drought in our previous blog (http://www.lpicea.com/responses-of-ecosystem-water-use-efficiency-to-water-supply-condition-in-tibetan-plateau/). Here we discuss the response to CO₂ enrichment.

Vegetation, as an essential part of terrestrial biosphere, plays an important role in regulating carbon and water exchange between land and air. The process of carbon capture in plants mainly occurs during the process of photosynthesis in leaves, and photosynthesis itself being a process in which water and carbon participate and regulate each other. The amount of photosynthates produced by consuming a unit of water is called plant water use efficiency (WUE). WUE reflects the relationship between photosynthetic carbon capture and water consumption of plants, and is an important index to measure the coupling of carbon and water in surface ecosystem.

At leaf scale, instantaneous water use efficiency is defined as the ratio of photosynthetic rate to transpiration rate. Similarly, water use efficiency at the ecosystem scale is defined as the following narrative:

At present, it is generally believed that an increase in atmospheric CO₂ concentration can produce “fertilization effect” on terrestrial ecosystem, that improves plant photosynthetic rate and increases plant water use efficiency. On the one hand, CO₂ is an important substrate for vegetation photosynthesis. Additionally, increased CO₂ concentration can improve the carboxylation rate of photosynthase (Rubisco enzyme) and inhibit the oxidation reaction of RuBP, thus promoting the accumulation of photosynthates in plants. On the other hand, the increase of CO₂ concentration reduces the opening degree of vegetation stomata, reduces the water consumption of vegetation, and thus improves the water use efficiency of vegetation.

Free-air CO₂ enrichment experiments (FACE), flux site data and satellite remote sensing observation all show that the increase of CO₂ concentration promotes photosynthesis of plants and enhances the function of terrestrial carbon sink. Ainsworth, E.A. & Long, S.P. (2005) showed that the stomatal conductance of plants decreased by about 20% at high CO₂ concentration (475ppm-600ppm). Keenan et al. (2013) found that the water use efficiency of temperate and boreal forests in the northern hemisphere has been greatly improved in the past two decades due to the effect of CO₂ fertilization. De Kauwe et al. (2013) used 11 existing models to simulate the results of forest FACE experiments, and the results showed that there was a positive correlation between CO₂ concentration and WUE. Based on satellite observation data and runoff records, Ukkola et al. (2016) found that the increase of WUE caused by the increase of CO₂ concentration will also have an impact on hydrological cycle and runoff on a large scale.

Although the increase of CO₂ concentration promotes the carbon sequestration capacity and water use efficiency of ecosystem in the short term, the long-term effect of this promotion is unknown. Whether the “fertilization effect” is limited by nutrients and other factors, and whether plants will adapt to the continuous increase of high-concentration atmospheric CO₂, is not full understood.

Thus far, understanding the global impact of the continuous increase of atmospheric CO₂ concentration observed in field experiments is limited because they are aimed at a single vegetation type or done at local scale, with difficulty to scale up to the large ecosystem scale. Going forward, it is suggested that the effects of increased CO₂ concentration on water use efficiency and carbohydrate cycle in terrestrial ecosystems can be better understood and predicted by combining spatio-temporal scale deduction with individual plant responses.

Reference：

Ainsworth, E. A. & Long, S. P. What have we learned from 15 years of free-air CO₂ enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO₂. New Phytol 165, 351–372 (2005).

Keenan, T. F. et al. Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise. Nature 499, 324–327 (2013).

Kauwe, M. G. et al. Forest water use and water use efficiency at elevated CO₂: a model-data intercomparison at two contrasting temperate forest FACE sites. Global Change Biol 19, 1759–1779 (2013).
Ukkola, A. M. et al. Reduced streamflow in water-stressed climates consistent with CO₂ effects on vegetation. Nat Clim Change 6, 75–78 (2016).