Trees Use Less Water with More CO2 Emissions

Scientists are increasingly worried for the Earth's forests as climate change stresses out plants with warming temperatures, affecting their growth and development. But new research may offer hope, as trees apparently cope by using less water with more carbon dioxide (CO2) emissions in the air.

Increased atmospheric CO2 concentrations have already caused large-scale physiological responses of European forests. In particular, the efficiency of water-use of trees, which is coupled to the uptake of CO2 during photosynthesis of leaves and needles has changed significantly. According to the study, published in the journal Nature Climate Change, European broadleaf and coniferous trees have increased their water-use efficiency since the beginning of the 20th century by 14 percent and 22 percent, respectively.

During the natural process of photosynthesis, trees take up CO2 from the air and in turn lose water vapor (H2O) through tiny pores of their leaves or needles, so-called stomata. This gas exchange between trees and the atmosphere is regulated through the opening widths (aperture) of their stomata. Although wider apertures of the stomata allow plants to absorb more CO2 molecules, it promotes an increased loss of water vapor (transpiration) into the atmosphere. Meanwhile, the opposite holds for narrowed apertures.

"Assuming that the trees demand for CO2 does not change, they can reduce the aperture of the stomates of their leaves and needles under increasing atmospheric CO2 concentrations. This should lower the rates of transpiration and minimize the tree's water loss," study co-author Gerhard Helle, at the GFZ German Research Centre for Geosciences, said in a statement. "Nevertheless, a 5% increase in European forest transpiration was calculated over the twentieth century. This can likely be attributed to a lengthened growing season, increased transpiration due to a warmer environment, and an enhanced leaf area."

These findings can help scientists better understand how Earth's changing climate will impact forests in the future, and in addition help improve model scenarios of future climate development and provide a more reliable assessment of the global water cycle.

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