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Is the water use efficiency of land plants changing?

Prof Emanuel Gloor (SoG), Dr Roel Brienen (SoG)

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The globe’s vegetation is closely intertwined with the global carbon and hydrological cycles, and changes in plant functioning can thus have substantial consequences for climate. Plants fix atmospheric CO2 via photosynthesis, and release carbon to the atmosphere when respired or through death of plant material. Imbalances between these two processes have the potential to affect atmospheric CO2 levels substantially, for example through increases in total plant mass. Such changes can be relevant at the global scale because the total amount of organic carbon in vegetation and soils is several times larger than the amount of CO2 in the atmosphere.

Uptake of atmospheric CO2 by plants is via stomata, which are minute, valve-like openings on leaves. Stomata not only regulate CO2 uptake but also control water. Water from the soil is pulled to the leaves and lost to the atmosphere via transpiration through stomata. Plants thus act as water pumps and recycle several times per year the amount of water in the atmosphere, thereby affecting the hydrological cycle.

Growing conditions on our planet are changing fast. The key ingredient for plant growth, atmospheric CO2, has increased by nearly 50 percent compared to pre-industrial levels and keeps raising. Because of greenhouse warming land temperatures steadily increase, and precipitation and humidity patterns are changing too. It is thus expected that plants will react and adapt their habits of functioning. One main expected effect is increases in plant water use efficiency. Plant water use efficiency is the amount of carbon gained by plants per amount of water that is lost. Due to increased levels of atmospheric CO2 plants need less opening(s) of stomata to obtain the same amount of carbon, or alternatively plants may not change their stomatal opening(s), thus possibly leading to greater carbon uptake. Both changes are observed: plants change stomatal conductance under higher CO2 and plants grow faster under higher CO2.

Such changes could significantly affect the carbon and hydrological cycle, for example increases in plant productivity could slow down the rate of atmospheric CO2 increase, while decreases in recycling of soil water back into the atmosphere affects precipitation. It is thus of great interest to have a clear understanding whether water use efficiency is indeed changing. 

Various methods have been employed to measure changes in water use efficiency. One of the methods includes the use of carbon isotopes. Plants discriminate against 13CO2, and the level of discrimination depends on how widely stomata open on average. Time trends of 13CO2 discrimination, from e.g. tree rings, or herbarium material, are therefore indicative of plant responses over time to e.g. CO2 increases. 

This project aims to produce firm conclusions about changes in water use efficiency in vegetation by using three lines of investigation which when combined together should provide a more convincing assessment of this question than has been possible sofar. 

Firstly, we propose to analyse precipitation and riverine discharge records of selected large basins in the world, like the Amazon basin, to establish to what extent changes in discharge and precipitation patterns are consistent with changes in forest water use efficiency.

Secondly, we propose to explore what the global atmospheric 13CO2 record reveals about changes in vegetation isotopic discrimination over. Atmospheric 13CO2 is steadily decreasing because fossil fuel 13CO2 is depleted but the rate of decrease seems to be smaller than expected based simple carbon cycle box models of atmosphere, oceans and land vegetation. We propose to follow up on these analyses using similarly simple models.

Finally, while naïve use of tree core based isotope records will not yield reliable estimates of water use efficiency changes a more sophisticated approach may be feasible. Trying to devise and apply more reliable methods based on such data, possibly also involving herbarium and/or a field work component.

You will work under the supervision of a strong team of earth system dynamics experts from the Schools of Geography and Earth and Environment. Applicants should have a strong interest in global environmental problems, and preferably some background in a quantitative science (math, physics, engineering, environmental sciences). 

Click here for a full project description.

Related undergraduate subjects:

  • Applied mathematics
  • Atmospheric science
  • Earth science
  • Ecology
  • Environmental biology
  • Environmental science
  • Geophysical science
  • Natural sciences
  • Physical geography
  • Plant science