Transpiration – the evaporation of water from plants –  is one of the dominant forces in the Earth’s water cycle. To get a sense of how it will change in response to rising CO2 concentrations, we need to better understand the role played by plant stomata in regulating this flux.

This information can then be factored into climate models to improve our projections of future climate change on the water cycle and the potential knock-on impacts across the rest of the climate system.

A key to our understanding of this process is insight into the thickness of the layer of air which surrounds the leaf. Water leaving the plant through the stomata (transpiration) must pass through this layer of air. This layer of air is affected by properties of the leaf such as leaf size and shape, but also factors such as wind speed and the height of the vegetation.

When water diffuses rapidly through this layer of air, climate researchers describe this as the leaf surface as being “well coupled” to the atmosphere and transpiration is said to be strongly controlled by stomata. In contrast, when the leaf surface is said to be decoupled (or poorly coupled) from the surrounding atmosphere, transpiration is more strongly controlled by the available energy (radiation).

To get an insight into the difference between this interaction in the real world and how it is represented in climate models the researchers first performed an extensive literature review to see previous estimates of these impacts.

They then compared these representations with estimates derived from FLUXNET data, which consists of massive observational datasets taken from networks around the world that record real world exchanges of carbon dioxide, water vapour and energy between plants and the atmosphere.

When they compared the results derived from this observational network with the results synthesised from the literature, the researchers found substantial differences.
As a result of this work they have suggested a new benchmarking metric that could be used to test existing hypotheses embedded in climate models. They have also mapped a path forward involving using further detailed observations that can then be used to improve the way coupling/decoupling processes are currently represented in climate models.

Paper: De Kauwe, M. G., Medlyn, B. E., Knauer, J., and Williams, C. A.: Ideas and perspectives: how coupled is the vegetation to the boundary layer?, Biogeosciences, 14, 4435-4453, https://doi.org/10.5194/bg-14-4435-2017, 2017.