Vegetation is the main regulator of water, carbon and energy fluxes between land and atmosphere via transpiration and photosynthesis. Under conditions of soil dryness or extreme heat, plants close their stomata, i.e. the cells that mediate these exchanges. Here we refer to this process as vegetation stress. The consequent reductions in transpiration and photosynthesis imply changes in the water, carbon and energy cycles. Conversely, extreme stress can cause plant mortality and increase the risk of fires and pathogen outbreaks. This makes vegetation stress a key feature to determine terrestrial carbon uptake, land water availability, land-atmosphere feedbacks, and even general climate dynamics. However, Land Surface Models (LSMs) parameterize the detrimental effects of soil-water deficit (water stress) and extreme temperatures (heat stress) on transpiration and photosynthesis very crudely, usually based on empirical relationships resulting from a handful of past experiments. This leads to errors in model predictions of energy, carbon and water budgets.
However, meeting the seeming requirement of accurate large-scale observations of plant stress may already be possible. The GOME-2 instrument (onboard MetOp and OCO-2) can sense chlorophyll fluorescence, emitted by the chemical reactions that occur during photosynthesis, thus it is (a priori) sensitive to plant stress. Recent studies concentrated on investigating forest primary production using GOME-2 data. Here, we propose a different use: to uncover how vegetation stress impacts the ecosystem's transpiration.
In particular, we will: (a) explore the use of GOME-2 fluorescence to develop a vegetation stress dataset; (b) validate our observations and estimates against in-situ measurements of fluorescence, and compare them to transpiration, photosynthesis and soil moisture from three distinct ecosystems: Amazonia, Eastern Australia and Sahel; (c) disentangle the drivers of large-scale fluorescence using radiation (CERES), satellite soil moisture (AMSR-E, ASCAT, SMOS, SMAP), vegetation water content (AMSR-E, SMAP) and land-surface temperature (MODIS); (d) apply the new stress observations into GLEAM, a simple land surface model designed to derive transpiration estimates from satellite data; (e) develop a new version of the GLEAM global transpiration dataset. Results will progress towards improving the understanding of large-scale ecosystem transpiration and the implications of vegetation stress for global biochemistry, hydrology and climate. Finally, STR3S goals are in line with the European Space Agency (ESA) top priorities, in anticipation to the launch of the fluorescence-dedicated FLEX mission.
