Published on 21 March 2022
Vegetation does not only release oxygen
Vegetation releases volatile organic compounds (VOCs), in other words, gaseous molecules comprising carbon and hydrogen atoms. At a global level, biogenic VOC emissions dominate anthropogenic emissions by a factor of ten. Among BVOCs (Biogenic Volatile Organic Compounds), one specific compound is released in much higher proportions than the others: isoprene (C5H8). It is estimated that 350-800 millions of tons of isoprene are emitted worldwide per year. However, due to its short lifetime (about 1 hour) and extreme variability of its concentration in the atmosphere, the accurate appraisal of isoprene emissions is extremely challenging.
Although harmless in itself, isoprene contributes to the formation and growth of fine particles and in polluted areas, to ozone. Isoprene is relatively ubiquitous, but is mainly emitted by trees under warm and sunny conditions, especially in the Tropics. Emissions also depend on the type and abundance of vegetation or the water availability in soil, among others.
Because of its significance for air quality and climate, scientists have been developing modelling tools to study this molecule. Within the STEREO III project ALBERI (Assessing Links between Biogenic Emissions and Remotely-sensed photosynthesis Indicators), jointly carried out by the Royal Belgian Institute for Space Aeronomy, the H-Cel lab of the Ghent University in partnership with the Department of Earth System Science at the University of California, the goal is to better describe the response of isoprene emissions to vegetation changes and to droughts, the focal point of this exploration project. So let’s have a look at how dry conditions affect isoprene.
An insight into the tools and knowledge
Without satellite observations of isoprene, laboratory and measurement campaigns in natural conditions provide valuable insights but lack the global picture. To study isoprene, we use biogenic emission models and satellite observations of formaldehyde (HCHO), a gas that forms mainly as a result of oxidation of isoprene in the atmosphere. A sophisticated model developed by the project experts allows to link isoprene to HCHO by accounting for the relevant reactions and transport processes of chemical compounds.
Global distribution of modelled isoprene fluxes in July 2018. The Tropics contribute 80% to the total annual flux.
Laboratory studies have shown that, when plants suffer moderate water stress due to limited water availability in soils, isoprene emissions increase. In order to mitigate water losses through transpiration, plants partially close their stomata (tiny cavities on their leaves for exchange with the atmosphere) which increases the leaf temperature, and consequently increases isoprene production. Synthesizing isoprene is rather expensive in terms of energy cost for plants so it must be worth the trouble. While certain BVOCs, such as monoterpenes, are associated to the pleasant smells of pine and lemon scents, the primary role of isoprene is to protect the plants against thermal damage. Severe or prolonged drought leads to decreased isoprene emission due to reduced photosynthesis. Besides, certain plants are better drought-adapted than others.
Challenge: what drives the isoprene response to drought?
Laboratory studies allowed to establish a simple, linear link between isoprene and water stored in soils. In recent years, isoprene fluxes at the top of a 32-m tower overlooking a forest in central U.S. were monitored experimentally for two consecutive summers concomitant to two drought events. Those observations were used to improve the description of drought in the emission model. The generalization of this update at larger scales was found difficult, however, as it relies on very uncertain inputs such as the threshold point that marks the onset of isoprene reduction and the highly heterogeneous properties of the soil. Moreover, one study site is not representative of the diverse environments that occur worldwide.
Satellite observations of formaldehyde observed by the TROPOMI instrument in July 2018. Enhanced columns correspond to the isoprene-emitting regions.
Other possible avenues should be explored. Those include the potential of satellite solar-induced chlorophyll fluorescence (i.e., radiation that plants emit when they are photosynthetically active) for drought detection, and the use of new isoprene measurements. Isoprene is indeed now observed from space, and in Belgium, isoprene emissions will be continuously monitored from next June 2022 at the Vielsalm site in the Belgian Ardennes. Research to be continued…
ALBERI, a springboard for future research
Our knowledge on isoprene emissions allows us to better assess the formation of ozone and fine particles and their impact on air quality and climate. In a future when the world will be experiencing more frequent and intense drought, it is one of our primary concerns to understand the response of isoprene emissions to environmental stress factors such as water stress.
The ALBERI project highlighted the strengths and limitations of using highly heterogeneous soil properties as drought indicators. And, with the avenue of new observations of satellite solar-induced fluorescence and isoprene, as well as in situ canopy flux measurements, ALBERI will be a springboard for future research that will help the scientific community to narrow down the uncertainties in isoprene emissions. Moreover, since enhancements in isoprene emissions are indicative of an onset of thermal and water stress, isoprene satellite monitoring could be explored as a potential tool to track vegetation stress.