For a long time now, scientists have been describing nature’s scents on the basis of measurements of their main ingredients, volatile organic compounds (VOCs). Emitted by plants, fungi, bacteria and all animals for communication purposes, such compounds are a form of language.

After years spent deciphering their messages, CREAF researcher Ana Maria Yáñez has just published the most comprehensive, up-to-date review of knowledge about biogenic VOCs in the journal Global Change Biology. Entitled Amazonian biogenic volatile organic compounds under global change, it deals specifically with the biogenic VOCs emitted by the Amazon Rainforest, one of the major global sources of such compounds.

"The Amazon speaks to us in its silent language and tells us that it is not well, that it is too hot, that it lacks water and that it is losing trees." Ana María Yáñez

Over a 23-year period, according to the review, deforestation, fires and climate change have brought about an increase in the concentration of sesquiterpenes and a fall in isoprene levels in Amazonia’s air. The rainforest emits the compounds in question in situations of stress. Heat causes a sharp rise in sesquiterpene evaporation, which is a means of protection against high temperatures. Isoprene levels fall, meanwhile, when biomass (trunks and leaves) is lost due to deforestation or fires. The ratio between the two has been used to gauge the Amazon Rainforest’s health. “Amazonia is talking to us in its silent language and it’s telling us that it’s unwell, that it’s too hot, needs more water and is losing trees”, says Yáñez, the review’s lead author.

Sesquiterpenes are biogenic VOCs with 15 carbon atoms, while isoprenes, which are more common and abundant, have five carbon atoms. Both types of compounds are extremely reactive and have a high capacity for aerosol formation, albeit to a lesser degree in the case of isoprenes.

Chemical compounds and heat

The review looks at every factor that influences the cocktail of Amazonian scents and shows that human activity is altering them all. “The composition and the quantity of chemical compounds emitted by living organisms, especially plants, depend on heat to a large extent, but are also conditioned by humidity, solar radiation, the vigour of plant growth, the species present, the time of year, and whether or not plants are leafing or flowering. Consequently, climate change and deforestation are altering the planet’s fragrance, especially in Amazonia.”

On top of towers like this one in the middle of the Amazon, there are the instruments used to capture the volatile organic compounds emitted by plants. Author: Ana María Yáñez.

The review encompasses more than 240 scientific journal articles on biogenic VOCs. Furthermore, it uses mathematical models and satellite observations and data to verify information.

Smells that govern the world

To humans, the chemical substances with unpronounceable names found in high concentrations in Amazonia smell like cut grass, pine trees or damp earth, among other odours. Over and above offering an insight into the health of forest systems, VOCs have a key ecological role. As chemical substances essential to communication between living organisms, they facilitate interaction between species, helping to attract pollinating insects, protect against herbivores, and generate the aroma of fruits to be eaten so that seeds can be spread, for instance. Changes to the scents involved could result in living organisms misinterpreting messages, reducing their capacity for communication, reproduction, protection, etc.

Deforestation, fires and climate change have increased the concentration of sesquiterpenes and isoprenes, two compounds that are generated by stressful situations.

VOCs are also vital in relation to climate and water regulation. “Changes in concentrations of VOCs in Amazonia – the decrease in isoprene levels, for example – could affect rainfall patterns and the radiation that reaches Earth’s surface”, remarks Yáñez. “VOCs are involved in cloud formation and create aerosols that modulate the amount of solar radiation that makes it to the surface of the planet”, she concludes.

Reference:

Yáñez‐Serrano, A.M., Bourtsoukidis, E., Alves, E.G., Bauwens, M., Stavrakou, T., Llusià, J., Filella, I., Guenther, A., Williams, J., Artaxo, P., Sindelarova, K., Doubalova, J., Kesselmeier, J., Peñuelas, J., 2020. Amazonian biogenic volatile organic compounds under global change. Glob. Chang. Biol. gcb.15185. https://doi.org/10.1111/gcb.15185