How forests smell -- a risk for the climate?

Plant odours penetrate the atmosphere

Plants produce a variety of organic compounds to communicate with each other and with their environment. These are known as biogenic volatile organic compounds (BVOCs), such as terpenes, which give plants their characteristic scent and help to repel pests. As well as acting as chemical signals, these substances play a role in regulating climate, air quality and atmospheric chemistry. This is because these BVOCs emitted by plants form biogenic secondary organic aerosols (BSOAs) in the air, i.e. particles in the atmosphere. These aerosols in turn affect air quality, cloud formation and the climate.

MyDiv Experiment: Measurements in plots with different tree species

But how do emissions and concentrations of aerosols in the air change as biodiversity declines or plants are stressed by drought? The interdisciplinary team led by scientists Dr Anvar Sanaei and Professor Alexandra Weigelt from Leipzig University and other researchers from TROPOS and iDiv investigated this question. The scientists collected the data at the MyDiv tree diversity experimental site. The site, near Bad Lauchstädt in Saxony-Anhalt, covers around two hectares and has 80 plots with ten tree species growing together in monocultures or mixtures of different species. For the study, the team spent almost two weeks collecting air samples from ten of the 11x11 metre plots, which grow four tree species -- rowan, wild cherry, common ash and sycamore -- in different combinations.

Fewer plant odours, fewer risks

"In the field, we measured BVOCs and BSOA compounds in ten plots of varying tree diversity. Our results show that the amount of BVOCs decreases with increasing biodiversity in most cases," says Dr Anvar Sanaei, first author of the study and postdoctoral researcher at the Institute of Biology at Leipzig University. It is estimated that global BVOC emissions from vegetation will increase by around a third as a result of climate change and higher temperatures. "There are considerable uncertainties here: these precursor gases can form particles, which in turn can become cloud droplets. Whether BVOCs ultimately cool or warm the atmosphere depends on many factors and is difficult to predict. However, more biodiversity and fewer BVOCs would reduce the changes in the atmosphere and thus also the risks of climate change -- including changes in precipitation," adds Professor Hartmut Herrmann from TROPOS. The second part of the study shows how difficult it is to investigate these complex processes in the field: the team was unable to establish any clear correlations for BSOAs, which could be partly due to environmental influences, as the conversion of BVOC gases into BSOA particles takes a certain amount of time. At just under two weeks, the measurement campaign was also comparatively short. That is why the team wants to continue the research -- not least because many questions remain unanswered.

More stress, more plant odours?

Previously, it was thought that species-rich forests and grasslands released more gases into the atmosphere than species-poor ones. The reason for this was thought to be that species-rich systems produce more biomass because they can utilise resources such as light, water and nutrients more efficiently. More biomass then also means more leaf surface area from which the gases can be emitted. "Our new results, however, suggest that the situation may be due to the fact that plants in species-rich forests and grasslands are under less stress. Compared to monocultures, they face fewer herbivores and less heat and drought. But this is just a hypothesis for now. Much more research is needed to better understand how biodiversity affects the atmosphere, where we need to look more closely at the microclimate, above- and below-ground stress on plants, and many other factors in long-term experiments," says Professor Nico Eisenhauer from iDiv.

Biology + climate research + chemistry = A team fit for the future

What made this study so special was that different disciplines worked together, combining atmospheric and biological measurements. "Only with knowledge from biology, climate research and atmospheric chemistry can we decipher how plant emissions are linked to biodiversity and the atmosphere. Our study highlights the need for experiments at the local and regional scales and the development of models to improve our understanding of biosphere-atmosphere interactions," says senior author Professor Alexandra Weigelt from the Institute of Biology. She adds that this is a prime example of the Breathing Nature research project, for which the University submitted a draft proposal back in May as part of the Excellence Strategy. After all, answers to the pressing questions of our time can only be found by transcending disciplinary and institutional boundaries.