How Forests Breathe

Heat, drought, and pests cause stress in plants. The composition of their “breath” is changing – which also has an impact on air quality and climate. Eva Pfannerstill is investigating these changes with a new measuring method.

The colourful screensaver on display in Dr. Eva Pfannerstill’s office shows the famous “Mesa Arch” in Utah – a small reminder of her three years spent as a postdoc in Berkeley (USA). During her time there, she measured the air over Los Angeles before moving to Jülich’s Institute of Climate and Energy Systems (ICE-3) as a young investigator group leader in October 2023.

In Germany, she now studies the air above forests instead of the air above cities. Her most important tool is a new measurement technique she developed at Berkeley. The combination of a state-of-the-art mass spectrometer and a special calculation method provides a wealth of measurement data on over 400 substances in the atmosphere. In addition to the typical plant substances, these include vehicle emissions, solvents, and cleaning and personal care products. “The great thing is that we are not only able to measure the substances, but also trace the sources from which they originate,” says Pfannerstill. Ten measurement points per second can be recorded for the concentration of these substances, and the wind can be simultaneously analysed in three dimensions at the same frequency. “This allows us to quantify exactly which quantities of the measured substances are emitted at which location, for example whether they are blown in from the side or originate from the land surface,” explains the researcher.

Identifying the cause

Her method helped to determine why particulate matter and ozone pollution in Los Angeles increases with rising temperatures. “One of the causes is the temperature-dependent emissions that dominate the ‘breath’ of the metropolitan area,” says the expert. The main drivers of what occurs at high temperatures are terpenoids from plants, followed by evaporation from solvents. Both react with the nitrogen oxides from exhaust gases to form ozone and particulate matter (see infobox).

In upcoming measurement campaigns, Eva Pfannerstill aims to find out how many terpenoids are emitted by German forests – this time not from an aircraft, but from a Zeppelin NT airship. This has both advantages and disadvantages: “We can fly much lower – down to 100 metres above the ground – but we have less space in the zeppelin’s cabin,” the researcher says. One challenge will be to adapt the spectrometer to the limited space available. “Here we can benefit from the experience gained from Jülich’s previous zeppelin campaigns and from the expertise of Jülich engineers,” stresses Pfannerstill, who is not the only Jülich scientist to collect data during measurement campaigns with the zeppelin.

With our method, we are not only able to measure the substances, but also trace the sources from which they originate
Dr. Eva Pfannerstill, Jülich Institute of Climate and Energy Systems (ICE-3)

Dr. Georgios Gkatzelis for example, uses the same mass spectrometer to detect organic trace gases in the atmosphere of urban regions. Prof. Hendrik Fuchs and Dr. Anna Novelli, meanwhile, use an instrument to measure how reactive the detected gases are. And Prof. Uwe Rascher’s team is contributing an instrument that detects the fluorescence of plants. This indicates whether the plants are under stress before the human eye can detect any changes.

The researchers will also work together with local forestry authorities. “They can tell us, for example, whether the forests we fly over are suffering from a massive insect infestation. Plants emit different substances depending on whether they are suffering from drought or insect infestation,” says Pfannerstill. Taking all factors into account.

Finding the pattern

The chemist has already conducted preliminary tests in the Jülich atmospheric simulation chamber SAPHIR and its coupled plant chamber facility SAPHIR-PLUS. During these tests, young beech and oak trees had to withstand temperatures of up to 40 °C and increased ozone levels: “That wasn’t good for the trees,” Pfannerstill summarizes. In addition, a controlled insect infestation is planned in the chamber.

Plants release gaseous scents into the air. The stress reactions triggered in plants by heat, drought, and pests alter the quantity and composition of the scents. These include volatile hydrocarbons and terpenoids such as isoprene. Exactly how this mix of molecules varies under different types of stress is the subject of intensive research. The quantity of the substances is important, as some of them react in the presence of nitrogen oxides from vehicle emissions or other emissions to form ozone and contribute to the formation of aerosols. Aerosols can have a cooling effect on the climate, but they can also turn into harmful particulate matter.

“The SAPHIR-PLUS experiments show us whether there is a certain ‘response pattern’ of the trees to the different stimuli. We will be looking for these signals later during the zeppelin campaign,” she explains. The data from the campaign will be an important addition to existing climate models, since previous calculations of the stress response of forests are often solely based on laboratory measurements with a few small trees, Pfannerstill adds.

That’s what makes Jülich research so special. We can collect data on a small scale in a controlled manner in the laboratory and then compare it with data from large-scale measurement campaigns in the real world. In doing so, we want to highlight ways in which the air quality in cities and the health of forests can be improved in a changing Earth system,” explains Pfannerstill.

Text: Brigitte Stahl-Busse / Pictures: Forschungszentrum Jülich: Sascha Kreklau