Air pollution remains one of the main side effects of rapidly advancing industrial development globally. A major challenge with pollution is that we still do not fully understand how pollutants affect the functioning of natural systems. Air pollutants produced by burning of fossil fuels, including tropospheric ozone (O3) and nitrogen oxides (NOX), have a complex relationship with volatile organic compounds (VOCs), a group of airborne chemicals that plants emit as a consequence of normal metabolism and especially in response to stress. For example, O3 is known to oxidise (break apart) a variety of VOCs and is formed in the troposphere from NOX and VOCs in the presence of sunlight. VOCs also have an important informative function in the biosphere and can mediate between-plant interactions.
Most studies on between-plant interactions have been conducted with herbaceous plants, with trees often being overlooked due to various challenges with their height, slow growth speed and lab space constraints. However, trees form the basis of all forest ecosystems, which cover around 32% of total Earth land cover. What is more, all forests around the globe are affected by pollution to a certain extent because O3 and NOX can travel long distances from their place of origin (from industrial centres or traffic, for example) along air currents.
I joined the Environmental Ecology Research Group (EERG) in Finland as a postdoctoral researcher in 2022. The big recent discovery at the time showed that Scots pine (Pinus sylvestris) seedlings can interact with each other using herbivore-induced plant volatiles (HIPVs; a specific subset of VOCs) and gain resistance to damage by the large pine weevil (Hylobius abietis) in carefully controlled laboratory conditions (Yu et al. 2022; https://doi.org/10.1098/rspb.2022.0963). However, extrapolating laboratory-based results into real world conditions and claiming that VOC-mediated interactions readily occur in Scots pine in the field is not so straightforward. For example, VOCs are transported by air, so variations in air currents could disrupt the interactions; variable temperature, light and moisture conditions could affect the interactions as well. Air pollution could also impair these interactions. However, between-plant interactions can also occur belowground, so that is also important to consider.
I was curious to test these laboratory-based findings in the field.
As luck would have it, at the beginning of my postdoctoral employment in July 2022, together with my colleagues at the EERG, I attended the International Congress of Entomology (ICE) 2020 in Helsinki, Finland, which was postponed for 2 years due to the COVID-19 pandemic.
There, I met the collaborators of the group from the UK, James Ryalls and Robbie Girling, who mentioned that they have a newly established site at the University of Reading for studying the effects of O3 and diesel exhaust pollution in field conditions, which they named the Free-Air Diesel and Ozone Enrichment (FADOE) facility.
Perfect! The place for studying between-plant interactions in the field was settled. Together with James Blande, the leader of the EERG, we discussed the plans for fieldwork in the spring/summer of 2023. I wanted to establish an experimental setup that could stay in the FADOE for years and could potentially be studied in future experiments. Therefore, I decided to plant the trees in big pots, so that they can have both lines of interaction available and thus mimic the real conditions of trees growing freely in the field.
When I arrived in Reading, in April 2023, I helped James Ryalls install a new diesel exhaust generator. We also spent a couple of days with Neil Mullinger, who built the FADOE facility, to repair the systems that relay O3 and diesel exhaust exposure to the rings.
The main thing that surprised me was the size of the FADOE facility. It is so much larger than one might imagine based on the aerial photographs. The pollution rings are quite big, the distance between them is larger than I realised and the whole facility is hard to see from a single viewpoint.
After filling the pots with the soil, and planting the seedlings in May, I was almost ready to start the experiment. It looked like I wouldn’t be able to do all the data collecting by myself, so I called James Blande over a plate of English breakfast and asked him “Is there any chance to send a student assistant over here to help me? Is Oliver Welling available?”
He was and therefore in early June 2023 we travelled together from Finland to Reading and worked hard to conduct fieldwork and collect the data as originally planned.
We collected VOCs from the seedlings before and after weevil herbivory, along with measuring photosynthetic traits and the damage to the bark of seedlings.
Thankfully, the weather forecast was accurate, so we didn’t have issues with the rain during fieldwork. However, it was quite warm and sunny, so we needed to protect ourselves from the sunlight. I didn’t know that English summers can be so hot! Sunscreen was not an option (too many VOCs from the cream could have contaminated our samples), so we had to employ other, more creative solutions.
By early July 2023, we finished the fieldwork and we returned to Finland. Now it was the time to analyse the data. It took about a year until I had the story ready to present. After some internal presentations in the EERG, I had a draft of the manuscript written in mid-2024 and after a few back-and-forth with the coauthors, started the publishing process in early 2025.
So, what did we find out?
Well, it turns out that seedlings that are exposed to damaged neighbours suffered lower rates of bark damage after being exposed to the large pine weevils, by almost 50%, expressed higher rates of photosynthesis and higher emission of green leaf volatiles (compounds that are associated with mechanical damage) following a weevil attack. Therefore, Scots pine seedlings really do activate the internal defence mechanisms and modify their metabolism in response to exposure to damaged neighbours, even in field conditions. The O3 and diesel exhaust pollution (rich in NOX) did not disrupt these interactions, although they disturbed the photosynthetic traits and VOC emission of seedlings.
Our work highlights the innate capacity of plants to regulate their metabolism in response to cues from their neighbours, which withstands the test of environmental disturbance. Plants have an amazing ability to adjust to their environment, which we are still only beginning to understand. Considering the diversity of plants and animals on Earth, the path forward in the field of Chemical Ecology is open with seemingly endless possibilities.