Ozone – the invisible threat to Europe’s forests

Ambitious measures to tackle ozone precursors could reduce impact on forests by 30 per cent. This would boost biomass growth and significantly enhance carbon storage.

In the previous issue of Acid News, I explored ground level ozone’s impact on crops. Here, we turn our attention to forests, where the effects are equally troubling. Research on ozone’s impact on forests began in the 1950s, revealing that trees are as sensitive to ozone as annual plants. A review found that 90% of 165 woody plant species studied were negatively affected by ozone. [1]

Understanding ozone’s impact on trees isn’t straightforward. Due to their long lifespans and large size, most studies have relied on ozone exposure experiments with young trees in controlled environments, such as open-top chambers. While these studies provide valuable insights, they don’t fully reflect the complexity of mature forest ecosystems.

To bridge this gap, researchers have turned to epidemiological studies, which analyse extensive datasets to uncover relationships between ozone exposure and tree health. One such study in Switzerland monitored over 7,000 European beech trees (Fagus sylvatica) and 6,500 Norway spruces (Picea abies) across 163 forest plots over nearly three decades. This rich dataset, spanning diverse climates, altitudes, and ozone exposure levels, allowed researchers to validate experimental findings and uncover new insights. The study confirmed that ozone exposure significantly harms mature tree growth under forest stand conditions. For European beech, results aligned with earlier experimental findings, while Norway spruce was found to be even more sensitive than previously thought. [2]

To assess the risk for ozone damage, scientists and policy makers use the concept of “critical levels”. The UNECE Air Convention sets the critical level for forests at an AOT40 (Accumulated Ozone Exposure Over a Threshold of 40 parts per billion) of 10,000 µg/m³·hours from April to September. Monitoring data show that most European forests regularly exceed this level, with severe exceedances in 2006 and 2018 for example (see figure 1). [3]

Figure 1. Exposure of forest areas to ozone (AOT40) in EEA member countries in 2005 to 2022. Green represents areas below the critical level of 10,000 µg/m³·hours.

Although AOT40 is widely used in policy, there’s a growing shift toward using the Phytotoxic Ozone Dose (POD), which accounts for how much ozone a plant actually absorbs through its stomata, which are tiny pores on the leaf surfaces that the plant can open and close. This approach highlights that ozone impacts have been overestimated in drier Mediterranean regions and underestimated in humid northern Europe. However, even with POD adjustments, ozone damage in southern Europe remains severe, while northern forests are more affected than previously thought (see figure 2). [4]

Figure 2. Ozone stomatal flux (POD1) for beech (top) and spruce (bottom) in 2020. Only green areas are below critical levels.

How does this translate to actual volumes? In a not yet published study researchers have estimated annual losses of the growth of forests. It was found that among the EU member states it is estimated that ozone reduces the stem volume growth rates of trees by up to 30 per cent. Even the least affected member states miss out some forest growth due to ozone damage. [5]

The growth of trees is also directly related to their ability to sequester carbon and act as a carbon sink. If reduced ozone exposure could boost annual biomass growth by just a few percent that could flip the margins and significantly enhance carbon storage, since the increase in the carbon stocks of the living biomass of forests depends on the difference between growth and harvest rates. This could ultimately help EU member states meet their LULUCF targets, which is something most of them struggle with.

Another approach is to quantify the loss in forest growth in monetary terms. So far there have only been a few studies. For example, the potential economic loss was estimated at 940 million SEK per year (82 million euro) for Sweden’s forest owners for the period 2014–2017 [6] and 31.6–57.1 million euro for Italy’s forests in 2015 [7]. The studies indicate that the economic loss could be in the same order of magnitude as for crops.

Can we mitigate these losses? The answer lies in reducing emissions of ozone precursors: nitrate oxides and volatile organic compounds (VOCs) including methane (CH₄). A recent study analysed scenarios for Europe, showing that under current legislation, ozone’s impact on deciduous forest growth would drop by only 8% by 2050. However, adopting known technical measures and policies aligned with the Paris Agreement could reduce impacts by up to 32%. [8]

Not included in these models are more dynamic effects. Ozone pollution doesn’t just harm plants directly – it also influences interactions between plants and insects. In the 1990s, researchers found that ozone impacts on how plants allocate their resources, which make them less competitive and more vulnerable to pest attacks. Even earlier, in the 1960s, scientists observed that ozone exposure made western pines in southern California more susceptible to deadly bark beetle infestations. Other studies have revealed that ozone levels can alter plant to plant competition favouring less sensitive species. These studies hint at larger, ecosystem-wide ripple effects caused by ozone pollution. [1]

Protecting forests from ozone damage is not a cause that often captures headlines. This is partly because it doesn’t fit easily into the typical news narrative. Challenges include:

• Ozone is an invisible gas.
• Its formation involves a complex series of chemical reactions.
• High ozone levels have persisted for decades.
• Even when visible damage appears on leaves, it can be hard for non-experts to distinguish from other issues, such as pests or drought.

Yet this issue deserves greater attention, as it intersects with many other challenges. Lowering ozone levels can boost ecosystem resilience, enhance carbon sequestration, and protect a valuable economic resource. With decisive policy action, we can limit the damage and secure the future of thriving forests.

References:
[1] Agathokleous, Evgenios & Saitanis, Costas. (2023). Effects of Ozone on Forests. 10.1007/978-981-15-2760-9_24.
[2] Braun, S. & Rihm, B. & Schindler, C. (2022). Epidemiological Estimate of Growth Reduction by Ozone in Fagus sylvatica L. and Picea abies Karst.: Sensitivity Analysis and Comparison with Experimental Results. Plants. 11. 777. 10.3390/plants11060777.
[3] EEA (2024), Exposure of forest areas to ozone in EEA member countries, https://www.eea.europa.eu/en/analysis/maps-and-charts/exposure-of-forest...
[4] Vlasáková, L., Marková, J., Tognet, F., Horálek, J., Colette, A. (2022). Evaluation of European-wide map creation of flux-based ozone indicator POD for selected tree species (Eionet Report – ETC HE 2022/23). European Topic Centre on Human Health and the Environment.
[5] Personal communication with Per-Erik Karlsson
[6] Karlsson, P.E., Pihl Karlsson, G., Danielsson, H., Langner, J. & Pleijel, H. 2019. Economic evaluation of the positive effects that would arise for production in forestry and agriculture in Sweden in the absence of exposure to anthropogenically caused the formation of groundlevel ozone. IVL Report C 460. In Swedish with an English summary.
[7] Sacchelli, S., Carrari, E., Paoletti, E. et al. Economic impacts of ambient ozone pollution on wood production in Italy. Sci Rep 11, 154 (2021). https://doi.org/10.1038/s41598-020-80516-6
[8] van Caspel, W., Klimont, Z., Heyes, C. & Fagerli, H. (2024). Impact of methane and other precursor emission reductions on surface ozone in Europe: Scenario analysis using the EMEP MSC-W model. 10.5194/egusphere-2024-1422.