Maxime Cailleret and colleagues from the Swiss Federal Institute for Forest, Snow and Landscape Research (Switzerland) recently published a review titled ‘Ozone effects on European forest growth – Towards an integrative approach‘. Maxime tell us more about the paper below.

Tropospheric ozone is a key greenhouse gas responsible for 5-16% of the global temperature change since preindustrial times, and is the second-most-important air pollutant in causing morbidity impacts to human health (after particulate matter; Ainsworth et al. 2012). Furthermore, ground-level ozone is currently considered to be more damaging to forest vegetation than any other air pollutant, impacting various ecosystem functions and services.

Ozone is absorbed by leaves through stomata, and has a negative impact on various cellular and molecular processes, damaging leaves (Figure 1) and reducing foliar carbon gain, which in turn may also reduce growth of individual trees. This has been shown by several studies (e.g., Wittig et al. 2009) and is commonly accepted by the scientific community.

Figure 1: Leaves from European beech (Fagus sylvatica L.), European ash (Fraxinus excelsior L.) and poplar (Populus x berolinsesis) damaged by ozone (Photo: M. Schaub). 

In our essay review published in Journal of Ecology (Cailleret et al. 2018), we present advantages and limitations of the approaches available for ozone risk assessment, and challenged this general agreement based on the following arguments.

1. Most previous studies analyzed individual saplings growing under controlled conditions (chamber experiments), and their results are not necessarily valid for populations of mature trees in real forests. Indeed, the environmental conditions within the canopy of seedlings differ from adult stands, and sensitivity to ozone may change with ageing. Also, mature trees have considerable storage pools (up to four times the carbon of the leaved canopy), and their growth is more dependent on the climatic conditions during the growing season than on the total amount of carbon available (i.e., ‘sink-driven’).

2. Ozone effects on stand growth interact with effects of climate, CO₂ fertilization, atmospheric deposition of nitrogen or other pollutants, stand structure, soil properties, or biotic stressors. For instance, high ozone concentrations and uptake frequently go along with high air temperatures and nitrogen deposition, which both can be related to increased tree growth in various geographical regions. In consequence, ozone effect may become statistically non-significant when considering the interdependency among all these environmental drivers, as recently highlighted by Ferretti et al., (2018) using long-term forest monitoring data from alpine forests in Trentino, Northern Italy.

Figure 2: In mixed forest stands, species-specific ozone effects on forest growth may be compensated by changes in competition intensity and in species composition (Photo: A. Rigling)

There are several defence, acclimation, and compensatory mechanisms that act at different spatial and temporal scales and may reduce ozone impacts. In the short-term, trees can allocate resources to build antioxidant enzymes. In the long-term, ozone effects on forest growth may also be compensated by changes in competition intensity and species composition (see Mina et al., 2017 and the associated blog post). For instance, the decrease in growth of a canopy tree may benefit to its neighbours. Also, as temperate oaks are less sensitive to ozone and more light-demanding than European beech, they may profit from the reduction in competition intensity by beech under high ozone levels (Figure 2). This change in competition regime among species is key in a climate change context, as the predicted increase in temperatures should favour oaks to the detriment of beech, and thus indirectly reduce ozone impacts on forest growth.

On this basis, we postulate that statistical or process-based models that simulate the development of forest ecosystems (and briefly reviewed in our paper) tend to overestimate ozone impacts on forest growth – as they most likely do also for CO₂ fertilization. These inaccuracies may have major implications for the determination and applicability of critical levels used by the UNECE to protect forest vegetation (Convention on Long-range Transboundary Air Pollution) and to adapt forest management strategies in the face of global change.

Figure 3. Summary of the approaches and data sources available to assess ozone impacts on forest growth.

To derive more realistic critical levels and – more generally – to improve our assessment of ozone effects on forest ecosystems, we provided research gaps and advocated the need (i) for further experimental and long-term monitoring studies to better quantify species-specific ozone uptake by forests and its impacts on their growth under real forest condition, and (ii) to develop physiological and demographic modelling tools that can upscale these impacts at longer temporal and larger spatial scales, from the tree to the landscape (Figure 3).

Maxime Cailleret, Swiss Federal Research Institute WSL, Switzerland.

The authors of the paper are Maxime Cailleret, Marco Ferretti, Arthur Gessler, Andreas Rigling, Marcus Schaub of the Swiss Federal Institute for Forest, Snow and Landscape Research. Read the full paper here: Ozone effects on European forest growth – Towards an integrative approach