The effects of tropospheric ozone on oaks: A global meta-analysis

Tropospheric ozone (O-3) levels are still elevated in many regions of the world including Northern Hemisphere forests areas, and are predicted to increase further due to anthropogenic activities and climate change. Oaks are major woody angiosperms in the Northern Hemisphere in terms of biodiversity, ecological dominance, and economic values. This meta-analysis shows overwhelming evidence of the O-3 effects on 51 growth, anatomical, biomass, physiological and biochemical parameters of 14 deciduous or evergreen oak species distributed all around the Northern Hemisphere. Although no large impacts were observed on biomass, suggesting an O-3 tolerance by oaks, some impairments were found at physiological level that might negatively affect carbon sequestration and water vapour transfer to the atmosphere. This outcome suggests the need to incorporate this phenomenon into future projection studies dealing with how atmospheric change and forest biomes will interact in effecting climatic change. Among the antioxidants used by oaks to respond to O-3, phenols seem to have a crucial role. Deciduous species resulted more affected by O-3 than evergreen ones, as well as oaks native to Eurasia, in comparison with those from North-America. Experiments performed in less controlled environments showed more O-3 deleterious effects, especially under higher AOT40 levels, but negative impacts were also reported for acute O-3 exposures. Most of the reviewed studies with additional treatments to O-3 exposure investigated the interaction(s) between O-3 and drought, but the negative effects induced by drought seemed not to be exacerbated by the pollutant. However, more combined experiments on the impact of O-3 and co-occurring stressors on woody species are necessary. Another major issue is the lack of experiments on adult trees. To better understand O-3 impacts, and to reinforce the strength of O-3 impact predictions, O-3 controlled experiments on young individuals should be combined with long-term experiments on mature trees grown in open-air conditions. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

Tropospheric ozone continues to impact Northern Hemisphere oak species, affecting growth, physiological processes, and biochemical parameters, with deciduous species and oaks native to Eurasia being more susceptible. Further research is needed to assess the combined effects of O3 and other stressors on woody species, as well as the lack of experiments on adult trees.