Responses to Airborne Ozone and Soilborne Metal Pollution in Afforestation Plants with Different Life Forms

HIGHLIGHTS: What are the main findings? Ozone and metal stress caused injuries that were partly similar but differed in their tissue and cell location irrespective of the species. Combined ozone and metal stress showed few interactions. What is the implication of the main finding? The conifer efficiently blocked the metals in its roots and was more tolerant of ozone stress, resulting in a biomass reduction rating in response to ozone and metal stress: conifer < ruderal forb < deciduous tree. With the current increases in environmental stress, our findings outline the relevance of “slow-return” species strategies, with low productivity but enhanced stress tolerance. ABSTRACT: With the current increases in environmental stress, understanding species-specific responses to multiple stress agents is needed. This science is especially important for managing ecosystems that are already confronted with considerable pollution. In this study, responses to ozone (O3, ambient daily course values + 20 ppb) and mixed metal contamination in soils (MC, cadmium/copper/lead/zinc = 25/1100/2500/1600 mg kg(−1)), separately and in combination, were evaluated for three plant species (Picea abies, Acer pseudoplatanus, Tanacetum vulgare) with different life forms and ecological strategies. The two treatments elicited similar stress reactions, as shown by leaf functional traits, gas exchange, tannin, and nutrient markers, irrespective of the plant species and life form, whereas the reactions to the treatments differed in magnitude. Visible and microscopic injuries at the organ or cell level appeared along the penetration route of ozone and metal contamination. At the whole plant level, the MC treatment caused more severe injuries than the O3 treatment and few interactions were observed between the two stress factors. Picea trees, with a slow-return strategy, showed the highest stress tolerance in apparent relation to an enhancement of conservative traits and an exclusion of stress agents. The ruderal and more acquisitive Tanacetum forbs translocated large amounts of contaminants above ground, which may be of concern in a phytostabilisation context. The deciduous Acer trees—also with an acquisitive strategy—were most sensitive to both stress factors. Hence, species with slow-return strategies may be of particular interest for managing metal-polluted sites in the current context of multiple stressors and for safely confining soil contaminants below ground.