Rising atmospheric CO2 concentration partially masks the negative effects of elevated O3 in silver birch (Betula pendula Roth).

This review summarizes the main results from a 3-year open top chamber experiment, with two silver birch (Betula pendula Roth) clones (4 and 80) where impacts of 2x ambient [CO2] (EC) and [O3] (EO) and their combination (EC + EO) were examined. Growth, physiology of the foliage and root systems, crown structure, wood properties, and biological interactions were assessed to understand the effects of a future climate on the biology of silver birch. The clones displayed great differences in their reaction to EC and EO. Growth in clone 80 increased by 40% in EC and this clone also appeared O3-tolerant, showing no growth reduction. In contrast, growth in clone 4 was not enhanced by EC, and EO reduced growth with root growth being most affected. The physiological responses of the clones to EO were smaller than expected. We found no O3 effect on net photosynthesis in either of the clones, and many parameters indicated no change compared with chamber controls, suggesting active detoxification and defense in foliage. In EO, increased rhizospheric respiration over time and accelerated leaf senescence was common in both clones. We assumed that elevated O3 offsets the positive effects of elevated CO2 when plants were exposed to combined EC + EO treatment. In contrast, the responses to EC + EO mostly resembled the ones in EC, at least partly due to stomatal closure, which thus reduced O3 flux to the leaves. However, clear cellular level symptoms of oxidative stress were observed also in EC + EO treatment. Thus, we conclude that EC masked most of the negative O3 effects during long exposure of birch to EC + EO treatment. Biotic interactions were not heavily affected. Only some early season defoliators may suffer from faster maturation of leaves due to EO.