Increasing Tropospheric Ozone Level Reduced Birch (Betula Pendula) Dry Mass within a Five Years Period

Soil-growing randomized mixtures of ten European silver birch (Betula pendula Roth) clones, showing different ozone sensitivity, were exposed to ambient air (control) or 1.4–1.7x ambient (elevated) ozone over five growing seasons using free-air fumigation (FACE) approach. During the last season, the juvenile trees were measured for growth, net assimilation rate and starch content. In elevated-ozone plants, significant effects were observed as 21–28% reduced new leaf development, 44.8% lower dry mass of leaves and 33.8% lower dry mass of roots, as well as 7.6% lower RGR of leaves and 27.8% lower RGR of roots, leading to 16% lower root/shoot ratio. In addition, net assimilation rate and starch content were slightly (8.9% and 14.3%) reduced in ozone-stressed plants. The results indicated cumulative ozone-induced growth reductions over five years. Ozone-stressed trees with declined root growth may become susceptible to other environmental stresses such as water and nutrient deficiency, and lose belowground competitiveness, which may affect tree survival.