Differences of Betula origins in ozone sensitivity based on open-field experiment over two growing seasons

One-year-old seedlings of nine different white birch seed origins (Betula p endula Roth, Betula platyphylla Sukatchev var. japonica Hara, two crossings of plus trees of B. pendula, and five two-way hybrids between B. pendula, Betula resinifera Britt., and B. platyphylla) were exposed to ambient ozone (control) and 1.3 x ambient ozone concentrations over two growing seasons. At the end of each growing season, the plants were measured for leaf, stem and root growth; visible leaf injuries; leaf senescence; and ribulose 1,5- bisphosphate carboxylase-oxygenase, starch, soluble protein, chlorophyll, c arotenoid, and nutrient concentrations to determine the differences in ozon e sensitivity among these species and hybrids and the relationship of ozone sensitivity to changes in growth pattern and tolerance to other abiotic st resses. There was a large variation in growth among seedlings of different birches (species and hybrids). In most birches, elevated ozone exposure res ulted in altered resource allocation at the expense of roots, leading to re duced root/shoot ratio. In fast-growing birches, stimulated stem height and foliage growth in the second growing season indicated compensatory growth, which was accompanied by accelerated senescence of old leaves. Seedlings o f B. platyphylla and those from crossings with Finnish plus trees (bred for growth) showed highest susceptibility to ozone. The hybrids between Alaska n B. resinifera and Finnish B. pendula were intermediate in ozone sensitivi ty, whereas the hybrids between Japanese B. platyphylla and Finnish B. pend ula genotypes were of improved tolerance compared with parents. The results suggest that ozone-induced disturbances in carbon allocation favouring sho ot growth pose a real risk factor for birch establishment, production, and sustainable forestry because of potential deteriorating belowground process ess and long-term tree vitality.