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 pendula 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, carotenoid, and nutrient concentrations to determine the differences in ozone sensitivity among these species and hybrids and the relationship of ozone sensitivity to changes in growth pattern and tolerance to other abiotic stresses. There was a large variation in growth among seedlings of different birches (species and hybrids). In most birches, elevated ozone exposure resulted in altered resource allocation at the expense of roots, leading to reduced 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 of B. platyphylla and those from crossings with Finnish plus trees (bred for growth) showed highest susceptibility to ozone. The hybrids between Alaskan B. resinifera and Finnish B. pendula were intermediate in ozone sensitivity, whereas the hybrids between Japanese B. platyphylla and Finnish B. pendula genotypes were of improved tolerance compared with parents. The results suggest that ozone-induced disturbances in carbon allocation favouring shoot growth pose a real risk factor for birch establishment, production, and sustainable forestry because of potential deteriorating belowground processess and long-term tree vitality.