Physiological responses of birch (Betula pendula) to ozone:: a comparison between open-soil-grown trees exposed for six growing seasons and potted seedlings exposed for one season

Physiological responses of 4-year-old potted saplings of an O-3-tolerant clone of Betula pendula Roth to short-term ozone (O-3) exposure (one growing season) were compared with those of 6-year-old open-soil-grown trees of the same clone fumigated with O-3 for six growing seasons. In the 2001 growing season, both groups of plants were exposed to ambient (control) and 1.6x ambient (elevated) O-3 concentration under similar microclimatic conditions in a free air O-3 exposure facility. Growth, net photosynthesis, stomatal conductance, stomatal density, visible foliar injury, starch and nutrient concentrations, bud formation and differences in O-3 responses between lower, middle and upper sections of the canopy were determined. The potted saplings were unaffected by elevated O-3 concentration, whereas the open-soil-grown trees showed a 3-38% reduction in shoot growth, a 22% reduction in number of overwintering buds, a 26-65% decrease in autumnal net photosynthesis, 30% and 20-23% reductions in starch and nitrogen concentrations of senescing leaves, respectively, and disturbances in stomatal conductance. The greater O-3 sensitivity of open-soil-grown trees compared with potted saplings was a result of senescence-related physiological factors. First, a lower net photosynthesis to stomatal conductance ratio in open-soil-grown trees at the end of the season promoted O-3 uptake and decreased photosynthetic gain, leading to the onset of visible foliar injuries. Second, decreased carbohydrate reserves may have resulted in deleterious carry-over effects arising from the reduced formation of over-wintering buds. Finally, the leaf-level O-3 load was higher for open-soil-grown trees than for potted saplings because of slower leaf senescence in the trees. Thus, O-3 sensitivity in European white birch increases with increasing exposure time and tree size.