© 2003 Heron Publishing—Victoria, Canada
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
Elina Oksanen
Department of Ecology and Environmental Science, University of Kuopio, POB 1627, 70211 Kuopio, Finland (Elina.Oksanen@uku.fi) / Received June 20, 2002; accepted September 20, 2002; published online May 15, 2003
Summary
Physiological responses of 4-year-old potted saplings of an O3-tolerant clone of Betula pendula Roth to short-term ozone (O3) exposure (one growing season) were compared with those of 6-year-old open-soil-grown trees of the same clone fumigated with
O3 for six growing seasons. In the 2001 growing season, both groups of plants were exposed to ambient (control) and 1.6× ambient
(elevated) O3 concentration under similar microclimatic conditions in a free air O3 exposure facility. Growth, net photosynthesis, stomatal conductance, stomatal density, visible foliar injury, starch and
nutrient concentrations, bud formation and differences in O3 responses between lower, middle and upper sections of the canopy were determined. The potted saplings were unaffected by
elevated O3 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 O3 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
O3 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 O3 load was higher for open-soil-grown trees than for potted saplings because of slower leaf senescence in the trees. Thus,
O3 sensitivity in European white birch increases with increasing exposure time and tree size.
Keywords:
bud formation, carry-over effects, free air exposure, growth, net photosynthesis, scaling, stomatal conductance.
