Impact of increased springtime O3 exposure on Scots pine (Pinus sylvestris) seedlings in central Finland

Abstract

Three-year-old Scots pine (Pinus sylvestris L.) seedlings were exposed to ambient or elevated ozone (O3) concentrations in open-air exposure fields in central Finland in 1995–97. Three different treatments were applied in 1996 and 1997: ambient air, elevated O3 (1.3–1.5×ambient) during the growing season (June–September) and elevated O3 in March–September, i.e. the growing season including the springtime O3 exposure. The ambient mean O3 concentrations were 40% higher in springtime (March–May) compared to the concentrations during the growing seasons. Maximum O3 concentrations were measured in April or early May, whereas a clear increase in the stomatal activity of the seedlings was observed by the middle of May. This suggests a low intake of O3 by conifers despite the higher O3 concentrations in spring. Stomatal conductance, and contents of chlorophyll and ribulosebisphosphate carboxylase/oxygenase (Rubisco) in current-year needles were not significantly affected by any O3 treatment. Only a slight decrease in current-year shoot growth, slight increase in the abscission of 2-year-old needles and increased electron density of chloroplast stroma by springtime O3 exposure suggest a rather small contribution of elevated springtime O3 concentrations to total O3 damage under current climatic conditions in Finland. However, the increases in springtime O3 concentrations may enhance the cumulative effects of O3 during long-term O3 exposures.