Uptake of ozone and its impact on silver birch

Abstract: This thesis deals with the impact of ground-level ozone (O3) on silver birch, Betula pendula. The impact is studied from an ecophysiological perspective, focusing on how the O3 molecules enter the leaves through the stomata and how the plant responds to O3 by changing its acquisition and allocation of resources. By relating the plant responses to the stomatal uptake of O3, this thesis contributes to the ongoing work within the Convention on Long-Range Transboundary Air Pollution (CLRTAP) to establish uptake-based Critical Levels for the impact of O3 on plants. The O3 effects on the growth and leaf senescence of young birch trees were studied in a two-year open-top chamber experiment at Östad, 50 km northeast of Göteborg, Sweden. O3 inhibited photosynthesis and growth, and changed the carbon allocation to the disadvantage of the roots. The leaves in elevated O3 were shed prematurely, without an efficient resorption of nitrogen to perennial parts of the plant. In order to estimate the stomatal uptake of O3, different types of stomatal conductance (gs) models were parameterised for juvenile as well as for mature birch. Multiplicative stomatal models were more successful in predicting gs as compared to a combined stomatal-photosynthesis model. Juvenile trees had higher maximum gs and a less pronounced stomatal closure in warm and dry air, as compared to mature trees. As a result, the capacity for stomatal uptake of O3 was higher in juvenile trees than in mature trees. Stomatal closure in response to increasing evaporative demand prevented the twig xylem pressure from falling below –1.7 MPa in both juvenile and mature birch. The gs models were used to estimate the stomatal uptake of O3 in nine European O3 experiments with birch. By relating the O3-induced reduction in biomass production in these experiments to different indices of O3 exposure, it was shown that the biomass reductions were more closely related to the stomatal uptake of O3 than to the external exposure. Critical levels for the adverse impact of O3 on birch are determined and discussed in relation to the Critical Levels within the CLRTAP. O3 causes adverse effects on growth and leaf senescence of birch at levels that are frequently exceeded in southern Sweden. The predicted increases in both the use of birch in Swedish forestry and the concentrations of ground level O3 suggest that the role of O3 as a stress factor for Swedish forests is likely to increase in the future.