CHARACTERISTICS OF OZONE UPTAKE IN URBAN FOREST CANOPY BASED ON SAP FLOW MEASUREMENTS

In this study, the differences in the ozone (03) uptake in Firms bungeana, Platycladus orientalis, Koelreuteria paniculata and Ginkgo biloba were studied by the sap flow technique. In the growing season (May to October), the flow density reached its maximum value in May, which was 2.63 times that in October. The rank order of the mean value of sap flow density in the growing season was Koelreu-feria paniculata (0.00087 g.cm(-.2.)s(-1)) > Pinus hungeana (0.00052 g.cm(-2.)s(-1)) > Ginkgo biloba (0,00047 g.cm(-2).sat) > Platycladus (0,00013 g.cm(-2).s(-1)); ozone concentration ranged from 15 mu g.m(-3) to 123,67 mu g.m(-3) diurnally, and the diurnal pattern of variation resembled a curve with a single peak. Furthermore, the ozone concentration was positively correlated with temperature (R-0.96). The variation in ozone canopy conductance (G(O3)) was greater than the variation in F-O3 for the different tree species. The rank order of G(O3) and F-O3 of the different tree species was Ginkgo biloba > Koelreuteria paniculata > Platycladus orientalis > Pinus bungeana; the average F-O3 of the different tree species was 6.42 +/- 1.23 nmol.m(2).s(-1), and the mean value of G(O3) was 9.10 +/- 2.14 mmol.m(-2).s(-1). G(O3) and F-O3 for Koelreuteria paniculata and Ginkgo biloba were higher than those for Pinus bungeana and Platycladus orientalls; VPD and Ta were positively correlated with ozone concentration (R = 0.95 and 0.86, respectively). G(O3) decreased as VPD and Ta increased, and F-O3 increased as VPD and Ta increased; there were differences among different tree species, and the rank order was as follows: Koelreuteria paniculata > Ginkgo biloba > Platycladus orientalis > Pinus hungeana. The ozone uptake rate was greater in broad-leaved trees than in conifers, indicating that Koelreuteria paniculata and Ginkgo biloba were more sensitive to ozone pollution. Therefore, coniferous trees such as Pinus hungeana and Platycladus orientalis should he preferentially employed for urban greening given that they are more ozone-resistant tree species.