In this study,the differences in the ozone (O3) uptake in Pinus 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 Koelreuteria paniculata (0.00087 g-cm"2-s"1) > Pinus bungeana (0.00052 g-cm"2-s"1) > Ginkgo biloba (0.00047 g-cm"2-s"1) > Platycladus orientalis (0.00013 g-cm'2-s'1); ozone concentration ranged from 15 pg m"3 to 123.67 pg-m"3 diumally,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 (G03) was greater than the variation in F03 for the different tree species.The rank order of G03 and F03 of the different tree species was Ginkgo biloba > Koelreuteria paniculata > Platycladus orientalis > Pinus bungeana; the average F03 of the different tree species was 6.42±1.23 nmol m^ s"1,and the mean value of G03 was 9.10±2.14 mmol-m"2-s"1.G03 and F03 for Koelreuteria paniculata and Ginkgo biloba were higher than those for Pinus bungeana and Platycladus orientalis; VPD and Ta were positively correlated with ozone concentration (R = 0.95 and 0.86,respectively).G03 decreased as VPD and Ta increased,andFo3 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 bungeana.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 bungeana and Platycladus orientalis should be preferentially employed for urban greening given that they are more ozone-resistant tree species.
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