Vertical gradient of needle ozone uptake within the canopy of Cryptomeria japonica

Leaf ozone uptake through the stomata is an important index for the ozone risk assessments on trees. Stomatal conductance (g_s) and ozone concentration ([O₃]), determinants of the leaf ozone uptake, are known to show vertical gradients within a tree canopy. However, less is known about the within-canopy vertical gradient of leaf ozone uptake. This study was aimed to elucidate how the vertical gradient of [O₃] and g_s affect needle ozone uptake within a canopy of mature Cryptomeria japonica trees in a suburban forest at Tokyo, Japan. For this purpose, a multilayer gas exchange model was applied to estimate the vertical gradient of needle g_s and the accumulated ozone uptake during the study period (POD₁, Phytotoxic Ozone Dose above a threshold of 1 nmol m^-2 s^-1). In addition, we also tested several scenarios of vertical gradient of [O₃] within the canopy for sensitivity analysis. The POD₁ was declined from the top to the bottom of the canopy. This tendency strongly depended on the vertical gradient of g_s and was hardly affected by the changes in simulated vertical reductions of the [O₃]. We further assessed the photosynthesis of sunlit needles (needles absorbing both direct and diffuse light) and shaded needles (needles only absorbing diffuse light). The photosynthesis of shaded needles in the upper half of the canopy made a great contribution to the entire canopy photosynthesis. In addition, given that their POD₁ was lower than that of sunlit needles, ozone may affect sunlit and shaded needles differently. We concluded that these considerations should be incorporated into modeling of the calculation of ozone uptake for mature trees to make accurate predictions of the ozone effects on trees at the canopy scale.