Abstract: [Objective] This paper aims to investigate the effects of irradiances with different tree sizes of Quercus aliena var. acuteserrata on the carbon-water balances.[Method] The upper part and lower part of the crown of adult trees, saplings in understory and saplings in gaps were used as the materials in this study, and the hydraulic parameters (including water potential, and soil-to-plant hydraulic conductance), gas exchanges, photosynthetic light response curve, and chlorophyll fluorescence parameters were measured by using PMS1000 pressure chamber, LI-6400XT photosynthesis system, PAM-2500 fluorometer, respectively. Nonstructural carbohydrate (NSC), composed of starch and soluble sugars, concentrations of different tissues were measured with the anthrone-sulfuric acid method. In additon, the functional traits of leaves and shoots (leaf size, specific leaf area, Huber value) were also measured.[Result] 1) The upper crown of adult trees had the lowest twig pre-dawn water potential and midday water potential, and the leaf stomatal density was highest, stomatal length was lowest among the four experimental materials. The net photosynthetic rate, stomatal conductance, water use efficiency and dark respiratory rate of the upper crown were significantly lower than those of saplings in gaps. 2) The NSC concentrations in leaves and phloem of the upper crown were significantly higher than that of saplings in gaps, and NSC concentration in xylem of the upper crown was significantly lower than saplings in gaps, suggesting that hydraulic limitation with increasing tree height would reduce the cell turgor and magnify xylem embolism. 3) The light availability of the upper crown of adult trees and saplings in gaps was much greater, and their net photosynthetic rate, stomatal conductance, transpiration rate, water use efficiency, light compensation point, PSⅡ maximal photochemical efficiency (Fv'/Fm') and PSⅡ actual photochemical efficiency (Φ_PSⅡ) were significantly higher than the shaded lower part of the adult tree crown and saplings in understory. Especially the seriously-shaded environment in understory significantly reduced the photosynthetic capacity and carbon sequestration of saplings. The leaf and root NSC concentrations of saplings in understory were significantly lower than that of the lower crown leaf and adult tree root, respectively.[Conclusion] The hydraulic limitation in the upper part of adult tree crown was remarkable. Compared to saplings in gaps, the NSC in leaves and phloem of the adult tree upper crown accumulated due to a decrease in growth caused by hydraulic limitation, and frequent embolism refilling consumed much NSC in xylem of the adult tree upper crown. The irradiance was the main factor for modulating the photosynthesis of Q. aliena in terms of irradiation and tree size. Carbon starvation resulting from seriously-shaded environment in understory might be the major reason for Q. aliena regeneration difficultly.

Key words: hydraulic limitation hypothesis, water potential gradient, hydraulic conductance, nonstructural carbohydrate, cell turgor, carbon starvation