光强和树体大小对锐齿栎树木水、碳平衡的影响

Effects of Irradiation and Tree Size on the Carbon-Water Balances of Quercus aliena var. acuteserrata

【目的】测定锐齿栎在不同光强下不同大小树木的水分状况、光合作用和非结构性碳(NSC)差异,探索光强和树体大小对锐齿栎树木水、碳平衡的影响。【方法】以锐齿栎成年树树冠上部、树冠下部、林下幼树和林窗幼树为研究对象,用PMS1000压力室、LI-6400XT光合仪、PAM-2500荧光仪等仪器分别测定水势等水力参数、气体交换参数、光合光响应曲线参数和叶绿素荧光参数,用蒽酮-硫酸法测定不同组织NSC浓度,另外还测定叶片大小、比叶面积、Huber值等叶片和枝条的功能性状。【结果】1)在4个研究对象中,成年树树冠上部的小枝凌晨水势和中午水势均是最低,且叶片气孔密度最大、气孔长度最小;成年树树冠上部叶片的净光合速率、气孔导度、水分利用效率和暗呼吸速率显著低于林窗幼树。2)成年树冠层上部叶片和韧皮部的总非结构性碳(TNC)浓度显著高于林窗幼树,而木质部TNC浓度显著低于林窗幼树,说明树高造成的水力限制会降低细胞膨压并增大木质部栓塞风险。3)与光有效性低的树冠下部和林下幼树相比,光有效性高的成年树树冠上部和林窗幼树具有更大的净光合速率、气孔导度、蒸腾速率、水分利用效率、光补偿点、PSⅡ有效光化学效率(F'_v/F'_m)、PSⅡ实际光化学效率(ΦPSⅡ);尤其是更为遮荫的林下环境显著降低了林下幼树的光合潜能和固碳,叶片和根的TNC浓度显著低于冠层下部叶片和成年树的根。【结论】锐齿栎成年树冠层上部存在明显的水力限制,造成生长下降使得成年树冠层上部叶片和韧皮部的TNC相对积累,而木质部经常性的栓塞修复大量消耗木质部TNC;光照因素对锐齿栎光合作用的塑造起主要作用;林下遮荫环境导致的碳饥饿可能是宝天曼锐齿栎树种难以更新的主要原因。

[ 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 PS II actual photochemical efficiency （ФpsⅡ） 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.