Canopy stomatal conductance (gs) is an important biological parameter to measure water vapor flux in canopy atmosphere interface. Exploring characteristics of canopy stomatal conductance and its responses to environmental factors can offer theoretical evidence for conducting mechanism based study of vapor exchange process in forest canopy. SF-L thermal dissipation probe was adopted to measure sap flow density (Js) of Platycladus orientalis in 2014. The environmental factors including photosynthetically active radiation (PAR), vapor pressure deficit (VPD) and air temperature (T) were simultaneously observed. The canopy stomatal conductance was calculated, and the responses of gs to environmental factors were analyzed. The results showed that the daily variation of sap flow density was of two peak pattern curve, and the Js in growing season was higher than in non-growing season. And the bigger DBH was, the higher Js would be. The daily variation of canopy stomatal conductance was similar to the canopy transpiration of per leaf area (EL), which was of two peak pattern curve. The gs and EL in growing season were higher than those in non growing season. The canopy stomatal conductance and air temperature presented parabolic relation, and gs reached peak valley at around 10 ℃. Taking the value of 400 μmol·m-2·s-1 as the PAR thre shold, when PAR was under this threshold, gs was positively correlated with PAR. When PAR was beyond this threshold, the correlation between gs and PAR would be small. The relationship between canopy stomatal conductance and vapor pressure deficit was negative logarithmic function. As VPD increasing, gs decreased gradually. Higher air temperature and photosynthetically active radiation, and lower vapor pressure deficit contributed to higher canopy stomatal conductance of P. orientalis and could promote greater canopy transpiration.
冠层气孔导度（gs）是衡量冠层大气界面水汽通量的重要生物学常数，研究其特征及对环境因子的响应，能为开展森林冠层水汽交换过程的机理性研究提供理论依据.于2014年利用SFL热扩散式探针测定了侧柏的树干液流密度（Js），同步监测光合有效辐射（PAR）、饱和水汽压差（VPD）、气温（T）等环境因子，计算侧柏的冠层气孔导度特征并分析其对各环境因子的响应.结果表明：侧柏液流密度的日变化总体呈双峰曲线，生长季高于非生长季，且胸径越大液流密度越大；冠层气孔导度日变化与单位叶面积冠层蒸腾（EL）趋势相近，均呈双峰曲线，生长季的冠层气孔导度和蒸腾较非生长季略高.侧柏冠层气孔导度与空气温度呈抛物线关系，在10 ℃左右冠层气孔导度达到峰谷；光合有效辐射以400 μmol·m-2·s-1为界，小于该阈值两者呈正相关关系，大于该阈值则冠层气孔导度受其影响较小；与饱和水汽压差呈负对数函数关系，随饱和水汽压差增大而逐渐降低.较高的空气温度和光合有效辐射、较低的饱和水汽压差有利于侧柏形成较大的冠层气孔导度，进而促进冠层蒸腾.
Keywords: Platycladus orientalis; canopy stomatal conductance; photosynthetically active radiation; sap flow density; vapor pressure deficit.