作 者 ：郑元,赵忠*,周慧,周靖靖
期 刊 ：生态学报 2010年 30卷 23期 页码：6399~6408
Keywords:Robinia pseudoacacia, photosynthesis, canopy, slope aspect, spatial heterogeneity,
摘 要 ：林木冠层是森林与外界环境相互作用最直接的部分，冠层光合作用是研究森林生产力的基础。为了深入了解冠层内部光合作用的差异性，以陕西省永寿县马莲滩流域阳坡和阴坡立地的刺槐林为研究对象，对比分析了光合速率（An）、蒸腾速率（E）、水分利用效率（WUE）、气孔导度（gs）、羧化效率（Vc）、水汽压亏缺（VPD）、气孔限制值（ls）、光合有效辐射（PAR）、空气温度（Ta）在树冠不同层次、不同方位，以及不同坡向之间的差异性。结果表明，刺槐树冠不同层次的光合作用差异性显著，大部分光合生理生态指标表现为：上 > 中 > 下。对于阳坡刺槐，VPD、Ta、gs、E是影响不同层次An的主要因子；对于阴坡刺槐，VPD、E、PAR是影响不同层次An的主要因子。光合作用在刺槐树冠的不同方位没有显著差异，大多数光合指标变化很小，E、ls、PAR、Ta是影响不同方位An的主要因子。对于刺槐冠层内部的任何层次或方位，阴坡刺槐具有更高的日均An、E、Vc、VPD、ls，而阳坡刺槐具有更高的日均WUE、gs、PAR、Ta。阳坡刺槐树冠中层西方和阴坡中层东方的日总光合速率值，可以分别代表阳坡和阴坡刺槐整个冠层的日总光合速率。研究认为，在冠层水平模拟和估计森林生产力时，必须考虑冠层光合作用的空间异质性，对于从单木到林分的尺度推演和模型拟合具有重要的意义。
Abstract:Forest canopy is a direct part of a forest interacting with its environment. At the plantation level, there is a huge spatial heterogeneity of forest canopy. At different canopy levels and canopy orientations, energy transmission, solar radiation and water evaporation differ significantly, which may cause the spatial heterogeneity of canopy photosynthesis. Obviously, the spatial heterogeneity of canopy photosynthesis should be considered when forest primary productivity is estimated. However, little information is currently available about the spatial heterogeneity of canopy photosynthesis. To better understand the spatial difference in photosynthesis at the canopy level, we analyzed the net CO2 assimilation rate (An), transpiration rate (E), water use efficiency (WUE), stomatal conductance (gs), carboxylation efficiency (Vc), vapor pressure deficit (VPD), stomatal limitation (ls), photosynthetic active radiation (PAR), air temperature (Ta) at different canopy levels and orientations of black locust (Robinia pseudoacacia) plantations located at sunny and shady slopes at the Maliantan valley of Yongshou county, Shaanxi Province. Photosynthetic parameters at different canopy levels of black locust plantations have showed significantly different values. Irrespective of slope aspects, values for An, E, gs, Vc, and PAR follow the pattern: the upper canopy is more than the middle canopy which is more than the lower canopy. In contrast, values for VPD, ls, and Ta show the pattern: the lower canopy is more than the middle canopy which is more than the upper canopy. The contributions of main photosynthetic parameters to An at different canopy levels or orientations of black locust plantations were determined using path analysis. For the black locust plantation on the sunny slope, VPD, Ta, gs, E are the main factors to impact An at different canopy levels. For the black locust plantation on the shady slope, VPD, E, PAR are the main factors to impact An at different canopy levels. No marked difference was observed in photosynthesis at different orientations of black locust canopies both on sunny and shady slopes. Values of most photosynthetic parameters change little and E, ls, PAR, Ta are the main factors to influence An at different orientations of the canopies. For any canopy level and/or orientation, the black locusts have higher daily mean An, E, Vc, VPD, ls on the shady slope, while they show greater daily mean WUE, gs, PAR, Ta on the sunny slope. The daily total An of black locust plantation at the west middle canopy level on the sunny slope and at the east middle level on the shady slope can represent the daily total An of the whole canopies on the sunny and shady slopes, respectively. These results suggest that spatial heterogeneity of canopy photosynthesis in black locust plantations should be considered when forest productivity is simulated at the canopy level, which may have a great implication for the simulation of forest productivity from a single tree to the plantation level. Our observations also imply that black locust plantations on the shady slope present better photosynthetic capacities than those on the sunny slope, which may lead to greater afforestation survival rates and/or higher primary productivities of this species on the shady slope compared to those on the sunny slope on the Loess Plateau.
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