Fan S-M, Goulden M L, Munger J W, Daube B C, Bakwin P S, Wofsy S C, Amthor J S, Fitzjarrald D R, Moore K E, Moore T R
Division of Applied Sciences, Harvard University, Cambridge, MA, USA.
, 29 Oxford Street, 02138, Cambridge, MA, USA.
Oecologia. 1995 Jun;102(4):443-452. doi: 10.1007/BF00341356.
Measurements of net ecosystem CO exchange by eddy correlation, incident photosynthetically active photon flux density (PPFD), soil temperature, air temperature, and air humidity were made in a black spruce (Picea mariana) boreal woodland near Schefferville, Quebec, Canada, from June through August 1990. Nighttime respiration was between 0.5 and 1.5 kg C ha h, increasing with temperature. Net uptake of carbon during the day peaked at 3 kg C ha h, and the daily net uptake over the experiment was 12 kg C ha day. Photosynthesis dropped substantially at leaf-to-air vapor pressure deficit (VPD) greater than 7 mb, presumably as a result of stomatal closure. The response of ecosystem photosynthesis to incident PPFD was markedly non-linear, with an abrupt saturation at 600 μmol m s. This sharp saturation reflected the geometry of the spruce canopy (isolated conical crowns), the frequently overcast conditions, and an increase in VPD coincident with high radiation. The ecosystem light-use efficiency increased markedly during overcast periods as a result of a more even distribution of light across the forest surface. A mechanistic model of forest photosynthesis, parameterized with observations of leaf density and nitrogen content from a nearby stand, provided accurate predictions of forest photosynthesis. The observations and model results indicated that ecosystem carbon balance at the site is highly sensitive to temperature, and relatively insensitive to cloudiness.
1990年6月至8月，在加拿大魁北克省谢弗维尔附近的一片黑云杉（Picea mariana）北方林地，通过涡度相关法对生态系统净碳交换、光合有效光子通量密度（PPFD）、土壤温度、气温和空气湿度进行了测量。夜间呼吸作用在0.5至1.5千克碳每公顷每小时之间，并随温度升高而增加。白天碳的净吸收量峰值为3千克碳每公顷每小时，整个实验期间的每日净吸收量为12千克碳每公顷每天。当叶-气蒸汽压亏缺（VPD）大于7毫巴时，光合作用大幅下降，这可能是气孔关闭的结果。生态系统光合作用对入射PPFD的响应明显呈非线性，在600微摩尔每平方米每秒时突然饱和。这种急剧饱和反映了云杉树冠的几何形状（孤立的圆锥形树冠）、频繁的阴天条件以及与高辐射同时出现的VPD增加。由于森林表面光照分布更加均匀，阴天期间生态系统的光利用效率显著提高。一个以附近林分的叶密度和氮含量观测数据为参数的森林光合作用机理模型，对森林光合作用进行了准确预测。观测结果和模型结果表明，该地点的生态系统碳平衡对温度高度敏感，而对云量相对不敏感。
