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黄土高原地区“小老树”现象多出现在成年树,幼树相对较少.为探讨树龄影响“小老树”形成的机制,以该地区“小老树”发生面积最大的树种小叶杨为例,研究了3个不同树龄(低龄:13~15 a;中龄:31~34 a;高龄:49~54 a)树木的生长、光合、水力学特性.结果表明:随树龄增加,小叶杨枯稍长度显著增加,叶片光合速率、气孔导度和蒸腾速率显著下降,整株水力导度也显著下降,不同测定时间的光合速率、气孔导度与整株水力导度呈显著正相关,表明树龄增加引起的光合速率下降可能与整株水力导度下降有关.与中、低龄相比,高龄小叶杨枝干和叶片抵抗空穴化的能力(P50)更强,但通过脆弱性曲线估算的不同树龄正午时的枝干栓塞程度和叶片水力导度无显著差异,表明高龄小叶杨土-根系统水流阻力的增加可能是导致其整株水力导度降低的重要原因.
In the Loess Plateau area, the phenomenon of “small old tree ” appears in the adult tree, the young tree is relatively less.To explore the mechanism of the age effect “small old tree ”, take the area "small old tree The largest species of Populus simonii was taken as an example to study the growth, photosynthesis and hydraulics of three tree species with different age (13 to 15 years old, 31 to 34 years old and 49 to 54 years old) .The results showed that: The photosynthetic rate, stomatal conductance and transpiration rate decreased significantly with the increase of tree age, and the hydraulic conductivity of the whole plant also decreased significantly. The photosynthetic rate, stomatal conductance and hydraulic conductivity The results showed that the decrease of photosynthetic rate caused by the increase of tree age may be related to the decline of hydraulic conductivity of the whole plant.Compared with the middle and low age, No significant difference was observed between the degree of embolism at the noon and the hydraulic conductivity of leaf at different ages, which indicated that the increase of water flow resistance in the young soil-root system may be the important reason for the decrease of hydraulic conductivity of the whole plant.