中文摘要：

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：13～15a；中龄：31～34a；高龄：49～54a）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．

英文摘要：

‘Old and dwarf trees＇ on the loess plateau region mainly occurred among mature trees rather than among small trees. To elucidate the mechanism of tree age on ‘ old and dwarf trees＇ for- mation, taking Populus simonii, a tree species that accounted for the dwarf trees＇ on the loess plateau, as an example, the largest portion of ‘old and growth, photosynthesis and hydraulic traits of P. simonii trees with different ages （young： 13-15 years, mid-aged： 31-34 years, and old： 49-54 years） were measured. The results showed that the dieback length increased, and net photo- synthetic rate, stomatal conductance, transpiration rate, and whole plant hydraulic conductance de- creased significantly with the increasing tree age. Both net photosynthetic rate and stomatal conduc- tance measured at different dates were significantly and positively related to the whole plant hydrau- lic conductance, suggesting that the decreasing photosynthetic rate of old trees was possibly caused by the declined hydraulic conductance. Although the resistance to cavitation in stems and leaves was stronger in old trees than in young and mid-aged trees, there were no differences in midday native stem embolization degree and leaf hydraulic conductance based on the vulnerability curve es- timation, suggesting that the increased hydraulic resistance of the soil-root system is probably the most important reason for decreasing the whole plant hydraulic conductance of old trees.