Abstract: ‘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’ formation, taking Populus simonii, a tree species that accounted for the largest portion of ‘old and dwarf trees’ on the loess plateau, as an example,  the 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 photosynthetic rate, stomatal conductance, transpiration rate, and whole plant hydraulic conductance decreased 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 hydraulic 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 estimation, 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.