The land degradation has been accelerated in arid and semi-arid area due to the climate change and human activities. Mongolia has been considered as one of countries which under the effect of desertification. Although reforestation was conducted in Mongolia with the aims of combating desertification, the reforestation rate was very low due to the limited funds. Furthermore, the drought considered as a main constraint for plant growth in desertification area. Therefore, developing the effective reforestation scheme (e.g. an irrigation interval) is important for successful reforestation. Populus sibirica was widely used for reforestation in Mongolia because it grows fast and has resistance to drought and salinity. However, there were only limited studies about physiological responses of P. sibirica to drought. Two-years old P. sibirica was planted in Elsen-tasarkhai, an arid region in Mongolia, 2014. Three different irrigation regimes (well-watered (WW), irrigation every 2 days, 14 L tree-1 week-1; moderate drought (MD), irrigation every 5 days, 5.6 L tree-1 week-1; severe drought (SD), irrigation every 7 days, 4 L tree-1 week-1) were applied with drip irrigation system from June to August, 2015. The survival rate, height relative growth rate, leaf water potential and physiological traits were measured during growing season.
Even though the applied drought treatment did not affect the survival rate, the relative growth rate was significantly decreased in MD and SD in accordance with the reduction of carbon assimilation. As the drought intensity increased, the leaf water potential was significantly decreased in MD and SD. The photosynthetic rate, transpiration and stomatal conductance was significantly reduced as the drought intensity increased. However, the intercellular CO2 concentration was increased in SD. These results indicated that the decline of photosynthesis was caused by not only the stomatal aperture but also the restrictions in carbon fixation. The chlorophyll fluorescence was measured in order to evaluate electron transport in the light reaction of photosynthesis. The quantum efficiencies per RC and CS were significantly increased in SD. Also, the total chlorophyll content was significantly decreased in SD. These data indicated that P. sibirica dissipated the exceed energy effectively to prevent from photoinhibition. From these data, P. sibirica could tolerate the applied drought stress effectively. Although the optimal irrigation regime could be concluded as 4L∙week-1∙tree-1 in this study, further study with longer irrigation intervals are needed.