In order to photosynthesize, land plants must open their stomata to exchange small amounts of CO₂ at the cost of losing a lot of water vapor through transpiration. This gas exchange is unequal in that more water is lost than CO₂ is taken in. In general, fast growing species are characterized by their high photosynthetic capacity, most of which is achieved at the expense of consuming large amounts of water. This trade-off poses a serious challenge to forestry in the selection of fast-growing tree species in arid and semiarid areas. The ideal forestry species for drier climates should maintain a high photosynthetic capacity as well as low water consumption for transpiration, but such species are very rare. In this study, the LI-6400 (LICOR, Inc. Lincoln, NE, USA) portable photosynthesis system was employed to measure diurnal changes in photosynthetic and transpiration indices in sexually mature Pinus densiflora var. zhangwuensis and P. sylvestris var. mongolica (18 years old). In addition, the cut-branch transpiration method was used to compare stomatal and cuticular transpiration in order to evaluate the sensitivity of stomatal opening and closing and to characterize the photosynthetic productivity and water consumption for transpiration in these two species in different growing seasons. P. densiflora var. zhangwuensis had a higher photosynthesis rate (P_n) and a lower transpiration rate (T_r) than P. sylvestris var. mongolica under the same conditions. In May and July, the diurnal changes of P_n and T_r in P. densiflora var. zhangwuensis formed double-peaked curve as a result of mid-day stomatal limitation, while it formed single-peaked curve in October. The daily variation of P_n and T_r in P. sylvestris var. mongolica exhibited a monopeak curve throughout the growing season. The net photosynthesis per day in P. densiflora var. zhangwuensis was higher than in P. sylvestris var. mongolica; values for the former were 163.4 (May), 211.1 (July), and 183.6 (October) percent of the latter. The photoresponse parameter measurements showed that, in different months, the maximum rate of photosynthesis of P. densiflora var. zhangwuensis was greater than that of P. sylvestris var. mongolica, and the former also had a higher light saturation point and a lower light compensation point. The stomatal conductivity (G_s) and T_r of P. densiflora var. zhangwuensis were lower than those of P. sylvestris var. mongolica when measured at random times. The stomatal and cuticular T_r of P. densiflora var. zhangwuensis were lower, and in the same drought conditions, its stomata were deeper and had a higher sensitivity for opening and closing. Water use efficiency (WUE) of P. densiflora var. zhangwuensis was 2.29 times that of P. sylvestris var. mongolica. Analysis of the correlation between G_s and WUE showed that P. densiflora var. zhangwuensis could maintain high WUE when G_s levels were high. In May and July, when P. densiflora var. zhangwuensis exhibited midday photosynthetic depression, its G_s level was low, usually in the 90-200 mmol·m^-2·s^-1 range. At the same time, its WUE was higher than that of P. sylvestris var. mongolica, indicating that P. densiflora var. zhangwuensis could effectively conserve water by closing its stomata rapidly at midday, so as to maintain its high WUE. These results implied that P. densiflora var. zhangwuensis can improve its WUE, yielding its higher P_n and lower T_r, and achieves its fast growth in arid and semiarid regions through its sensitive stomatal response and leaf xeromorphism.