Abstract: To explore water balance and influencing factors in typical plantation ecosystems under drought conditions in northern China, we analyzed water balance components and influencing factors in Pinus tabuliformis and Platycladus orientalisplantations in Beijing mountain region in a dry year (2014). The results showed that the rainfall in 2014 was only 47% of longterm mean value (1958-2011). Transpiration accounted for the highest proportion of total water loss in both forests. The stand transpiration of P. tabuliformis and P. orientalis plantations was 247.16 and 259.89 mm, respectively. The other water loss pathways ranked following the order: soil evaporation (P. tabuliformis, 87.26 mm; P. orientalis, 60.02 mm) > canopy interception (P. tabuliformis, 56.63 mm; P. orientalis, 50.23 mm) > litter evaporation (P. tabuliformis, 9.99 mm; P. orientalis, 8.30 mm). Soil water was at a state of the annual deficit but was replenished during the second half of the growing season. The annual water deficit of the P. tabuliformisand P. orientalis stands was 81.24 and 67.45 mm, respectively. Stand water loss was significantly affected by environmental factors. Meanwhile, transpiration was significantly correlated with the stand diameter at breast height. Canopy interception was affected by canopy closure and wind speed. In conclusion, with precipitation as the single water source, water balance in both P. tabuliformis and P. orientalis stands cannot be met during a dry year. Therefore, these stands will face substantial survival challenge associated with precipitation reduction under climate change.

Key words: seasonal dynamics, redundancy analysis., urban forest, soil meso and microfauna, densitygroup index