The photosynthetic and stomatal characteristics and chloroplast ultrastructure of poplar were discussed in shelterbelts of black soil region in Heilongjiang Province, northeastern China. The poplars were treated with the root excisions mulched with film from the trunk 0.5, 1 and 2 m, compared with no treatments (CK). The results indicated that net photosynthesis rate of treated poplars decreased significantly compared with CK. The decreasing degree enhanced with the increasing root excision strength after one month. Three months after root excision, the net photosynthesis rate of the poplars almost returned to CK values. The net photosynthetic rate of treated poplars declined, with the stomatal resistance of leaves increasing and the intercellular CO2
concentration decreasing. These phenomena suggested that the decreasing net photosynthetic rate was mainly caused by stomatal limitation. The stomatal density of the upper and lower epidermis of the poplar leaves was obviously decreased than CK after one month. Two months later, with the stomatal density increasing continuously, the lower epidermis of the poplar leaves was significantly higher than CK. After one month of treatments, chloroplast ultrastructure of poplar leaves was significantly changed. With the root excision strength increasing, the phenomena were as followings: the chloroplast showed expansion, the plasmolysis was more evident, lamellae of chloroplast were blurred, starch grain volumes became smaller and the quantity was reduced, the number of the osmiophilic granules had increased enormously. Chloroplast ultrastructure of the treated poplar leaves was recovered well and the number and volume of starch granules in the chloroplasts were significantly increased in the treatments of root excision mulched with film from the trunk 1 and 2 m after three months. The structural and functional injuries of treated poplar leaves were reversible. The photosynthetic characteristics, the stomatal morphology and the chloroplast ultrastructure of poplar leaves had adapted to the water stress causing by root excision mulched with film. These changes were correlated with the intensity of root excision. In summary, the treatments of root excision mulched with film in the distance from the trunk 0.5, 1 and 2 m are reasonable.