Abstract:

Pruning on 4-year-old plantations of Populus tomentosa Carr. with three degrees in Weixian (site I) and Bazhou (site II) of Hebei Province was studied in this paper. The three pruning degrees were low-grade P1, medium-grade P2, strength-grade P3 and the unpruned for control. Tree height, diameter at breast height, physiological parameters including net photosynthetic rate and transpiration rate in different canopies were measured, in order to explain the impact of pruning from the physiological mechanism. The results were: 1) Pruning inhibited the growth of diameter, in proportion to the increasing intensity of pruning. Difference in P1 and P2 were not significant with CK, while, P3 was significantly different with CK in site I, whereas not significant with CK in site II; 2) Pruning speeded the height growth, and P1,P2 were conspicuously with CK(p＜0.05); 3) The leaf area index(LAI) changed significantly after pruning, and leaf density of pruned Populous stand increased while the LAI of CK decreased. Thus, pruning can extend growing stage of trees; 4) Pruning improved net photosynthesis rate and transpiration rate of leaves in the middle and upper of crown, but these indices in the bottom crown were low at the same time. Considering from the growth，photosynthetic and physiological indices of Populus tomentosa Carr., branches in the bottom of canopy should be pruned after canopy closing.