| As a basic measure for forest management, pruning has an important role on improving tree trunk shape and timber quality.This research focused on the pruning in Poplar ( Populus×euramericana cv.'74/76') plantation in Shandong and Hebei province, and the growth factors and physiological responses were measured so that the scientific pruning model which suitable to these stands can be found . The pruning treatments included control (unproved), low-intensity (1/6 crown was pruned), medium intensity (2/6 crown was pruned), high-intensity (3/6 crown was pruned), and severe-intensity p4 (4/6 crown was pruned). Tree total height and the diameter at breast height (DBH) were measured for all trees both before and after the pruning. The radial increment were measured, and the relative volume growth rate was calculated. The measured physiological parameters including net photosynthetic rate, transpiration rate, stomata conductance, leaf area index, sap flow velocity etc. The results indicated that:(1) The difference of relative tree height growth rate in different pruning treatment was not significant (P<0.05), this demonstrated that the nutrition for apex growth mainly came from the branches and leaves of top layer; the difference of relative growth rate of DBH was significant (P<0.01), that is, the higher pruning intensity, the lower DBH relative growth rate.(2) The influence from different pruning intensity on the net photosynthetic rate and other physiological parameters was significant.①The pruning improved net photosynthesis of the single leaf on the middle and top layer of crown in some degree, and with the increased pruning intensity, the net photosynthetic rate also increased.②At the same time, pruning increased the transpiration rate, particularly around the noon time, some pruning treatments trees can still keep the relatively higher transpiration rate.③The results showed that pruning reduced the saturation water deficit of individual foliage, and increased the leaf stoma conductance, which was propitious to improving net photosynthesis and transpiration rate.④As to the water-using efficiency of single leaf, the differences form all pruning treatments were not obviously, even some water-using efficiency of individual leaf from pruned was lower than that from control.(3)The difference of relative volume growth from different pruning intensity is significant. That means the suitable pruning intensity can increase the volume growth rate, but excessive pruning is disadvantageous to the volume growth.(4) The Effect of different Pruning intensity on the effective leaf area index was significant. Along with the increasing of pruning intensity, the effective leaf area index decreased gradually.(5) The difference of trunk flow rate under different pruning intensity was significant. Based on the data in Site I, the data of trunk flow rate from relative high-intensity pruning trees were just as half as these of control trees in May, June, and July. This indicated that the pruning can effectively reduced the transpiration water volume of whole individual tree in some degree. Although there was no difference between the pruned trees and unpruned trees in terms of the instantaneous water use efficiency at single leaf level, as to the flow rate at whole tree level, the pruning effectively reduced water consumption of trees, and improved the water use efficiency.(6) The radial growth of the analyzed trunk discs at different heights was different along with the various pruning intensity. Based on the analysis data from the trunk height from 0 to 11 m, the radial growth decreased in the lower part and increased at the higher part along with the increase of pruning density.(7) Pruning effectively decreased the taper of trunk, that means the relative taper of logs became smaller, and the trunks became full and straight, this increase the timber volume.
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