EFFECT OF PARTIAL DEFOLIATION ON GROWTH AND CARBON EXCHANGE OF 2 CLONES OF YOUNG POPULUS-TRICHOCARPA TORR AND GRAY

A controlled environment study was performed to test the effect of two levels of a single partial defoliation on growth, dry weight partitio ning, and gas exchange of two contrasting clones of Populus trichocarp a Torr. & Gray. One clone was from a moist habitat in western Washingt on and the other from a dry habitat in eastern Washington. Trees were partially defoliated (40% or 80% of leaf area removed) at the 25-leaf stage. Height, diameter, leaf area, and rates of net photosynthesis, d ark respiration, transpiration, and leaf conductance were determined a t the time of defoliation and 3 wk, and 5 wk after defoliation. Biomas s distribution was determined following harvest 5 wk after defoliation . Defoliation caused reductions in height, diameter, and biomass in bo th clones compared to control plants. However, reductions were not pro portional to level of defoliation, suggesting compensatory growth was occurring. Total biomass was reduced by 30% and 27% in the western and eastern Washington clones respectively by a 40% defoliation, and 43% and 35% following an 80% defoliation. A 40% defoliation caused an incr ease in the proportion of biomass in lateral branches of trees of the western Washington clone. An 80% defoliation caused an increase in the proportion of biomass in lateral branches of both clones. Both defoli ation treatments reduced leaf area on the main stem but increased leaf area on lateral branches. Both levels of defoliation reduced root bio mass of both clones. Specific leaf weight of leaves produced after def oliation in both clones increased. Partial defoliations of 40% and 80% had no effect on net photosynthesis of either clone. Rates of dark re spiration in both clones were either unaffected by defoliation or slig htly reduced.