PRODUCTION PHYSIOLOGY AND MORPHOLOGY OF POPULUS SPECIES AND THEIR HYBRIDS GROWN UNDER SHORT-ROTATION .2. BIOMASS COMPONENTS AND HARVEST INDEX OF HYBRID AND PARENTAL SPECIES CLONES

Growth and biomass components were studied during 4 consecutive years of short-rotation culture in western Washington, U.S.A., for four popl ar clones, including Populus deltoides Bartr., Populus trichocarpa Tor r. & A. Gray, and their interspecific hybrids. Results confirm previou s observations indicating the high productive potential of hybrid clon es of P. trichocarpa x P. deltoides; at the end of the second year the maximum rate of total biomass production reached 21 Mg . ha(-1). year (-1) and at the end of the fourth year maximum aboveground biomass pro duction was 35 Mg . ha(-1). year(-1). In two of the hybrid clones test ed, 11-11 and 44-136, large differences in biomass distribution among tree's components and in the pattern of growth were evident, as indica ted by harvest index and root/shoot ratios. An examination of the biom ass distribution coefficients demonstrated that there were temporal ch anges and clonal differences in the activity of biomass sinks within a tree. Again, hybrid clones had contrasting patterns in the first year : clone 11-11 apportioned more biomass into leaves and branches, where as clone 44-136 apportioned more biomass into roots, which contributed to rapid leaf and stem growth early in the second growing season. Our results suggest that clonal differences in total biomass, in allocati on to different tree components, and in harvest index have important i mplications for future ideotype-based breeding programs with Populus.