GEOGRAPHICAL VARIATION AND CORRELATION IN SPECIFIC GRAVITY AND TRACHEID LENGTH OF JUVENILE WOOD OF MASSON PINE STUDY ON THE INHERITANCES AND CORRELATIONS OF WOOD QUALITY CHARACTERISTICS OF POPULUSבPOPULARIS‘IMPROVED CLONES GENETIC VARIATION IN GROWTH AND WOOD PROPERTIES AND MULTIPLE-TRAITS SELECTION OF POPLAR CLONES Heterosis of Populus×xiaozhuania cv.‘popularis’ Improved Hybrids F1 and Cluster Analysis for Clones according to Genetic Distance Study on Wood Quality Characteristics of Populus davidiana and Its Variation of Intra-tree