Genetic variation and productivity of Populus trichocarpa and its hybrids .9. Phenology and Melampsora rust incidence of native black cottonwood clones from four river valleys in Washington

A common-garden study of Populus trichocarpa Torr. and Gray was initiated in 1985 when material from 128 trees was collected along two mesic (Hoh and Nisqually) and two xeric (Dungeness and Yakima) river valleys. In spring 1986 cuttings were used to establish two replicate plantations, at Puyallup and at Wenatchee, WA. For 2 years trees were assessed for timing of spring flush, autumn budset and leaf fall, and incidence of Melampsora occidentalis leaf rust. Seasonal patterns varied with trait, plantation, year, and origin of clones. Significant differences were found mainly between clones from the lower and upper elevations of the Nisqually (LN, UN) and Yakima (LY, UY). At both plantations, clones from the cooler UN flushed later than those from the LN. In contrast, cool canyon climates of some sites along the LY resulted in later spring flush of lower than upper clones. Factors affecting patterns of autumn phenology differed between trees from those river valleys. Later budset in LN vs. UN trees was again related to cold affecting the growing season length, but earlier budset and leaf fall in LY than UY clones at Puyallup was brought about by higher rust infection. The greater rust susceptibility of LY material likely reflects lower rust pressure in the more arid lower Yakima valley, Soil moisture deficit and the presence/absence of rust at the plantations also contributed to autumn phenology patterns. Genetic variances were large for all traits at Puyallup. In Yakima trees, the large group component of the genetic variance (elevational group, clone) in leaf fall and rust points to a steep selection gradient along the Yakima. A similarly steep gradient seems also to influence spring flush along the Nisqually. Heritabilities indicate moderately to very strong genetic control in these traits. The patterns of genetic variation have implications for short-rotation intensive-culture forestry and management of natural populations.