GROWTH-RATES AND PHENOLOGY OF FAST-GROWING AND SLOW-GROWING FAMILIES OVER AN ENTIRE GROWTH PERIOD IN BETULA-PENDULA ROTH

Differences in growth rate and growth rhythm, and their contribution t o stem volume were analyzed in fast- and slow-growing full-sib familie s of silver birch (Betula pendula). Observations were made on the basi s of a 15-year-old progeny test. Significant differences between fast- and slow-growing family groups were noted for growth rates at almost all development periods over the entire growing season, for growth ces sation and for the length of growth period. Growth rates at the middle periods of the growing season and the total length of the growth peri od were strongly correlated with annual growth or total stem volume. T otal stem volume was mainly attributable to growth rates during a cert ain period in the middle of the growing season, here named ''growth ef ficiency'' (GE). It suggests that direct selection for this yield comp onent may be the most efficient way of breeding for high yield.