EFFECTS OF DEFOLIATION, NITROGEN NUTRITION, AND TEMPERATURE ON LEAFING AND ROOT CARBOHYDRATES OF BIRCH SEEDLINGS

Difficulties were observed in the leafing of mature birch, alder, and willow species in southern Finland in the spring 1989, especially at l ush growing sites. The suspected reason was freezing of the buds in th e preceding autumn caused by disturbed bud dormancy. The hypothesis wa s tested experimentally with 1-year-old Betula pendula Roth seedlings, investigating the effects of partial defoliation, nitrogen nutrition, and temperature. Defoliation clearly weakened both the growth and ove rwintering of the seedlings, altered the carbohydrate relations in the ir roots, and markedly delayed leafing in the spring. The earlier in t he summer the defoliation was performed, the more conspicuous were its effects, which were also emphasized by the fertilization. A short gre enhouse treatment in autumn had no significant effect, however, and it is thought that the partial death of the defoliated top shoots may ha ve been caused by frost drying. The main reason for the late leafing s eems to be the delayed development of the buds as a consequence of ear ly defoliation. On the other hand, the growth-promoting effect of nitr ogen fertilization and the premature branching of the buds evidently d isturbed the carbohydrate balance in the roots and thus the developmen t of the whole seedlings.