© 1995 Heron Publishing—Victoria, Canada
Dormancy release and chilling requirement of buds of latitudinal ecotypes of Betula pendula and B. pubescens
T. Myking and O. M. Heide
Department of Biology and Nature Conservation, Agricultural University of Norway, P.O. Box 5014, N-1432 Ås, Norway / Received November 16, 1994
Summary
Bud burst and dormancy release of latitudinal ecotypes of Betula pendula Roth and B. pubescens Ehrh. from Denmark (≈ 56° N), mid-Norway (≈ 64° N) and northern Norway (≈ 69° N) were studied in controlled environments.
Dormant seedlings were chilled at 0, 5 or 10 °C from October 4 onward and then, at monthly intervals from mid-November to
February, batches of seedlings were held at 15 °C in an 8-h (SD) or 24-h (LD) photoperiod to permit flushing. A decline in
days to bud burst occurred with increasing chilling time in all ecotypes. In November, after 44 chilling days, time to bud
burst was least in plants chilled at 0 and 5 °C. The difference diminished with increasing chilling time, and in February,
after 136 chilling days, bud burst was earliest in plants chilled at 10 °C. Long photoperiods during flushing significantly
reduced thermal time after short chilling periods (44 and 74 days), but had no effect when the chilling requirement was fully
met after 105 or more chilling days. No significant difference in these responses was found between the two species. In both
species, chilling requirement decreased significantly with increasing latitude of origin. Bud burst was normal in seedlings
overwintered at 12 °C, but was erratic and delayed in seedlings overwintered at 15 and especially at 21 °C, indicating that
the critical overwintering temperature is between 12 and 15 °C.
We conclude that there is little risk of a chilling deficit in birch under Scandinavian winter conditions even with a climatic
warming of 7–8 °C. The likely effects of a climatic warming include earlier bud burst, a longer growing season and increased
risk of spring frost injury, especially in high latitude ecotypes.
Keywords:
birch, bud burst, chilling temperature, photoperiod.
