© 1997 Heron Publishing—Victoria, Canada
Effects of duration of a simulated winter thaw on dieback and xylem conductivity of Betula papyrifera
R. M. Cox and J. W. Malcolm
Canadian Forest Service–Atlantic Forestry Centre, Natural Resources Canada, P.O. Box 4000, Fredericton, N.B. E3B 5P7, Canada / Received May 23, 1996
Summary
Stems or roots + stems of potted, 2-year-old paper birch (Betula papyrifera L.) were subjected to simulated winter thaws of various durations in climate-controlled chambers. The simulated thaws induced
dieback of shoots of the treated plants. Although the stem thaw treatment did not significantly increase dieback, there were
significant (P < 0.05) correlations between growing degree days above 4 °C and both shoot dieback and percent reduction in conductive xylem.
All trees that received > 60 growing degree days (GDD) > 4 °C died back to some extent. Plants in the root + stem thaw treatment
that received more than 60 GDD > 4 °C showed a significant (P < 0.05) increase in dieback and a significant (P < 0.05) loss of conducting xylem after a period of growth and recovery in the greenhouse, especially in the xylem of 1-year-old
stems. Furthermore, higher correlations between GDD > 4 °C during a thaw and both the extent of dieback and the loss in conductive
xylem were found in trees subjected to the root + stem thaw treatments than in trees exposed only to the stem thaw treatments
(P < 0.05). The root + stem thaw treatments also resulted in highly significant relationships (P < 0.05–0.001) between loss in conductive xylem and dieback. The occurrence of dieback in response to winter thaws, and its
close correlation with irreversible losses of xylem conductivity due to embolisms, coupled with an inability to refill the
xylem because of root damage, support the view that these processes may be key factors in initiating birch decline.
Keywords:
bud expansion, embolism, paper birch, root damage.
