EFFECTS OF DURATION OF A SIMULATED WINTER THAW ON DIEBACK AND XYLEM CONDUCTIVITY OF BETULA-PAPYRIFERA

Stems or roots + stems of potted, 2-year-old paper birch (Betula papyr ifera L.) were subjected to simulated winter thaws of various duration s 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 degrees C and both sh oot dieback and percent reduction in conductive xylem. All trees that received > 60 growing degree days (GDD) > 4 degrees C died back to som e extent. Plants in the root + stem thaw treatment that received more than 60 GDD > 4 degrees 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 xyl em of 1-year-old stems. Furthermore, higher correlations between GDD > 4 degrees 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 s ignificant relationships (P < 0.05-0.001) between loss in conductive x ylem and dieback. The occurrence of dieback in response to winter thaw s, and its close correlation with irreversible losses of xylem conduct ivity due to embolisms, coupled with an inability to refill the xylem because of root damage, support the view that these processes may be k ey factors in initiating birch decline.