Responses of xylem cavitation, freezing injury and shoot dieback to a simulated winter thaw in yellow birch seedlings growing in different nursery culture regimes

Seedlings of yellow birch originating from the same seed source were treate d with two levels of fertilizers during two growing seasons. The lower leve l of fertilizers, such as 50 (11:41:08 N:P:K ratio, 100 (20:08:20) and 35 p pm of nitrogen (08:20:30) were applied as the starter. grower and finisher, respectively. The higher level ones consisted of 75, 150, and 100 ppm of n itrogen in the same fertilizers. After ? years growth, seedlings treated wi th the higher level of fertilizers, had Fewer lateral branches, greater hei ght and larger stem diameter. After natural hardening from November to Febr uary, seedlings were subjected to 0, 5, 10, 19 and 27 days of a simulated w inter thaw followed by 10 weeks at - 10 degreesC. After the thaw-refreeze t reatments, series of measurements were carried out. Stem xylem cavitation a nd root freezing injury significantly increased with thaw duration regardle ss of levels of fertilizer treatments. Stem freezing injury also significan tly increased with thaw duration in all stem segments of seedlings treated with the higher level of fertilizers, but only in the top segments for the seedlings provided with the lower level of fertilizers. However, seedlings treated with the higher level of fertilizers developed cavitation more quic kly. After I month of growth in the greenhouse following the thaw and freez e treatment, both types of seedlings showed significant decrease in the len gth of new shoot growth and increase in percent length of shoot dieback wit h thaw duration. The length of new shoots, however, was always greater for the plants treated with the higher level of fertilizers. No difference of s hoot dieback between the seedlings of the two different nursery treatments were observed. Correlation analyses showed that the length of new shoots wa s highly related to root and stem freezing injury, while dieback was best c orrelated with root freezing injury and stem xylem cavitation regardless of the levels of fertilizer treatments. It was concluded that (1) the higher level of fertilizer applied during the culture of yellow birch seedlings ca n accelerate xylem cavitation and dehardening in the stem following freeze- thaw events; (2) stem xylem cavitation was unlikely the cause of stem freez ing injury; and (3) root freezing injury and stem xylem cavitation are the most reliable measurements for predicting dieback of potential planting sto ck, but both root and shoot freezing injury are relate well to regrowth of new shoots in stock exposed to prolonged thaw. (C) 2001 Elsevier Science B. V. All rights reserved.