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 treated with two levels of fertilizers during two growing seasons. The lower level of fertilizers, such as 50 (11:41:08 N:P:K ratio), 100 (20:08:20) and 35 ppm 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 nitrogen in the same fertilizers. After 2 years growth, seedlings treated with the higher level of fertilizers, had fewer lateral branches, greater height and larger stem diameter. After natural hardening from November to February, seedlings were subjected to 0, 5, 10, 19 and 27 days of a simulated winter thaw followed by 10 weeks at −10°C. After the thaw-refreeze treatments, series of measurements were carried out. Stem xylem cavitation and root freezing injury significantly increased with thaw duration regardless of levels of fertilizer treatments. Stem freezing injury also significantly 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 quickly. After 1 month of growth in the greenhouse following the thaw and freeze treatment, both types of seedlings showed significant decrease in the length of new shoot growth and increase in percent length of shoot dieback with thaw duration. The length of new shoots, however, was always greater for the plants treated with the higher level of fertilizers. No difference of shoot dieback between the seedlings of the two different nursery treatments were observed. Correlation analyses showed that the length of new shoots was highly related to root and stem freezing injury, while dieback was best correlated 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 can accelerate xylem cavitation and dehardening in the stem following freeze–thaw events; (2) stem xylem cavitation was unlikely the cause of stem freezing injury; and (3) root freezing injury and stem xylem cavitation are the most reliable measurements for predicting dieback of potential planting stock, but both root and shoot freezing injury are relate well to regrowth of new shoots in stock exposed to prolonged thaw.