Key message In Canadian boreal forests, trembling aspen trees form frost rings in subzero temperatures after the high-degree days required for tree-ring formation, which coincide with a warmer spring than normal. Analysis of traumatized tissues within a tree trunk is key to understanding the past environmental conditions in which the tree has grown. To clarify the environmental conditions that cause frost rings to form over trembling aspen (Populus tremuloides Michx.) tree trunk xylem, we analyzed the relationships between the patterns of frost ring occurrence and spring air temperature in the boreal forests of northwestern Canada. Transverse sections through the trunks of seven trees were collected at 1-m intervals between the ground and the treetop and observed under a light microscope. The frost ring severity was evaluated in the three principal directions: among trunk height and four directions (north, south, east, and west), and across years, comparisons were made among the seven trees, and its correlation with the air temperature was analyzed. As a result, a frost ring was formed in each of 13 years, and its severity was found to vary with the height, the direction, and the trees. Frost rings formed in response to late spring frost when the minimum temperature was ≤ − 4.4 °C and the frost duration was ≥ 4.6 h after 270 growing degree days (base 0 °C) were reached. High-severity frost rings occurred at 3- to 5-year intervals when the spring mean air temperature was unusually high. The results provide new insights into the effects of spring temperature fluctuations on the growth of P. tremuloides trees.
Takahashi et al. (Thu,) studied this question.