Variation in frost ring severity in the trunk of a trembling aspen tree growing in a boreal forest in northwestern Canada

Summary A trembling aspen ( Populus tremuloides ) tree growing in a boreal forest in northwestern Canada exhibited traumatised xylem tissues. The features indicated that the traumatised tissues comprised frost rings caused by late spring frost. To clarify frost ring formation characteristics, such as the year of occurrence, trunk height and direction, and severity, we analysed the xylem throughout the tree trunk. The sampled aspen tree (25 years old; tree height = 5.1 m; diameter at breast height = 3.1 cm) grew after a forest fire in 1981. Transverse sections of the tree trunk along four directions (north, south, east, and west) obtained at 20-cm intervals between the ground level and the treetop were dyed and observed via a light microscope. The spring air temperature between 0 and 1.5 m above the ground was analysed in years with frost ring formation. Frost rings occurred throughout the trunk in eight years between 1983 and 2007. Frost rings with high severity formed at 3–5-year intervals after 1986. The frost rings were less severe near the ground and treetop than in the intermediate parts. There was no significant difference in frost ring severity among the four directions. The spring air temperature increased substantially before it decreased below 0°C in 2003, and subzero temperatures were recorded at 1.5 m above the ground but not at ground level. These results suggest that frost ring formation varies from one year to the next and is affected by temperature variation with height. This study provides new insights into frost ring variation throughout the trunks of trembling aspen trees and the effect of air temperature differences with height on frost ring formation.