Frost hollows of the boreal forest as extreme environments for black spruce tree growth

Abstract

An extensive survey of spruce growth forms was conducted in frost hollows of the Parc national des Grands-Jardins located in the highlands of the Charlevoix region (Quebec). Frost hollows studied were situated within lichen woodlands, which are a favourable environment for the formation of cold air masses. The mean number of frosts recorded 1 m aboveground during the growing seasons of 1997–2002 indicates that there is no frost-free period in frost hollows. Based on the records of minimum temperature in two frost hollows, the critical threshold for frost-ring formation is a nocturnal temperature below 0 °C for 6.4 h with a climax at around –5.7 °C, i.e., at a cooling rate of 1.78 °C/h under the freezing point for at least 3.2 h. Frequent and severe nocturnal frosts slow down the colonization of black spruce in frost hollows, and are at the origin of the inverted tree line that surrounds frost hollows. Mean stem height of black spruce in frost hollows is 1.97 m ± 2.15. A comparative analysis of aerial photographs between 1950 and 1996 showed a 34% reduction in frost hollow area due to the progressive colonization of black spruce. Massive tree establishment occurred in frost hollows between 1970 and 1980 because of the reduced frequency of freezing temperatures during the growing season.