A quantitative definition of light rings in black spruce (Picea mariana) at the arctic treeline in northern Quebec, Canada

Light rings in black spruce (Picea mariana [Mill.] BSP.) at the arctic tree line are characterized by pale-colored latewood made of a single or very fe w latewood-cell layers with thin-walled cells. Their widespread occurrence and their high frequency greatly facilitate the cross-dating procedure in d endrochronological studies, In this study, black spruce tree-ring density a nd wood structure were analyzed for light ring characteristics along with t he mechanism of their formation according to ambient temperature. Light rin gs were quantitatively categorized into three classes based on the maximum tree-ring density using a normalized standard distribution. A light-ring ch ronology was established according to this classification. The results indi cate that the grade of light ring was positively related to the frequency o f fight rings obtained from visual light-ring chronologies. The following a natomic variables were examined: number of cell layers of latewood, number of cells of the whole ring, percentage of latewood in the total ring width, and mean latewood cell-wall thickness, Among these anatomic variables, the mean latewood cell-wall thickness represents the best quantitative descrip tor of a typical light ring as recognized by optical examination. The main causal factors of light rings are insufficient length of the growing season or cool summers.