Development of stunted black spruce (Picea mariana) clones in the subarctic environment: A dendroarchitectural analysis

Abstract

Black spruce (Picea mariana [Mill.] BSP.) is the dominant tree species across the North American boreal forest. In the northernmost parts of its natural range, the species thrives in the form of scattered or aggregated stunted clones (krummholz) in winter-exposed subarctic and arctic sites. In this study, we have examined the mechanisms and developmental sequence that allow the growth, maintenance and expansion of long-lived (several hundred years old to more than one thousand years old), stunted black spruce using architectural and tree ring analyses. The development of stunted black spruce clones is the result of the reiteration potential of the species through layering. Exposed snow-drift sites are favourable to layering because branches are maintained close to the soil surface; these site conditions promote layering through adaptive reiteration associated with dedifferentiation of growing axes. Because the lower branches are held near the soil surface and buried beneath the litter, the formation of adventitious roots is accelerated and facilitates the physiological autonomy of clonally-born units. Newly formed units with an expanding root system lead to self-pruning of inter-branch connections and physiological autonomy. The age structure of all physiologically autonomous units within a clone often exhibits an inverse J-shaped distribution suggesting a stable population of units with prevailing site conditions. The average life span of a unit was 170 ± 70 years, but individual units may reach an age of more than 450 years. The dynamics of establishment and death of units (fragmentation of layers) through adaptive reiteration is the mechanism ensuring the great longevity of prostrate clones. In the absence of fire disturbance, stunted black spruce clones can perpetuate indefinitely.