Development of black spruce growth forms at treeline

Abstract

Most treeline populations in northeastern Canada are monospecific stands of black spruce (Picea mariana [Mill.] B.S.P.), a hardy, cold-tolerant species able to withstand harsh climatic conditions under different growth forms. In the forest tundra, black spruce thrives in protected areas and exhibits a normal arborescent growth form, but in exposed sites, upright stems are damaged above the snowpack by snow abrasion and wind. In this study, the development of damaged growth forms was examined in a moderately exposed habitat. Five developmental stages were identified and described using detailed stem analysis of 13 spruce trees. Four different types of damaged growth forms were identified according to variations in supra-nival (above snow) stem height and number. At the site scale, the age structure of supra-nival shoots, based on a larger sample of 256 stems, was unimodal, suggesting a synchronous development of the spruce stand in which 46% of the shoots were initiated during the 1960s and 1970s. Subfossil trunks on the ground were all depressed trees, indicating that the former vegetation was a krummholz, not a forest. This indicates the recent development of the small-tree stand above the snowpack, probably triggered by recent milder conditions associated with snowier winters in the last decades.