Black spruce (Picea mariana) colonization of subarctic snowpatches in response to warmer climate

Abstract

Snowpatches are disjunct arctic ecosystems scattered across the subarctic, particularly on wind-protected lee slopes, where a thick snow cover accumulates during the winter. These snow-rich treeless ecosystems are affected by delayed snowmelt, causing shorter growing seasons. Snow-tolerant plants occupy the centre of subarctic snowpatches, whereas black spruce trees grow at the margins. Snowpatches have shown sporadic expansion and shrinkage phases from tree establishment and mortality linked to climatic trends. Field surveys in the subarctic of eastern Canada are showing an afforestation process occurring in snowpatches. The origin and nature of this afforestation were investigated based on the hypothesis that tree colonization and growth were closely associated with recent changes in climate. Snowpatches were categorized into three types based on border trees: forested, semi-forested and tundra-like. In eight randomly selected snowpatches of each type, snowpatch borders and areas covered by deciduous shrubs or trees were mapped and chionophilous plants and frost-associated soil disturbances were recorded. Radial (tree-ring) and vertical growth were measured on stems to evaluate the impact of climate and local factors on snowpatch black spruce populations. Two waves of spruce colonization occurred in all forested and semi-forested snowpatches in the 1960s and in the 1980s to present. Spruce establishment was more frequent and abundant in recent years, near the forest margins and on moss and barren seedbeds. Expanding shrub (dwarf birch) cover inhibited spruce seedling establishment. Tree establishment and growth were positively correlated with growing season temperature and negatively correlated with annual maximum snow depth. More recently established spruce seedlings exhibited faster vertical growth than those established in the 20th century. Synthesis. Due to warmer conditions and earlier snowmelt in eastern Canada, black spruce trees and dwarf birch shrubs are racing through subarctic snowpatches. This afforestation could change biodiversity in the subarctic and affect watershed dynamics through a change in snowmelt pattern. Subarctic snowpatches are climate-sensitive ecosystems of the forest–tundra landscape, forming ideal biotopes for snow-tolerant, arctic–alpine species. Further tree encroachment in snowpatches in this century is potentially a threat to plant diversity, especially chionophilous species that have no corridors to migrate towards arctic ecosystems.