Black spruce regeneration at the treeline ecotone: synergistic impacts of climate change and caribou activityThis article is a contribution to the series Tree recruitment, growth, and distribution at the circumpolar forest–tundra transition.

Abstract

Tree regeneration at treeline is often inhibited by harsh climatic conditions and ecological constraints such as the presence of a continuous lichen cover. The objective of this study was to verify if recent climate warming and increased caribou (Rangifer tarandus L.) activity, which destroys the lichen cover, could act synergistically to increase black spruce (Picea mariana (Mill.) B.S.P.) regeneration. We collected cones produced in 2006–2007 to compare seed viability with data from 1989 to 1995. We conducted experimental plantations (field and controlled conditions) to determine if germination and seedling emergence were higher in disturbed environments where mineral soil was exposed. We sampled naturally established seedlings to evaluate the relationship between the year of establishment and caribou activity and to compare growth in disturbed and undisturbed environments. We found that seed viability was significantly higher in 2006–2007 compared with 1989–1995. The number of germinated seeds per cone increased by a factor >1000. Germination and seedling establishment were higher in the disturbed environment (mineral soil). Finally, although seedling establishment in areas with continuous lichen cover was rather constant over the last 50 years, 73.5% of the seedlings recorded on bare mineral soil became established in the years following high caribou activity. Our results suggest that climate warming and caribou activity are likely to act synergistically to promote black spruce regeneration at treeline.