Spruce-seedbed interactions: is there facilitation of recruitment in the boreal forest-alpine tundra ecotone? (Mealy mountains, Labrador, Canada)

Abstract

As climates warm, black spruce (Picea mariana) is expected to expand its range into alpine tundra. Ecological factors such as seedbed and predation may influence recruitment on regional scales. In the boreal forest, seedbed-seedling competition dominates but in the climatically-stressed Mealy Mountains forest-tundra ecotone (Labrador, Canada) the Stress Gradient Hypothesis (SGH) predicts facilitation as the dominant plant interaction. We investigated mechanisms of seedbed facilitation (temperature, water, nutrients, physical structure) in Pleurozium schreberi, Cladina spp., and mineral soil, and examined links between predation and seedbed. Pleurozium seedbeds had strong positive effects on seedling growth (30.9%) and survival (55%). Conversely, seedlings recruited poorly on mineral soil, with lowest growth (20.5%) and survival (26%). Seed emergence was highest on Pleurozium (6.3%), and lowest on Cladina (0.3%). The physical structure of Pleurozium likely protects seedlings from exposure, temperatures extremes and predators. As climates warm and seed availability increases, Pleurozium may facilitate black spruce expansion.