Feathermoss seedbeds facilitate black spruce seedling recruitment in the forest-tundra ecotone (Labrador, Canada)

As climate warms, conifers are expected to expand their ranges into alpine tundra where ecological factors such as seedbed availability, and post-dispersal seed and seedling predation may control local recruitment. Seedbed composition may influence microhabitat, nutrients, physical structure, and predation level and, therefore, affect the success of conifer recruitment, thereby providing the template for future expansion. In the boreal forest, seedbed-seedling competition dominates such that seedbed removal increases black spruce recruitment. In the harsher climate of the Mealy Mountains boreal forest-tundra ecotone (Labrador, Canada) the Stress gradient hypothesis (SGH) predicts that facilitation may dominate seedbed-seedling interactions. This study investigated potential mechanisms of seedbed facilitation (temperature, water, nutrients, physical protection) in three conifer seedbeds (Pleurozium schreberi, Cladonia spp., bare soil) and examined whether seed predation and/or seedling herbivory varied among seedbeds over three years. Seed emergence was low overall (<10% on all treatments), but highest on Pleurozium (6.3%), followed by bare ground (4.6%) and Cladonia (0.3%). Facilitation was observed between Pleurozium and black spruce as seedling height increase (31%) and survival (55%) were highest; herbivory, seed predation and overwinter mortality were lowest compared to both Cladonia and bare ground seedbeds. Unlike in the closed canopy boreal forest, seedlings recruited poorly on bare soil as seedling height increase and survival were 20.5% and 26%. Temperature and water availability were similar across seedbeds, while nutrient availability was higher on Pleurozium. The physical structure of Pleurozium likely protects first to third-year seedlings from temperature extremes and predators. As climate warms and seed availability increases, Pleurozium may facilitate black spruce recruitment and treeline expansion.