LAUSR

Abstract The composition of the bryophyte layer influences boreal forest regeneration and growth through its effect on soil conditions. Canopy openings in boreal spruce stands can favor the expansion of Sphagnum spp. in the understory at the expense of feather mosses. We use an experimental approach in both paludified field and greenhouse fully-randomized conditions to examine the differential effects of these two ground cover types on black spruce germination and growth, specifically the role of nutrient limitation in generating these effects. We also tested the impact of ground cover shading, simulating the effect of a closed forest canopy on the ground layer, with the assumption that the stress induced to mosses, especially to Sphagnum spp., would have a beneficial impact on tree growth. The two moss types had no differential effects on spruce germination and 0–6-month-old seedling growth in the greenhouse. However, the growth of 2-year-old seedlings in the greenhouse was lower in Sphagnum spp. than in feather mosses. This negative effect was removed by fertilization, suggesting that soil nutrient availability could explain the seedling growth difference between moss types. Greenhouse 2-year-old seedlings also allocated a greater proportion of biomass to roots in Sphagnum spp. than in feather mosses. In the field, feather and Sphagnum spp. mosses had no differential effects on 3-year-old seedling growth, and ground cover shading did not have any short-term positive impact on spruce growth. Although they were not validated in the field, the results we obtained in the greenhouse suggest that the replacement of feather mosses by Sphagnum spp. mosses do not only affect spruce growth through the build-up of an organic layer often associated with low soil temperature and excess water, but also through more direct effects on nutrient availability. Therefore, silvicultural treatments that would favor Sphagnum spp. expansion at the expense of feather mosses, such as partial or total harvesting with protection of regeneration and soils, may result in subsequent tree growth problems even in sites with moderate organic layer accumulation.