Seed–seedling conflict in conifers as a result of plant–plant interactions at the forest-tundra ecotone

Abstract

Background: As the climate warms, plant interactions between shrubs and conifer seedlings may affect migration of boreal trees into alpine areas; however these interactions have not been widely tested across conifer life stages. Aims: Determine the role of shading by Betula glandulosa shrubs on early Picea mariana recruitment in the forest-tundra ecotone of the Mealy Mountains, Labrador (Canada). Methods: Four shrub treatments were established with varying degrees of shade (unaltered, trimmed, cleared, shaded). Emergence and growth of P. mariana in treatment plots were compared to control plots without shrubs. Results: Seedling emergence was facilitated; seedlings beneath shrubs emerged at greater levels (16% ± 5.1%) than in the control (0.5% ± 0.3%); however, evidence of facilitation post-emergence was weak, likely due to stronger interactions with seedbed species at this stage. Vertical growth of young seedlings was greatest in control and shaded plots, but needle production significantly decreased with increased shade suggesting that seedlings may compete with shrubs for light early in life. Conclusions: Although facilitation has been previously implicated as an important mechanism in stressful environments, our study indicates that the early life stages of some conifers at the tree line are not strongly facilitated by shrubs as has been previously suggested for adults.