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.

Keywords:

• Betula glandulosa
• dwarf shrubs
• facilitation
• Picea mariana
• recruitment
• tree-line migration

Acknowledgements

We thank Paul Marino and Keith Lewis for their comments and advice during this study and to all our colleagues for field support, and particularly Julia Wheeler and John Jacobs for providing unpublished data. Thank you to Alan Mark, Esther Lévesque, Andrew Trant, Julia Wheeler, and Brian Starzomski for valuable comments on earlier versions of this manuscript. Thanks to the Labrador Tree Nursery (Dept of Natural Resources) for use of their facility in Labrador, and providing seeds. We acknowledge funding from the Government of Canada Program for International Polar Year as part of the project PPS Arctic Canada. This project is a product under the International Polar Year (IPY) core project PPS Arctic as part of the IPY 2007–2008, which was sponsored by the International Council for Science and the World Meteorological Organization. Funding and logistical support was provided by Memorial University, IPY-CiCAT, Parks Canada Agency, and the Northern Scientific Training Program (NSTP), Department of Aboriginal Affairs and Northern Development Canada.