Similar tree seedling responses to shrubs and to simulated environmental changes at Pyrenean and subarctic treelines

Abstract

Background: Climate, land-use and disturbance regimes are key drivers of treeline dynamics worldwide, but local and regional spatio-temporal patterns indicate that additional factors play an important role. Some studies suggest that shrub-tree interactions control tree seedling recruitment patterns across the treeline ecotone, but little is known about the generality of this interaction.

Aims: We established an experiment in a treeline ecotone in the central Pyrenees to investigate the role of such interactions and other environmental factors on tree seedling growth and survival. It was based on a similar experiment we completed recently in the subarctic Scandes. By comparing local phenomena between both experiments we assessed the generality of the findings across regions with different biogeographic histories and species characteristics.

Methods: We followed the survival and growth of transplanted Pinus uncinata seedlings during three growing seasons in a multi-factorial design (forest vs. treeline, +/– shrub removal, +/– temperature increase and +/– nutrient addition).

Results: There was better seedling growth at the treeline compared to the forest, and the presence of shrubs prevented seedling winter damage and herbivory. Both temperature and nutrient increase enhanced seedling performance.

Conclusions: Although some particular details were exclusive to the Pyrenees or to the Scandes, the similarities indicated general patterns and help to interpret the underlying mechanisms of treeline dynamics in both regions. This convergence of responses points to the potential of the development of a robust mechanistically founded predictive framework for scaling up shrub impacts on treeline dynamics to other regions.

Keywords:

• competition
• environmental change
• facilitation
• herbivory
• plant–plant interaction
• Pyrenees
• shrub
• subarctic
• treeline dynamics

Acknowledgements

We thank the Parc Natural de l'Alt Pirineu for their support and all the people who have helped with the fieldwork; without their help this study would have not been possible. We acknowledge the constructive comments from two anonymous reviewers. We are grateful to Richard S.P. van Logtestijn (VU University, The Netherlands) for his help with the nutrient analyses in the laboratory, and to Albert Ferré (University of Barcelona), who made the map in Figure 1. O. Grau was financed by an FI contract from the Comissionat per a Universitats i Recerca of the Generalitat de Catalunya and the European Social Fund. The fieldwork in the Scandes was supported by the ATANS grant (EU Transnational Access Programme, FP6 contract 506004). T.V. Callaghan wishes to thank FORMAS for funding his contribution to this work through the project ‘Climate change, impacts and adaptation in the Subarctic; a case study from northern Swedish mountains’ (214-2008-188).