Spatial relations and population structure of a dominant tree along a treeline ecotone in the Tropical Andes: interactions at gradient and plant-neighbourhood scales

Abstract

Background: Studies in temperate mountains suggest that plant–plant interactions modulate tree establishment above the forest line. In tropical mountains worldwide this issue remains largely unexplored.

Aims: To analyse the population structure and local spatial relationships of a dominant tree at a species-rich tropical Andean forest line.

Methods: We determined changes in the population structure of Diplostephium venezuelense along an elevation gradient between continuous forest and open páramo and analysed plant community structure and superficial rock cover in the neighbourhood of saplings and adults at the upper forest line.

Results: Sapling and adult densities were highest in low-altitude páramos adjacent to the forest line and at the borders of small forest islands. Saplings showed local spatial aggregation, were positively associated with small boulders and low shrubs, and negatively associated with mosses and lichens. However, no spatial association was found between scattered adults in the páramo and saplings of other forest trees.

Conclusions: Complex species-specific local spatial interactions, suggesting both facilitative and antagonistic effects, seem to modulate the establishment of the dominant tree D. venezuelense at and above the upper forest line. Nevertheless, the establishment of other tree species above continuous forests does not appear to be facilitated by the canopy cover offered by the isolated D. venezuelense individuals established in open páramo environments.

Keywords:

• elevation gradient
• facilitation
• local spatial structure
• plant–plant interactions
• safe sites
• treeline dynamics

Acknowledgements

This work was carried out in the context of the LEAF research network (CRN II005) financed by the Inter American Institute for Global Change Research (IAI) and coordinated by Dr Guillermo Sarmiento, with additional support from the Consejo de Desarrollo Científico Humanístico y Tecnológico de la Universidad de Los Andes. We thank L. Ramírez, N.J. Marquez, W. Dugarte, A. Morales, M. Castañeda, C. Azocar, R. Dulhoste and J.E. Torres for their assistance during field work and three anonymous reviewers for excellent comments and suggestions. We are also grateful to the Mérida Cable Car System and the Instituto Nacional de Párques for facilitating access to the research site.