Shrubs as foundation species in a high tropical alpine ecosystem: a multi-scale analysis of plant spatial interactions

Background: Studies on plant-plant interactions in alpine ecosystems show a strong bias for temperate vs. tropical latitudes and pay limited attention to the implications of biophysical heterogeneity beyond the local plant scale. Aims: To evaluate the effects of a dominant shrub (Hypericum laricifolium) on community organisation, integrating multiple scales of analysis, in a high Andean paramo in Venezuela. Methods: We compared plant cover, species richness and Shannon diversity between plots placed under the shrub and outside (plant scale); quantified the spatial relations of Hypericum with its conspecifics, heterospecifics and rocks at different radial distances, using covariance functions (patch scale); analysed the effects of Hypericum shrubs on total species richness in the community (site scale); and evaluated the consistency of our results among four sites within the same region (between-site scale). Results: Average species richness, Shannon diversity, and the abundance of common forbs and conspecific shrubs were consistently higher inside the Hypericum shrub, while the exotic Rumex acetosella was more abundant in open areas. However, the aggregated spatial distribution of conspecifics and other species extended beyond the local plant scale and there were no clear effects of Hypericum shrubs on community-level species richness. Conclusions: Shrubs can function as foundation species in tropical alpine environments, modifying local community structure and alpha diversity, but not necessarily the species richness of the overall community. Our results stress the need of analysing multiple spatial scales to interpret the role of plant-plant interactions (facilitation/competition) in heterogeneous alpine ecosystems.