Positive community effects of nurse-plant shrubs along a tropical alpine elevation gradient: are they linked with plant water relations?

Shrubs are one of the most common plant growth forms acting as nurses, but their effects on community organization along elevation gradients and how these are linked with ecophysiological effects on beneficiaries remain poorly understood in alpine ecosystems. We aimed to evaluate the effect of a dominant shrub ( Hypericum laricifolium ) on local community structure, microhabitat conditions and the water relations of associated species along an elevation gradient (4100–4400 m) in the high tropical Andes of Venezuela. We compared species richness and diversity, total plant density and cover and the abundance of each species near and away from the shrubs. Incident radiation (PAR), air temperature (T air ) and humidity (ARH) and soil water content (SWC) under the shrub’s crowns and outside were measured. We selected species with positive and negative spatial associations with the shrub and compared their water status in individuals growing under shrubs and outside. Our focal shrub had net positive effects on local richness and diversity and vegetation cover, these effects being more pronounced in drier areas at lower elevations. PAR was lower, and SWC was higher under shrubs, and they had positive effects on the water potential and leaf-air water pressure deficits of other species at all elevations, regardless of their spatial association with this nurse. Hence, our results suggest that water stress could be an important driver of the facilitation effects of shrubs at a community scale in these tropical alpine ecosystems. However, the species-specific effects of these nurses, and how these change with elevation, could be linked other limiting factors that remain to be better understood.