Linking patterns and processes through ecosystem engineering: effects of shrubs on microhabitat and water status of associated plants in the high tropical Andes

Studies on alpine and semi-arid environments indicate that plants that act as ecosystem engineers improve microhabitat conditions and modify local plant abundance and diversity. However, few studies have linked these patterns with the physiological responses of associated species. We analyze the role of a dominant shrub (Hypericum laricifolium) as an ecosystem engineer in páramo ecosystems at two altitudes (3,715 and 4,300 m) in the Venezuelan Andes. Superficial soil temperatures, soil water content, and organic matter were compared under the crown and outside. We selected the species with positive and negative spatial relations with the shrub and compared their midday leaf water potentials and turgor loss points in individuals growing under shrubs and outside. Results show that H. laricifolium dampened temperature oscillations and increased soil water and organic matter contents, the effect being more pronounced at the drier, lower elevation site. While positively associated forbs showed an improvement in their water status when growing under the crown, the species with the lowest water potentials and higher water-stress tolerance were grasses with a negative spatial relation with the shrub; this was consistent at both elevations. Moreover, the effect of the shrub on the water status and abundance of the exotic herb Rumex acetosella changed from positive in the drier site to negative in the more mesic site. Our results provide mechanistic evidence for interpreting spatial association patterns between nurse plants and other species in the alpine tropics. We propose that stress-resistance strategies (tolerance vs. avoidance) and origin (native vs. exotic) influence interactions with ecosystem engineers.