Interactions between nurse-plants and an exotic invader along a tropical alpine elevation gradient: growth-form matters.
Conceptual models of how interactions with native species influence invasions emphasize competition, but recent evidence suggests facilitation can promote invasion in stressful environments. However, how nurse-plants with contrasting growth-forms and distribution interact with invaders remains unexplored, although it could offer insights on nurse/exotic interaction mechanisms. We asked whether shrub and cushion nurses differed in their effects on the exotic Rumex acetosella in sites at four elevations in the high tropical Andes (4100-4400 m), shrubs dominating the lowest sites and cushions the highest sites. During the dry season, we measured soil organic matter (SOM) and water content (SWC) under the shrub Hypericum laricifolium, the cushion Azorella julianii, and adjacent areas outside. We compared Rumex's performance under each situation, measuring midday leaf temperatures (Tleaf), vapor pressure deficit (VPD), minimum water potentials (Ψmin) and leaf nitrogen (Nleaf) and compared the number, size and proportion of fruiting ramets within sampling rings in each situation. SOM and SWC were higher at all elevations under cushions, then under shrubs and lower outside. Rumex's density was generally reduced under shrubs but increased on cushions. However, both nurses had positive effects along the gradient on Rumex's size, reproduction, water balance and Nleaf, shrubs having stronger effects on Tleaf and VPD and cushions on Nleaf. Our results indicate that alternating nurses influenced an invader's physiological performance to different extents via contrasting effects on shading and soil resources, leading to mixed competitive/facilitative effects of shrubs on the exotic's demography, while cushions had more consistent facilitative effects across elevations.