Pollination context effects in the high-mountain dimorphic Armeria caespitosa (Plumbaginaceae): Neighborhood is something more than density.

Frequency-dependent processes are relevant for flowering plant reproduction, especially for species with disassortative mating. In an individual-based study, we tested not only the effects of local density on reproductive success at small spatial scales, but also those of neighborhood quality. To test the neighborhood effects on the reproduction of Armeria caespitosa, a dimorphic Mediterranean high-mountain endemic, we introduce a novel pollination context (PC) index that considered the distance, floral display, and floral morph of neighbors at small scales (within 2 m from the focal plant), studying rock and pasture populations at both edges of the species altitudinal distribution. Reproductive success depended significantly on PC only at the low populations, suggesting that the PC effects are population-dependent and supporting the hypothesis that the neighborhood quality influences the reproductive success of A. caespitosa at least in stressful conditions. Moreover, fruit set was morph-dependent in the high-pasture population. The specific role of the spatial structure of compatible vs. incompatible morphs at small scales of the dimorphic self-incompatibility system in Armeria deserves further attention. Parameters other than plant density are useful for the study of small-scale density-dependent processes that affect pollination and other reproductive components, especially if they integrate neighborhood quality information at adequate spatial scales.