Pollen limitation and spatial variation of reproductive success in the insect-pollinated shrub Chuquiraga oppositifolia (Asteraceae) in the Chilean Andes

Abstract

Low temperatures, short growing seasons, and strong winds, which constrain the abundance and activity of insect pollinators, characterize alpine ecosystems. In northern hemisphere alpine environments, the reproductive output of several insect-pollinated plants has been reported to be pollen-limited. Using a supplemental hand-pollination experiment, we assessed the magnitude of pollen limitation (PL) in the obligate outcrossing insect-pollinated shrub Chuquiraga oppositifolia in the lower alpine scrub vegetation belt in the central Chilean Andes. We also assessed spatial variation in its reproductive success by comparing seed production among three additional sites. Hand-pollination resulted in a two- to three-fold increase in seed output above natural levels, thus demonstrating PL in this species. Nevertheless, percentage seed output remained low, increasing from 2.0 to 5.7%. Seed weight was reduced by 15% in hand-pollinated plants. Seed output was also low in the three additional sites, but did not differ among them, suggesting that low seed output is a spatially widespread phenomenon in this species. Our results, together with previous research, suggest that both pollinator visitation and abiotic resources (soil nutrients) constrain the reproductive output of this shrub. Longer-term research should unravel potential future vegetative and reproductive costs of current-year enhanced reproductive output and determine if PL is a recurrent phenomenon in the Chilean Andes.