INFLUENCE OF LOCAL FLORAL DENSITY AND SEX-RATIO ON POLLEN RECEIPT ANDSEED OUTPUT - EMPIRICAL AND EXPERIMENTAL RESULTS IN DICHOGAMOUS ALSTROEMERIA-AUREA (ALSTROEMERIACEAE)

Local density and sexual composition are two aspects of floral neighbo rhoods thought to influence pollination and seed output of recipient p lants. I characterized the floral neighborhood of 436 flowering ramets of Alstroemeria aurea, a southern Andean perennial, distributed among three sites. On each ramet, I measured total pollen receipt and seed output. The long-lived, bumblebee-pollinated flowers of A. aurea are s ynchronously protandrous with a given ramet being either all male or a ll female and thus incapable of self or geitonogamous pollination at t he ramet level. Even though each ramet changes sex over time, A. aurea forms floral neighborhoods that remain stable with respect to density and sex ratio during the span of a focal ramet female phase. Contrary to expectation, under field conditions neither local density nor sexu al identity explained significant amounts of variation in pollen recei pt. Density of neighboring flowering ramets marginally affected pollen receipt in two of the three populations but in opposite directions. D espite the absence of strong effects of neighborhood sexual compositio n on pollen receipt, the sexual identity of neighbors affected seed ou tput which suggests effects on the quality of pollination due to chang es in patterns of pollen flow. I also compared pollen loads on the sti gmas of artificially isolated ramets (6 m) with those on experimental fetal ramets surrounded by six close neighbors (20 cm) that were eithe r all male or all female. Here, pollen receipt by focal ramets in all- male neighborhoods was 1.3 times greater than in isolated ramets, and 3.8 times greater than in ramets in all-female neighborhoods. In these artificial neighborhoods, stigmatic pollen deposition increased signi ficantly over time. In nature, rates of bumblebee visits were higher i n female-biased (early-flowering) than in male-biased (late-flowering) co-occurring floral patches, Thus, spatio-temporal shifts in visitati on frequencies associated with the sexual composition of floral neighb orhoods might compensate for spatial variability in pollen availabilit y within populations and explain the discrepancies between empirical t md experimental results.