RESUMO
Fertile ramets of bumblebee-pollinated Alstroemeria aurea, a clonal perennial native to the temperate forests of the southern Andes, produce single terminal inflorescences that may bear two or more temporally non-overlapping whorls of flowers. While fruit set is commonly high (>80%) among early-opening flowers, it is usually low (<20%) among late-opening flowers within ramets. Using flowering ramets with two whorls of flowers, we examined experimentally the following related hypotheses. First, late flowers act as a reserve of ovaries, increasing their likelihood of setting seed when early fruits abort due to either pollen or resource limitation. Second, where early fruit abortion has occurred, plants may actively ensure pollination of late flowers by increasing their attractants and rewards. In a natural population, we simulated (1) lack of pollen deposition in early flowers, by excising their stigmas just before receptivity, and (2) resource limitation, by removing all the leaves from an experimental flowering ramet. Treatments were applied to individual ramets according to a 2 × 2 factorial design. We found that when early flowers failed to set fruit due to stigma excision, nectar secretion and particularly pollen receipt strongly increased in late flowers. Higher pollen deposition contributed significantly to the observed five-fold increase in seed output of late flowers. Fruit and seed set from early flowers were more negatively affected by defoliation than that from late flowers. Defoliation did not interfere with a ramet's capacity to increase late reproductive output when early reproduction failed. These results support the assertion that late flowers act as a reserve of ovaries helping a plant to cope with an unpredictable environment. These results also suggest that plants may actively increase pollinator visitation by opportunistically increasing flower rewards.
RESUMO
Local density and sexual composition are two aspects of floral neighborhoods thought to influence pollination and seed output of recipient plants. 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 synchronously protandrous with a given ramet being either all male or all female and thus incapable of self or geitonogamous pollination at the 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 sexual identity explained significant amounts of variation in pollen receipt. Density of neighboring flowering ramets marginally affected pollen receipt in two of the three populations but in opposite directions. Despite the absence of strong effects of neighborhood sexual composition on pollen receipt, the sexual identity of neighbors affected seed output which suggests effects on the quality of pollination due to changes in patterns of pollen flow. I also compared pollen loads on the stigmas of artificially isolated ramets (6 m) with those on experimental focal ramets surrounded by six close neighbors (20 cm) that were either 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 significantly over time. In nature, rates of bumblebee visits were higher in female-biased (early-flowering) than in male-biased (late-flowering) co-occurring floral patches. Thus, spatio-temporal shifts in visitation frequencies associated with the sexual composition of floral neighborhoods might compensate for spatial variability in pollen availability within populations and explain the discrepancies between empirical and experimental results.