RESUMO

Variation in plant reproductive success is affected by ecological conditions including the proximity of potential mates. We address the hypothesis that spatial distribution of sexes affects female reproductive success (RS) in the dioecious cycad, Zamia portoricensis. Are the frequencies of males, operational sex ratios, and distances to the nearest mate associated with RS in females? We studied the spatial distribution of sexes in two populations in Puerto Rico and compared RS of target females with the number of males and operational sex ratios. Population structure suggests regular successful recruitment. Adults, males, and females were randomly distributed with respect to one another. Reproductive success of females was highly variable, but was higher in neighborhoods with more males than females and generally decreased with increasing distance to the nearest male, becoming statistically significant beyond 190 cm. This possible mate-finding Allee effect indicates that pollinator movement among plants may be limited for this mutually dependent plant-pollinator interaction. Yet being close to male plants is a matter of chance, perhaps a factor generating the high intra-population genetic diversity in Z. portoricensis.

RESUMO

From studies in seasonal lowland tropical forests, bromeliad epiphytes appear to be limited mainly by water, and to a lesser extent by nutrient supply, especially phosphorous. Less is understood about the mineral nutrition of tropical montane cloud forest (TMCF) epiphytes, even though their highest diversity is in this habitat. Nutrient limitation is known to be a key factor restricting forest productivity in TMCF, and if epiphytes are nutritionally linked to their host trees, as has been suggested, we would expect that they are also nutrient limited. We studied the effect of a higher nutrient input on reproduction and growth of the tank bromeliad Werauhia sintenisii in experimental plots located in a TMCF in Puerto Rico, where all macro- and micronutrients had been added quarterly starting in 1989 and continuing throughout the duration of this study. We found that bromeliads growing in fertilized plots were receiving litterfall with higher concentrations of N, P, and Zn and had higher concentrations of P, Zn, Fe, Al, and Na in their vegetative body. The N:P ratios found (fertilized = 27.5 and non-fertilized = 33.8) suggest that W. sintenisii may also be phosphorous limited as are lowland epiphytes. Fertilized plants had slightly longer inflorescences, and more flowers per inflorescence, than non-fertilized plants, but their flowers produced nectar in similar concentrations and quantities. Fertilized plants produced more seeds per fruit and per plant. Frequency of flowering in two consecutive years was higher for fertilized plants than for controls, suggesting that fertilized plants overcome the cost of reproduction more readily than non-fertilized plants. These results provide evidence that TMCF epiphytic bromeliads are nutrient limited like their lowland counterparts.

Assuntos

RESUMO

Orchids parasitize their mycorrhizal fungi and are dependent on them for seed germination. Controversy reigns over how specific the mycorrhizal association is in tropical species. Although there is little experimental evidence to support any viewpoint, some variation is known to exist. We compared mycorrhizal specificity and performance in two phylogenetically related epiphytic orchids from Puerto Rico, Tolumnia variegata and Ionopsis utricularioides (Oncidiinae) by integrating two techniques: phylogenetic analysis of mycorrhizal fungi based on nuclear ribosomal internal transcribed spacer (ITS) sequences, and symbiotic seed germination experiments. Most of the mycorrhizal isolates from T. variegata fell into four different clades of Ceratobasidium, while most of those from I. utricularioides were restricted to a single clade of the same genus. Seeds of T. variegata germinated equally well with fungi from both T. variegata and I. utricularioides, but seeds of I. utricularioides germinated significantly better with its own isolates. Seeds of I. utricularioides germinated and developed faster than those of T. variegata. Both the molecular phylogeny and the seed germination experiments showed that T. variegata is a generalist in its association with fungal symbionts. In contrast, I. utricularioides is more specialized and more effective at exploiting a specific fungal clade. Our data are consistent with the theoretical trade-offs between specialized and generalized interactions.

Assuntos