Vegetative traits, floral biology, and mutualistic interactions in the tropical mountain shrub Ribes ciliatum (Grossulariaceae)

Plant species in tropical mountain ecosystems experience significant environmental variation across short spatial distances. These systems offer an opportunity to assess geographic variation in plant traits in relation to biotic and abiotic environments. We examined plant vegetative and reproductive traits in the shrub R. ciliatum across two native conifer forests in central Mexico. We measured plant height, number of branches, leaf size, flower production, floral morphology, floral longevity, and plant reproductive success in R. ciliatum populations found in fir forests (3100 m) and Hartweg's pine forests (subalpine habitat, 3700 m). We also quantified the level of pollinator dependence, pollinator visitation rates, and fruit consumption rates by frugivorous birds in both populations. Plants were shorter and had smaller leaves in the subalpine forest than in the fir forest. Floral longevity decreased in the subalpine forest, but flower production, flower size, and fruit weight remained similar across populations. Fruit set and pollinator visitation rates were higher in the subalpine forest; however, seed weight and fruit consumption rates did not differ between sites. Hummingbirds and bumblebees were the primary pollinators of R. ciliatum, and the bird Ptiliogonys cinereus was the most important fruit consumer at both sites. Fruit set was entirely dependent on pollinators. Reduced plant size in the subalpine forest coincides with a general pattern associated with high mountain environments. In contrast, reproductive characters had a closer relationship to the pollination environment. The absence of variation in flower size and shape was consistent with the similarity in the composition of the pollinator community. Meanwhile, compared to lower-elevation forests, the subalpine forest exhibited shorter-lived flowers and increased fruit set, associated with higher pollinator visitation rates. Ribes ciliatum is a key resource for pollinators and frugivores at a time of year when few other floral resources are available in these high-elevation mountain forests.