RESUMO
Numerous plant functional traits of ecophysiology and morphology associated with an epiphytic life history have promoted relatively high rates of evolutionary diversification and ecological success in tropical families such as the Orchidaeae, Polypodiaceae, Bromeliaceae, and Cactaceae. Epiphytic life histories are relatively uncommon in the Araceae and rare in the Cyclanthaceae which lack key functional traits for epiphytism. Only two lineages of Neotropical Araceae, Anthurium and Philodendron, include examples of epiphyte life histories. The evolution of a hemiepiphytic life history represented an important development for tropical Araceae by providing functional traits that have greatly expanded opportunities for adaptive radiation and ecological success as indicated by species richness and frequency of occurrence. The key adaptive trait allowing the diversification of hemiepiphytic Araceae was the development of heteroblastic growth of leaves and stems. Although hemiepiphytic life histories are present in the Cyclanthaceae, the family has undergone only modest speciation and limited ecological success in both its epiphytes and hemiepiphytes. Extensive sampling of more than 4600 trees from primary forest on four soil groups in northeastern Costa Rica have found a modest diversity of 15 species of epiphytic Araceae but only two species of epiphytic Cyclanthaceae. In contrast, 38 species of hemiepiphytic Araceae and 5 species of hemiepiphytic Cyclanthaceae were sampled, indicating relatively limited adaptive radiation of hemiepiphytic Cyclanthaceae and lower ecological success. Using summed values of frequency of occurrence as a measure of ecological success, epiphytic Araceae were 18 to 42 times more frequent than epiphytic Cyclanthaceae in swamp, alluvial, and residual soil forests. Summed frequencies of occurrence of hemiepiphytic Araceae were 7 to 13 times higher than those of hemiepiphytic Cyclanthaceae. The four soil groups were similar in their floristic composition of epiphytic and hemiepiphytic Araceae and Cyclanthaceae, but the frequencies of occurrence of both epiphytes and hemiepiphytes were, with few exceptions, highest on swamp soil plots, with alluvial soil plots slightly less favorable.
RESUMO
The Cyclanthaceae comprise a relatively small family of about 230 species and 12 genera in the Pandanales that is widespread in wet Neotropical forests. The great majority of species can be divided into three growth forms (understory herbs, epiphytes, and root-climbing hemiepiphytes) that share functional traits with similar growth forms present in the Araceae, a member of the Alismatales and not closely related. Our objectives were first to characterize the diversity, functional growth forms, and ecological traits of Cyclanthaceae at the La Selva Biological Station. Specific functional leaf and canopy traits of terrestrial herbs and epiphytes are very similar and associated with ecological success in both families. We further examined the functional traits of root-climbing hemiepiphytes, a specialized growth form that links the two families but rare in other families and argue that their specialized functional traits allow them to be considered as a distinct functional growth form. A key trait in distinguishing hemiepiphytes which are rare outside of the Cyclanthaceae and Araceae is the severance of the main stem hydraulic connection to the soil early in plant development. We used field data to examine the possible evolutionary pathways of developmental and ecological transition from terrestrial to hemiepiphyte growth forms. The broader ecological success of hemiepiphytic Araceae compared to Cyclanthaceae is hypothesized to result from the presence of heteroblasty in developing stems and leaves which allows more efficient utilization of complex canopy light environments of wet tropical forests.
