Heteroblasty in epiphytic bromeliads: functional implications for species in understorey and exposed growing sites.

Background and Aims: The functional relevance of heteroblasty, an abrupt morphological change in the ontogeny of a considerable number of angiosperm species, is still largely unresolved. During the ontogeny of many epiphytic Tillandsioids (Bromeliaceae), such a change occurs when small individuals transform into larger, tank-forming individuals that are capable of external water storage. Apart from its fundamental effect on plant water relations, the associated transition from narrow to broader leaves also affects plant architecture. The morphological changes and their effect on light interception may be especially relevant for heteroblastic species in the moist understorey, which are expected to be limited primarily by light. Methods: A functional structural plant model (Yplant) was used to construct digital replicas of atmospheric and tank-forming individuals of four species, two of them naturally growing in exposed conditions and two occurring in understorey sites. This allowed the determination of leaf display efficiencies as well as a systematic analysis of leaf architectural traits and their effect on light interception. Key Results: Modifying existing plant morphologies showed that broader leaves cause more self-shading within the plant. This supports the hypothesis that species from the light-limited understorey benefit from the early atmospheric life form through increased light capture. Modelling plant morphology that continuously followed the ontogenetic trajectories of the leaf architectural traits revealed that the rising total leaf number in atmospheric individuals constantly increased self-shading. Therefore, at a certain ontogenetic stage, a tipping point was reached when the tank form was even favourable in terms of light capture as it was associated with fewer leaves. Conclusions: The effects of changes in leaf morphology and leaf architecture on plant light capture may explain the common occurrence of heteroblastic species in the understorey of Neotropical forests, which does not negate a simultaneous positive effect of heteroblasty on plant water relations.

Host tree phenology affects vascular epiphytes at the physiological, demographic and community level.

The processes that govern diverse tropical plant communities have rarely been studied in life forms other than trees. Structurally dependent vascular epiphytes, a major part of tropical biodiversity, grow in a three-dimensional matrix defined by their hosts, but trees differ in their architecture, bark structure/chemistry and leaf phenology. We hypothesized that the resulting seasonal differences in microclimatic conditions in evergreen vs. deciduous trees would affect epiphytes at different levels, from organ physiology to community structure. We studied the influence of tree leaf phenology on vascular epiphytes on the Island of Barro Colorado, Panama. Five tree species were selected, which were deciduous, semi-deciduous or evergreen. The crowns of drought-deciduous trees, characterized by sunnier and drier microclimates, hosted fewer individuals and less diverse epiphyte assemblages. Differences were also observed at a functional level, e.g. epiphyte assemblages in deciduous trees had larger proportions of Crassulacean acid metabolism species and individuals. At the population level a drier microclimate was associated with lower individual growth and survival in a xerophytic fern. Some species also showed, as expected, lower specific leaf area and higher δ(13)C values when growing in deciduous trees compared with evergreen trees. As hypothesized, host tree leaf phenology influences vascular epiphytes at different levels. Our results suggest a cascading effect of tree composition and associated differences in tree phenology on the diversity and functioning of epiphyte communities in tropical lowland forests.