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

Abstract

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. 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. 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. 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.