Heteroblasty in Bromeliads: Its Frequency in a Local Flora and the Timing of the Transition from Atmospheric to Tank Form in the Field

The prevalence and functional relevance of heteroblasty, i.e., abrupt morphological changes in ontogeny in many angiosperms, remain obscure. To obtain a first estimate of the number of heteroblastic bromeliads, we used a simple metric to distinguish unambiguously heteroblastic from homoblastic species. We addressed ecological relevance by determining the time for transition between ontogenetic stages (atmospheric and tank)and by comparing the prevalence of heteroblasty in different habitats. Epiphytic Tillandsioideae were studied in a lowland and a montane forest in Panama. Repeated censuses allowed estimates of relative growth rates and the construction of ontogenetic trajectories for 14 species. Data on leaf index (leaf length/width)were taken for 23 species. Ontogenetic changes in leaf index reliably separate homoblastic and heteroblastic species. The early atmospheric stage in heteroblastic species lasts, on average, 3 yr, equivalent to 20(percent)of the average generation time. Heteroblastic species are as common in the understory as at exposed growing sites. This finding challenges the notion of heteroblasty as an adaptation to drought, because there are no differences in the proportion of heteroblastic species in wet and dry habitats. An alternative hypothesis is proposed.