Habitat conditions, spatial distribution and trichome morphology of different species of Tillandsia growing on trees on the Ilha Grande Island, Brazil

Epiphytes living in the canopy face the problem of being exposed to varying levels of air humidity, temperature and insolation, according to both local climate and growing site conditions. This study addresses interrelationships between a) vertical distribution of Tillandsia species on phorophytes of Ilha-Grande Island in tropical Brazil; b) micrometeorological conditions at the different epiphyte habitats and growing sites; c) diurnal changes in plant water content and d) morphological traits of Tillandsia leaves with respect to absorptive scales. For this purpose, five vertical zones of different height and exposure were defined within the canopy, which are characterized by specific epiphyte associations. It was found that the Tillandsia species are heterogeneously distributed within tree canopies. The most exposed sites are colonized almost exclusively by T. usneoides whereas less exposed zones are settled by different assemblages of all other considered Tillandsia species. Microsite conditions (temperature and humidity) were shown to be substantially different, with quite extreme conditions at the sites preferred by T. usneoides. Despite its apparently stressful habitat, T. usneoides showed the lowest decrease of daily water content. All considered Tillandsia species differed significantly with respect to size and density of their absorptive scales. Also, density and size of scales differed within the leaves in a species-specific manner. Tillandsia usneoides was distinctly different from all other considered species by showing the longest scales and a high scale density which was uniform over the leaves. The results demonstrate the unique ability of T. usneoides to cope with drought stress. The outcomes support the assumption that leaf and scale morphology of Tillandsia are correlated with habitat. Probably, a dense mat of filiform leaves covered with scales showing long wing cells - as shown by T. usneoides - is particularly suitable for harvesting atmospheric water.