Quantitative analyses of the factors driving the distribution of bryophytes in the phyllosphere of tropical rain forests are rare. In this study we sampled epiphyllous bryophytes on two leaves of each of 57 individuals belonging to four phorophyte species (Asplundia pittieri, Carludovica drudei, Costus laevis and Dieffenbachia concinna) at three adjacent, but microclimatically varied sites in the Esquinas forest, Costa Rica. Microclimatic parameters and phorophyte identities were correlated to differences in cover, diversity and species composition of bryophyte assemblages by means of Kruskal–Wallis tests, non-metric multidimensional scaling and indicator species analysis. High relative humidity and in particular daily fluctuations in relative humidity proved to be the most important factors for epiphyll distribution suggesting severe constraints of epiphyll colonisation by pronounced humidity fluctuations. Differences in air temperature and light availability as well as phorophyte identity were only weakly correlated with bryophyte cover and diversity. However, species composition of epiphyllous assemblages was related to all microclimatic variables as well as to phorophyte identity. The strong response of epiphyllous bryophytes to even subtle microclimatic variations suggests that undisturbed forest canopies and their control on microclimate may be essential for the development of epiphyll communities.

Among the hypotheses invoked to explain high species richness in tropical forests, the niche differentiation hypothesis has received observational and experimental support (Ashton 1969, Chesson 2000, Clark et al. 1999, Souza & Martins 2004, Svenning 2001, Terborgh & Mathews 1999). Habitat specialization with regard to edaphic factors and topography has been observed in several plant groups including trees, lianas, shrubs, ferns and palms (Clark et al. 1999, Ibarra-Manriquez & Martinez-Ramos 2002, Svenning 1999, Tuomisto & Ruokolainen 1993). Treefall gaps and light gradients have also been found to be important ecological factors affecting plant distribution and niche differentiation in the understory (Chazdon 1986, Poorter & Arets 2003, Terborgh & Mathews 1999). It is suggested that such habitat heterogeneity may not only maintain biodiversity but also may lead to the origin of new species through the process of parapatric speciation (Gentry 1989, Haffer 1997, Patton & Smith 1992).

The suitability of several methods for estimating light conditions in the understorey of tropical forests, and of different sampling schedules was evaluated. Light conditions at 16 understorey sites in a Panamanian lowland forest were continuously measured for 9 mo with quantum sensors and photodiodes. Light conditions at the sites were also assessed indirectly with hemispherical fisheye photographs, plant canopy analysis, 38-mm photographs, 24-mm photographs and a spherical densiometer. Estimates from all indirect methods, except the spherical densiometer, were highly correlated with the direct measurements. Short-term direct light measurements for a day or a week also correlated with long-term light conditions. The indirect measures differed by up to c. 70% from the direct measures relative to single site measurements. Hence, the indirect methods are inadequate where single site light conditions have to be assessed accurately. However, because light conditions encountered in the understorey varied up to 13-fold, the indirect methods were found to be well suited to rank understorey light conditions among a large number of sites. The results from frequent and infrequent sampling schedules differed only slightly, suggesting that taking indirect measures at the beginning and the end of a study offers a reasonable compromise between accuracy and sampling effort.

Primary tropical forests comprise a mosaic of mature, gap and building phase patches, resulting in great spatial variation in the distribution of foliage. Light may consequently penetrate into the forest interior over a wide range of angles. It thus seems possible that understorey tree species might be adapted for distinct understorey light conditions. At the Cocha Cashu Biological Station in Madre de Dios, Peru, there are two understorey treelets distinguished by contrasting crown architectures. One, Neea chlorantha (Nyctaginaceae), possesses a much-branched superstructure and displays a smoothly contoured shell of drooping elliptical leaves. The other, Rinorea viridifolia (Violaceae), displays planar arrays of horizontally-oriented obovate leaves held on whorls of stiffly radiating horizontal branches.

With the aid of hemispherical photography and the program CANOPY, the light environment at large within a 2.25-ha permanent tree plot, and above and below the crowns of Neea and Rinorea treelets, was investigated. Available light (measured as uncorrected indirect site factor (ISFU) in CANOPY) at control points showed a log-linear increase with height from 2.1 to 14.2 m. The relative amount of lateral illumination also increased with height. Photographs were taken just above and below the crowns of 50 Neea and 50 Rinorea treelets. Neea crowns were more effective at intercepting light from overhead sources, whereas Rinorea crowns were more effective at intercepting light from lateral sources. Adult Neea and Rinorea treelets occurred at locations in the forest where they were exposed to differing angular distributions of incident light, suggesting that the two species were engaged in a form of resource partitioning, a conclusion that is in conflict with the non-equilibrium model of tropical forest tree species diversity. It is suggested that tropical tree species diversity is, at least in part, a product of adaptive specialization to a spatially heterogeneous light environment.