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PREMISE OF THE STUDY: Regeneration niche differentiation promotes species coexistence and diversity; however, the ecological implications for the initial life phases of the majority of pteridophytes are unknown. We analyzed the sensitivity of gametophytes and juvenile sporophytes of the tree ferns Alsophila firma, Cyathea divergens, and Lophosoria quadripinnata to variation in light and water availability. METHODS: We evaluated gametophyte desiccation tolerance using saturated salt solutions and gametophyte solar radiation tolerance by direct exposure. We also transplanted juvenile sporophytes in environments with 7% and 23% canopy openness and two watering levels. KEY RESULTS: The response of photosynthetic efficiency and water content suggest that the gametophytes of the three species require high relative humidity, tolerate direct solar radiation for up to 30 min and that the response is not species-dependent. Sporophyte size and gas exchange were greater in the more open site, but decreased watering had a lesser effect on these variables in the more closed site. Relative growth rate correlated with the net assimilation rate and leaf weight ratio. Juvenile sporophytes of A. firma were more shade tolerant, while those of C. divergens and L. quadripinnata acclimatized to both environments. CONCLUSIONS: Specialization to humid habitats in the tree fern gametophyte restricts the species to humid forests, while differences in the plasticity of the sporophyte facilitate coexistence of the species. [ABSTRACT FROM AUTHOR]

In the forest canopy, seeds of epiphytic plants encounter heterogeneous environments created by a combination of factors such as solar radiation, humidity, and host characteristics. Germination requirements may explain the species distribution in the canopy; however, more knowledge is essential. Germination of Rhipsalis baccifera, a widespread tropical epiphytic cactus and representative of the humid montane forest in Mexico, was 80% or higher with far red, red, and white light and close to 0 in darkness. Germination was light saturated at very low photon flux density of only 13.5 μmol m-2 s-1. Germination decreased gradually at low water potentials and with increased storage time. After storage for 1 yr, no seeds germinated. Seeds have the ability to germinate in both the internal and external positions of the tree crowns. The germinative requirements of R. baccifera indicate that it could colonize a wide range of light conditions in the canopy; however, substrate humidity and seed age could limit germination. [ABSTRACT FROM AUTHOR]