Abstract
BackgroundEpiphytic bryophyte communities in the Amazon forest show a vertical gradient in species composition along the trunk of the host trees. The investigation of species traits related to this pattern has focused on the physiology of selected taxa with a clear preference for one of the extremes of the gradient. Although some species are indeed only found on the tree base or in the outer canopy, the vertical gradient is composed mainly by the variation in the abundances of species with a broader occurrence along the height zones. Therefore, this study approaches the differences among community assemblages, rather than among species, to test the role of morphological and dispersal traits on the establishment of the vertical gradient in species composition.MethodsA character state matrix was built for 104 species of the family Lejeuneaceae recorded as epiphytes in the Amazonian terra firme forests, and six binary traits supposed to influence species occurrence: dark pigmentation on leaves; ability to convolute leaves when drying; possession of thickened cell walls; reproduction mode (monoicous or dioicous); occurrence of asexual reproduction; and facultative epiphyllous habit. Based on a previous dataset on community composition along the vertical gradient, trait occurrences in random draws of the metacommunity was compared to trait occurrences in field data, in order to detect significant deviations in the different height zones.ResultsFour out of the six traits tested showed significantly higher or lower occurrence in the species composition of canopy and/or understory communities. Traits related to high dispersal ability did not vary much along the vertical gradient; although facultative epiphylls were overrepresented on tree base. Dark pigmentation and convolute leaves were significantly more frequent in the canopy communities, but also significantly less frequent in communities at the base of the tree.DiscussionDark pigmentation and convolute leaves seem to be advantageous for the establishment in the canopy zones. They may, respectively, prevent light damage and allow longer periods of photosynthesis. Interestingly, these traits occur randomly along the trunk, but are wiped out of communities on the tree base. In the relatively deep shade of the first meters of the understory, they possibly hamper net carbon gain, the first by darkening the leaf surface and the second by delaying desiccation—which can be damaging under high temperatures and low light. The fact that production of asexual propagules is not overrepresented in the most dynamic microenvironment along the gradient, the canopy, challenges current views of bryophyte life strategy theory.
Highlights
Watson published one of the first extensive discussions on the bryophyte morphological features that could be related to species occurrence (Watson, 1914), starting with: ‘‘when examining the bryophytes of the woodlands of Somersetshire (. . . ), I was desirous of obtaining information on the characters which enable some species to live in dry situations, whilst others could only exist when provided with a large amount of shade or moisture’’
Analyses that make use of the concept of metacommunity, under a species sorting framework (Leibold et al, 2004), allow testing whether species traits play a role in the process of community assemblage (Wang et al, 2017), an approach more frequently used for vascular plants (Kraft, Valencia & Ackerly, 2008; Lebrija-Trejos et al, 2010)
Trait information could be extracted from literature in most of the cases; the coding for convolute leaves and cell wall was omitted in two species each (Table 1)
Summary
The fact that the species studied are selected because they have a strong environmental preference prevents one to conclude how far the investigated character influences the establishment of the ecological gradient, usually formed by a much higher number of species than the ones tested. Epiphytic bryophyte communities in the Amazon forest show a vertical gradient in species composition along the trunk of the host trees. This study approaches the differences among community assemblages, rather than among species, to test the role of morphological and dispersal traits on the establishment of the vertical gradient in species composition. Dark pigmentation and convolute leaves seem to be advantageous for the establishment in the canopy zones They may, respectively, prevent light damage and allow longer periods of photosynthesis. The fact that production of asexual propagules is not overrepresented in the most dynamic microenvironment along the gradient, the canopy, challenges current views of bryophyte life strategy theory
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