The presence and abundance of species in communities at a given moment may be looked at in two ways: (1) emphasizing the quality or quantity of the preceding propagule supply or (2) within community interactions, such as competition. Several studies have attributed great importance to the former in the organization of epiphyte communities based on observed distribution patterns of epiphytes and the high variability between epiphyte communities at apparently identical habitats. On the other hand, the great cover and biomass of bryophytic epiphyte vegetation in the tropical montane rain forest suggests that competitive interactions may also be of importance. In this study, the structural development of epiphyte communities (assumed to be positively correlated with branch diameter) in the canopy of a Colombian Upper Montane Rain Forest is analyzed. Sampling of the epiphyte vegetation included recording the presence and relative abundance of species and the total biomass of the community. Access to the canopy was obtained using rope-climbing techniques. Epiphyte sampling of 21 high standing trees revealed that canopy communities are rich in non-vascular epiphytes comprising 120 taxa of bryophytes and 61 taxa of macro lichens. Four community types are recognized that correspond to a certain position from outer towards inner canopy. The types share many species and therefore form an organizational unit. Species richness of each of these four abstract types is similar at about c. 100 taxa. However, concrete stands of epiphyte vegetation on the thick inner canopy branches (diam. 21-80 cm) comprised significantly less taxa (p< 0.001) per unit surface area than those on the thinner middle canopy branches (5 ³diam. £ 20 cm), carrying on average 1.72 and 3.24 taxa/dm2, respectively. Richness was significantly higher again in the outer canopy (diam. < 5 cm) with 7.81 taxa/dm2. The opposite pattern would be expected when the time available for the immigration of species determines community richness. The decrease in variety with age is parallelled with an increase in biomass and a decrease in evenness. Thick branches and trunks typically carry large patches of individual plants of bryophytes, suggesting competitive replacement of species through 'horizontal' growth.
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