Epiphytes remain one of the most poorly-studied functional groups despite of their ecological relevance, high species richness and endemism, especially in Andean forests. Recently, it has been shown that tropical Andean forests on upper slopes can contrast markedly from forests on lower slopes in terms of key ecosystem processes such as nutrient cycling, productivity, stand structure and tree species composition. Although vascular epiphytes are a salient feature of tropical montane forest ecosystems, the effects of slope position on their diversity remain unknown. Moreover, because epiphytes are structurally dependent plants, it is important to explore how the three-dimensionality of the system affects diversity and in turn the effect of topography on diversity. Here we assessed the effect of topography (lower and upper montane forest, LSF vs. USF) on beta diversity of vascular epiphytes assemblages (VEAs) and how this effect varies according to sampling unit type (tree individuals vs. plots). Using single rope techniques we studied VEAs in 16 tree individuals (diameter breast height >20 cm) and 16 understorey plots (10 m2up to 2.5 m) distributed on ridges and ravines (i.e. near valley bottoms) at ca. 2230 m.a.s.l. in southern Ecuador.
Results/Conclusions
We registered over 17 000 epiphytic individuals, belonging to 31 families and 369 species of vascular epiphytes. Similarity profiles, based on the multi-assemblage abundance based overlap measurement, showed that: 1. Beta diversity was affected by sampling unit type, i.e. plots depicted significantly more similar VEAs than trees. 2. Differences among forest types were only shown by trees, i.e. VEAs at the USF were significantly more similar than VEAs at the LSF (about 20%), these differences where maintained for all aspects of diversity (composition and structure). A generalized linear model-based (GLM) approach for multivariate data showed that: 1. Topography has a significant effect on the variation among VEAs independently of sampling unit. 2. The strength of the effect varies according to sampling unit, i.e. topography has a stronger effect on VEAs found on plots than those found on trees, for which geographical distance was more important.
In conclusion, our study suggests that slope position has an important effect on species turnover of VEAs, but this effect may vary according to sampling unit type. Nevertheless, topography may be an important driver of high diversity of epiphytes in Andean forests.