Epiphytic biomass of a tropical montane forest varies with topography

Abstract

The spatial heterogeneity of tropical forest epiphytes has rarely been quantified in terms of biomass. In particular, the effect of topographic variation on epiphyte biomass is poorly known, although forests on ridges and ravines can differ drastically in stature and exposure. In an Ecuadorian lower montane forest we quantified epiphytic biomass along two gradients: (1) the twig-branch-trunk trajectory, and (2) the ridge-ravine gradient. Twenty-one trees were sampled in each of three forest types (ridge, slope, ravine positions). Their epiphytic biomass was extrapolated to stand level based on basal area-epiphyte load relationships, with tree basal areas taken from six plots of 400 m 2 each per forest type. Our results document the successional addition and partial replacement of lichens by bryophytes, angiosperms and finally dead organic matter along the twig-branch-trunk trajectory. Despite having the highest tree basal area, total epiphytic biomass (mean ± SD) of ravine forest was significantly lower (2.6 ± 0.7 Mg ha -1) than in mid-slope forest (6.3 ± 1.1 Mg ha -1) and ridge forest (4.4 ± 1.6 Mg ha -1), whereas maximum bryophyte water storage capacity was significantly higher. We attribute this pattern to differences in forest dynamics, stand structure and microclimate. Although our study could not differentiate between direct effects of slope position (nutrient availability, mesoclimate) and indirect effects (stand structure and dynamics), it provides evidence that fine-scale topography needs to be taken into account when extrapolating epiphytic biomass and related matter fluxes from stand-level data to the regional scale. © Copyright Cambridge University Press 2011.