Abstract:
1. Ridges of tropical mountains often differ strikingly from neighbouring ravines in terms of
forest structure, productivity and species composition. This heterogeneity is poorly understood
despite its critical role in biodiversity maintenance, carbon and nutrient budgets.
2. We examined measures of tree biomass and productivity, foliage and litter quality (nutrient
concentrations, specific leaf mass, phenolics), herbivory and leaf litter decomposition in each
six plots laid out in upper and lower slope position in a tropical montane moist forest in southeastern
Ecuador.
3. Productivity, quality of foliage and litter as well as herbivory were significantly lower in
upper slope position, and closely correlated with soil nutrient concentrations and accumulated
humus. The decomposition of upper slope leaf litter (decomposition rate k) was substantially
lower than in litter from lower slope forest, whereas the site of decomposition (slope position)
only had a marginal effect on the decomposition rate.
4. Our results suggest that the differences in stand structure, productivity, foliar quality, herbivory
and decomposition between slope positions are ultimately due to stronger nutrient limitations
in upper slope forest. We propose a general conceptual model that explains origin and
maintenance of contrasting forest types along topographical gradients through down-slope
fluxes of nutrients and water, and a nutrient-driven positive feedback cycle.
