Species richness-productivity relationships of tropical terrestrial ferns at regional and local scales

The species richness-productivity relationship (SRPR), by which the species richness of habitats or ecosystems is related to the productivity of the ecosystem or the taxon, has been documented both on regional and local scales, but its generality, biological meaning and underlying mechanisms remain debated. We evaluated the SRPR and three mechanistic hypotheses using terrestrial ferns in 18 study plots along an elevational gradient (500-4000 m) in Ecuador. We measured annual increases in above-ground biomass of 6175 fern individuals from 91 species over 2 years, and estimated plot-level tree productivity from increases in above-ground woody biomass of 560 trees. Analyses were conducted by (i) comparing plots along the elevational gradient (regional scale) and (ii) comparing plots within each elevational belt (local scale). Fern diversity was related to the productivity of the fern assemblages, but not to above-ground productivity of the trees. At the regional scale, we found a positive relationship of fern species richness to fern productivity that appeared to be determined by an increase in the number of fern individuals and niche availability. In contrast, at the local scale, this relationship was negative and likely driven by interspecific competition. Synthesis. Plot diversity of ferns appears to be limited by the number of available niches and competition to occupy these niches. At the local scale, this is reflected in a negative SRPR probably driven by competition, whereas with increasing scale, the positive influence of productivity emerges. This represents the first evidence that productivity and competition affect the diversity of tropical herb assemblages at the plot scale.