Richness patterns of ferns along an elevational gradient in the Sierra de Juárez, Oaxaca, Mexico: a comparison with Central and South America

The transition zone from the tropics to the subtropics in Mexico is an outstanding and poorly explored area of biotic overlap, and as such very interesting to evaluate how species richness patterns change in the latitudinal context. In order to assess alpha diversity patterns of ferns in this zone, we analyzed changes in their assemblages along an elevational gradient from 300 m to 3,000 m in the Sierra de Juárez, Oaxaca, and compared the results with similar studies conducted in Central and South America. We report the most detailed plot-based elevational gradient study on ferns in the region and provide detailed data on species abundances and elevational ranges. In 121 plots of 20 × 20 m, we found 195 fern species, with the highest species numbers recorded at mid-elevations (1,100–2,200 m) and lower richness at both gradient extremes. This hump-shaped pattern was mainly driven by epiphytes, which contributed about 40% to the total species richness and had wider elevational ranges than terrestrial species, which contributed 38% to the total richness, and were more or less constant at low to mid elevations about 2,500 m, followed by a gradual decline. Overall, richness per plot was low compared to other elevational gradients within the tropics closer to the Equator. This decline of species richness towards the subtropics and temperate regions appears to be rather abrupt within Central America, but additional data are needed to close the remaining gap of knowledge between Costa Rica and Mexico