Species richness and vertical distribution of ferns and lycophytes along an elevational gradient in Los Tuxtlas, Veracruz, Mexico

We studied species richness and community composition as well as the vertical distribution of ferns and lycophytes in three forest types along an elevational gradient in the region of Los Tuxtlas, Mexico. We established 23 plots of 20 m x 20 m at 150-1650 m, recording presence-absence of all species in eight vertical distribution zones (terrestrial and seven epiphytic zones). We found 84 species, including 79 ferns and five lycophytes. Species richness patterns differed markedly between different vertical zones, with the forest understory having a high diversity of epiphytes, many of which were specialized to this habitat. Overall species richness increased with elevation, but the richness of ground-living, terrestrial species showed the lowest elevational increase by only 50%, whereas epiphytic richness increased fourfold to tenfold. These differences presumably reflect the stronger influence of increasing environmental humidity on the water-limited epiphytic assemblages. We also found that the degree of habitat specialization of epiphytes decreased with elevation. This probably is due to decreasing tree height and increasing overall humidity, reducing the microclimatic differences between the forest interior and canopy. This elevational decrease in habitat specialization raises important questions as to, e.g., the strength of competitive interactions or the possibility that the evolutionary transition from terrestrial to epiphytic growth forms is more easily achieved at higher elevations.