Diversity of epiphyte ferns along an elevational gradient in El Triunfo Biosphere Reserve, southern Mexico

Background and aims - In the tropics, some studies have found that the richness of epiphytic ferns present a peak at mountain mid-elevations. However, it is not well understood how transitions from tropical to subtropical conditions affect this peak, and even less is known about beta diversity of epiphytic ferns. Thus, the objective is to understand the effect of climatic gradients on the variation of local richness of ferns and beta diversity patterns along an elevational gradient in a mountain system in southern Mexico. Methods - We sampled 32 trees, each in four elevational bands (100-2200 m). Alpha diversity patterns were analysed using linear regression models. We used the Morisita index to quantify species turnover between bands. An additive partitioning approach was used to analyse the degree to which individual trees, plots, and bands contributed to total species richness. We evaluated the influence of climatic variables on species composition via linear regression models. Key results - A total of 30 species in five families were recorded. Each family contributed in different magnitude to the elevational richness pattern, with Polypodiaceae dominating due to its richness and presence along the entire transect. Alpha diversity at the three scales (alpha(tree), alpha(plot), alpha(band)) increased with elevation and rainfall, and with decreasing temperature. Species turnover was high along the gradient, but was scale-dependent, with beta(transect) (65-75%) and beta(band) (14%) with the greatest contributing to total diversity. Although the contribution of the individual trees was lower, it increased with elevation. Conclusions - We emphasize the importance of including different scale levels in analyses of diversity along elevational gradients. In the region, cloud forest on the mountain peaks harbours the highest diversity of epiphytic fern communities. Due to a limited extent of this mountain range, the epiphyte ferns are susceptible to the effects of climate change.