Aim: To assess the range size patterns of ferns and lycophytes along elevational gradients at different latitudes in an ecographical transition zone and search for predictors of range size from a set of environmental factors.

Location: Mexico, from 15° to 23° N.

Taxon: Ferns and lycophytes.

Methods: All terrestrial and epiphytic species were recorded in 658 plots of 400 m² along eight elevational gradients. To test if the range size within assemblages increases with elevation and latitude, we calculated the latitudinal range using the northern and southern limits of each species and averaged the latitudinal range of all species within assemblages weighted by their abundances. We related climatic factors and the changes with latitude and elevation with range size using linear mixed-effects models.

Results: Species richness per plot increased with elevation up to about 1500-2000 m, with strong differences in overall species richness between transects and a reduction with increasing latitude. The mean weighted range size of species within assemblages declined with elevation, and increased with latitude, as predicted by theory. However, we also found marked differences between the Atlantic and Pacific slopes of Mexico, as well as low range size in humid regions. The best models described about 76-80 % of the variability in range size and included the seasonality in both temperature and precipitation, and annual cloud cover.

Main conclusion: Latitudinal and elevational patterns of range size in fern assemblages are driven by an interplay of factors favouring wide-ranging species (higher latitudes with increasing temperature seasonality; dryer habitat conditions) and those favouring species with restricted ranges (higher elevations; humid habitat conditions), with additional variation introduced by the specific conditions of individual mountain ranges. Climatically stable, humid habitats apparently provide favourable conditions for small-ranged fern species, and should accordingly be given high priority in regional conservation planning.

Study area: The Mexican transition zone is the complex area where the Neotropical and Nearctic biotas overlap, and in a strict sense corresponds to the mountain highlands of Mexico, Guatemala, El Salvador and Nicaragua (Halffter & Morrone, 2017). We here present data from eight elevational gradients at a range of 0 m to 3500 m elevation at 15-23° latitude N on both the Pacific and Atlantic (Gulf of Mexico) sides of Mexico (Figure 1, Table S4 in appendices). Three transects have been considered in previous studies: Los Tuxtlas (Krömer et al., 2013; Acebey, Krömer, & Kessler, 2017), Perote (Carvajal-Hernández & Krömer, 2015; Carvajal-Hernández, Krömer, López‐Acosta, Gómez‐Díaz, & Kessler, 2017), and Oaxaca (Hernández-Rojas et al., 2018). Los Tuxtlas including abundances was not published before (“Los Tuxtlas a”). Both transects from Los Tuxtlas were combined for the majority of the analysis.

Fern sampling: On each gradient, we sampled the fern assemblages at regular elevational intervals of 100-300 m (every 500 m at Perote), depending on accessibility. At each elevation, depending on the suitability of the slope, 4-8 plots of 20 x 20 m (400 m²) were sampled with a consistent, standardized methodology (Kessler & Bach, 1999; Karger et al., 2014). The plots were established in natural zonal forest, avoiding special structural features like canopy gaps, ridges, ravines, riparian areas, tree fall gaps, landslides, and other disturbed areas whenever possible, which all change microenvironmental conditions and have special fern assemblages. In each plot, all fern species and their abundances were recorded for terrestrial (soil, rocks, and dead wood) and for epiphytic substrates. Species with long creeping rhizomes were counted as patches. Epiphytes were sampled up to heights of 8 m with trimming poles and recorded at greater heights by using binoculars, climbing lower parts of trees, and searching recently fallen trees and branches within and adjacent to the plots (Gradstein, Nadkarni, Krömer, Holz, & Nöske, 2003; Sarmento Cabral et al., 2015).