Patterns of morphological leaf traits among pteridophytes along humidity and temperature gradients in the Bolivian Andes

Macroecological patterns of leaf traits can be used to assess adaptive responses of plants to environmental stress. Here we present the first such study on a large number of fern species (403) along gradients of elevation (temperature) and humidity. To assess how the representation of traits such as degree of lamina dissection, leaf length, leaf mass per area (LMA), trichome density, venation density, stomatal density, and of adaptive strategies such as poikilohydry vary at the community and species levels in response to changes in humidity and temperature in the Bolivian Andes, we (1) compared whole pteridophyte communities at 14 sites, and (2) analysed intraspecific variation of the morphological traits of 17 fern species along an elevational gradient at 1700–3400 m in humid forest. Among the fern communities of the 14 sites, leaf length decreased with elevation and aridity, LMA increased with elevation, and trichome density and venation density increased with aridity. The study of intraspecific variation among 17 species showed an increase of stomatal density with elevation in six of 11 species (filmy ferns lacked stomata), an increase of specific weight in 15 species, a decrease of trichome density in seven of 10 species (other species lacked hairs), and a decrease of venation density in seven of 10 cases. Some of these trends can be interpreted adaptively: leaf thickness appears to increase in situations with low nutrient availability rather than with low water availability, whereas a dense cover of scales or hairs serves as a protection against insolation or as a vehicle for the absorption of water in poikilohydric species. In arid areas, trichome density increased with elevation, while it decreased with elevation in cloudy and humid regions. For most traits, variation was more pronounced at the community than at the species level, except for stomatal density, which varied much more strongly within than between species. Several of these morphological and anatomical characters can be used to infer palaeoclimatic conditions based on fossil pteridophyte floras.