Abstract

Variation in leaf functional traits along environmental gradients can reveal how vascular epiphytes respond to broad- and small-scale environmental gradients. Along elevational gradients, both temperature and precipitation likely play an important role as drivers of leaf trait variation, but these traits may also respond to small-scale changes in light, temperature and humidity along the vertical environmental gradient within forest canopies. However, the relative importance of broad- and small-scale environmental gradients as drivers of variation in leaf functional traits of vascular epiphytes is poorly understood. Here, we examined variation in morphological and chemical leaf traits of 102 vascular epiphyte species spanning two environmental gradients along Cofre de Perote mountain in Mexico: (i) a broad-scale environmental gradient approximated by elevation as well as by species' lower and upper elevational limits, and (ii) small-scale environmental gradients using the relative height of attachment of an epiphyte on a host tree as a proxy for variation in environmental conditions within the forest canopy. We also assessed whether variation in morphological and chemical leaf traits along these gradients was consistent across photosynthetic pathways (CAM and C-3). Broad- and small-scale environmental gradients explained more variation in chemical traits (marginal R-2: 11%-89%) than in morphological traits (marginal R-2: 2%-31%). For example, leaf carbon isotope signatures (delta C-13), which reflects water-use efficiency, varied systematically across both environmental gradients, suggesting a decrease in water-use efficiency with increasing lower and upper elevational limits and an increase in water-use efficiency with relative height of attachment. The influence of lower and upper elevational limits on trait variation differed between photosynthetic pathways, except for leaf dry matter content and leaf nitrogen-to-phosphorus ratio. Contrary to our expectations, broad- and small-scale environmental gradients explained minimal variation in morphological leaf traits, suggesting that environmental conditions do not constrain morphological leaf trait values of vascular epiphytes. Our findings suggest that assessing multiple drivers of leaf trait variation among photosynthetic pathways is key for disentangling the mechanisms underlying responses of vascular epiphytes to environmental conditions. Read the free Plain Language Summary for this article on the Journal blog.