Broad- and small-scale environmental gradients drive variation in chemical, but not morphological, leaf traits of vascular epiphytes

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 were consistent across photosynthetic pathways (CAM and
C3). Broad- and small-scale environmental gradients explained more
variation in chemical traits (marginal R2: 11-89%) than in morphological
traits (marginal R2: 2-31%). For example, leaf carbon isotope signatures
(δ13C), 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.