Leaf structural adaptations in vascular epiphytes from the Atlantic rainforest along phorophyte vertical stratification

Abstract

Epiphytes are constantly exposed to light, water and nutrient stress, and thereby they frequently show anatomical adaptations in response to their habit. We aimed to investigate the variation in foliar anatomical traits of epiphytes along vertical stratification. We hypothesized that different species from varied groups would show a variation in foliar anatomical traits that would reflect the variation in microclimatic variables found across phorophyte strata. To address our hypothesis, we studied 23 vascular epiphyte species from different groups (ferns, magnoliids, monocots, and eudicots), occurring on ten phorophytes from a trail at an Atlantic Rainforest fragment in southeastern Brazil. We measured six microclimatic parameters at different phorophyte heights: light intensity, temperature, relative air humidity, altitude, evaporation rate, and wind speed. We then collected expanded-leaf samples for quantitative and descriptive anatomical evaluation, seeking to detect possible correlations between structural traits and microclimatic variations. We found that overall the analyzed species showed pronounced xeromorphic characters in response to epiphytic habit, such as thick outer periclinal wall of pavement cells, lignified epidermis, multiseriate epidermis, stomata below the level of pavement cells, water-storing tissues, and parenchymatous cells with wall thickenings, among others. In most epiphytes, we also found a trend for thicker mesophyll in plants exposed to higher light intensity along different phorophyte strata. The ecophysiological significance of these adaptations is discussed in detail. Our hypothesis was accepted, since we found that species from different groups showed a variation in foliar characters that reflected the variation found in microclimatic parameters along phorophyte strata. Some anatomical traits, e.g. thick mesophyll, were influenced by height differences along phorophytes and by the consequent variation in light intensity along vertical stratification, while other traits were directly related either to the epiphytic habit, especially to the water and nutrient deficits imposed by it, or to the taxonomic group, as seen in ferns.