Leaf wettability decreases along an extreme altitudinal gradient

The duration and amount of water captured on leaves and its functional significance is highly varied. Leaf surface wettability influences water absorption, gas exchange, pathogen infection, nutrient leaching, contamination by pollutants, self-cleaning properties and in freezing environments the probability of extrinsic ice nucleation. To test the impact of environment on the development of leaf wettability, this functional trait was measured in 227 dominant plant species along an extreme altitudinal environment gradient (186-5,268 m) on the wet and dry slopes of the Nepalese Himalayas. Plants from the understorey and open places in woodlands were also compared. Leaf wettability was assessed by droplet contact angle (θ), retention and leaf inclination measurement. With increasing altitude leaf wettability decreased significantly parallel to the observed atmospheric temperature decrease (0.5 K/100 m). Leaves from non-freezing tropical and subtropical origins were highly wettable (θ < 90°). Temperate leaves were non-wettable (110° < θ < 130°). Subalpine and alpine leaves were highly non-wettable (130° < θ < 150°) and adaxial pubescence occurred more frequently. Leaves taken from the understorey were more wettable but had a better droplet run off than leaves sampled in open places. In the semi-arid northern slopes (temperate to alpine) of the Himalayas leaf wettability was decreased in comparison to the southern humid side. The majority of the leaves had a low droplet retention <20°; higher values were linked to high non-wettability (θ > 130°) which was more often observed at high altitude. Good droplet run off at ±10° inclination was found in highly wettable leaves (θ < 90°) of tropical and subtropical origin and on leaves from the forest understorey. Structural properties for low wettability are developed in cold and dry environments and open sites with frequent dew formation as it appears to be an important functional trait to prevent a number of the negative effects adhering surface water may have.