The relationship between leaf hydrophobicity, water droplet retention, and leaf angle of common species in a semi-arid region of the western United States

Leaf hydrophobicity and water droplet retention are important functional traits among plant species and may be important selective strategies to increase water availability to root systems in arid and semi-arid environments. Hydrophobic canopies may decrease rainfall interception and increase throughfall to the soil. Leaf angle may also influence the ability of water droplets to channel off leaf surfaces. Water droplets may have a greater ability to drain off leaf surfaces with steeper leaf angles which decrease rainfall interception and increase throughfall to the soil. This study examined the relationships between leaf angle, leaf hydrophobicity, and water droplet retention in 11 species from a semi-arid region of Colorado in the western United States. Leaf hydrophobicity of the adaxial and the abaxial leaf surface was measured by calculating the contact angle between a water droplet and the leaf surface. Water droplet retention was measured as the angle at which a water droplet begins to move as a leaf is incrementally tilted from 0° to 90°. Leaf hydrophobicity was greater than leaf angle for 9 species and similar for 2 species. Leaf angle was greater than water droplet retention on the adaxial leaf surface for 8 species and similar for 3 species. Leaf angle was greater than water droplet retention on the abaxial leaf surface for 7 species and similar for 2 species. The relationship between leaf angle and leaf hydrophobicity and the relationship between leaf angle and water droplet retention were weak; however, the relationship between leaf hydrophobicity and water droplet retention was stronger. Results indicate that intercepted water droplets during a rainfall event will likely drain off the leaf surfaces from a majority of the species examined and contribute to throughfall because leaf angle is greater than water droplet retention in these species. The relationship between leaf angle and canopy hydrophobicity may be a significant influence on hydrological processes in arid and semi-arid regions.