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In California and other Mediterranean-type ecosystems, island species are typically exposed to more fog but less rain than mainland species. Because adaptations to absorb water from fog may conflict with those to minimize water loss, we hypothesized that island species should have greater fog absorption than their mainland congeners due to foliar uptake but at the cost of modifying other leaf structural and functional traits. To determine whether foliar water absorption is an adaptation to insularity, we compared seven physiological and anatomical leaf traits between congeneric island and mainland species of two genera, Ceanothus and Arctostaphylos, in a common garden in Claremont, California. We quantified leaf water potentials, maximum leaf water absorption rates, leaf hydrophobicity, leaf mass per area [LMA], succulence, stomatal density, and wax morphology. All taxa exhibited water permeability through their leaf surfaces, but only one of the three island taxa showed greater water absorption than their mainland counterparts. The island and mainland varieties of C. megacarpus were similar in water absorption and hydrophobicity, but the mainland variety had greater LMA, greater succulence, and thicker epicuticular wax. In Arctostaphylos, insularity promoted species-specific responses: A. catalinae had greater foliar absorption compared to the mainland species, whereas A. insularis displayed mesophytic traits such as hypostomatal morphology, horizontally oriented leaves and low LMA. Relative surface hydrophobicity was not linked to absorption rates, but the mainland species A. glauca had the most hydrophobic leaf surfaces in the study, achieved by their ornate epicuticular wax. Overall, island taxa displayed more mesophytic leaf traits than their mainland congeners. The results may have implications for biogeography in Mediterranean-type ecosystems that may be losing seasonal coastal fog with global change.

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© 2023 Humera Mirza

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