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Abstract

Barrier islands are a common geological feature found along the Atlantic and Gulf Coasts of the United States. Researchers have historically described barrier islands, and in particular dune habitats, as being physically controlled. Dune plant species experience high surface temperatures, low water-holding capacity of the substrate, sand movement, salt aerosols, high solar irradiance, and saltwater overwash. In this study, we investigated characteristics of three common plant species growing on the dunes of Sapelo Island, Georgia to understand how they survive and apparently thrive in the harsh, transient, and dynamic environment. Specifically, we examined leaf size, leaf thickness, stomatal frequency, internal leaf anatomy, and xylem water potential of Ipomoea brasiliensis, Croton punctatus, and Hydrocotyle bonariensis in relation to where individuals grew in the dune system. Leaves of plants of each species were smaller and thicker when growing on the front of dunes compared to the back of dunes. For each species, stomata were more abundant on the adaxial versus the abaxial leaf surfaces. In general, for each species, leaves growing on the front of dunes had a higher stomatal frequency than leaves growing on the back of dunes. Internal leaf anatomy for each species was equifacial with no appreciable differences in architecture between leaves growing on the front as compared to the back of dunes. Neither I. imbricata nor H. bonariensis experienced water stress over a typical summer day (the water potential of I. brasiliensis could not be measured due to the presence of latex). The difference in morphology of leaves of plants growing on the front of dunes as compared to the back of dunes illustrates classic phenotypic plasticity in relation to sun and shade leaves. The stomatal frequency and internal leaf anatomy of each species indicates that light is being processed by both the adaxial and abaxial leaf surfaces. Leaf succulence, presence of trichomes, and root architecture may allow C. punctatus to maintain a relatively high water potential. Translocation of water among ramets is a strategy used by H. bonariensis to avoid water deficit. Ipomoea brasiliensis, Croton punctatus, and Hydrocotle bonariensis demonstrate various leaf anatomy, morphology, and water conservation strategies that may serve as adaptations to the dune environment.

Acknowledgements

The authors are thankful for financial support provided by The Georgia Botanical Society through the Marie Mellinger Field Botany Research Grant. Financial support via a University System of Georgia STEM grant is also gratefully acknowledged. We thank The University of Georgia’s Marine Institute on Sapelo Island, and especially Gracie Townsend, for logistical assistance. The comments and suggestions of two anonymous reviewers are gratefully acknowledged.

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