David C. Housman, Mary V. Price, and Richard A. Redak. 2000. Coastal and desert phenotypes of Encelia farinosa: plasticity and adaptive genetic differentiation.. Oecologia, submitted.
Abstract. The perennial shrub Encelia farinosa occupies a geographic range that spans substantial variation in temperature and precipitation. Plants have adapted to this environmental variation in part through leaf characters: those exposed to hotter, drier conditions have smaller and more pubescent leaves. Small size and pubescence reduce leaf temperature and improve water-use efficiency, but this water economy is achieved at the cost of lower maximum photosynthetic and growth rates, which should result in shorter branch lengths and more compact growth form. We tested this expectation by characterizing the phenotypes of plants from natural populations in coastal vs. desert environments, and of their offspring grown in common gardens located in both environments. Coastal plants indeed had larger, less pubescent leaves and a more sprawling growth form, with lower branch densities and longer distance from the ground to the first branch-point. Damage from insect herbivores was greater in coastal than desert populations. Differences in leaf and growth form, but not in herbivory, persisted in coastal and desert offspring grown in both common gardens, suggesting a genetic basis. Leaf characters and growth form also showed plastic responses; all offspring had smaller, more pubescent leaves, shorter distance to the first branch, lower shoot density, and smaller size in the desert garden. Herbivore damage was greater for all offspring in the coastal garden. Offspring from coastal populations were no larger than desert offspring when grown in the coastal garden, but were significantly smaller in the desert garden, suggesting poor adaptation to desert conditions. Our results confirm that leaf size and pubescence are heritable characters that respond plastically and adaptively to environmental temperature and moisture, and are associated with pronounced variation in growth form.