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.