Variation in leaf dissection and leaf energy budgets among populations of Achillea from an altitudinal gradient

The degree of leaf dissection differs dramatically among populations oftheAchillea millefolium complex along an altitudinal gradient in the Sierra Nevada. The purpose of this study was to determine whether there was a genetic basis to differences in leaf shape among populations, and also to determine the importance of genetic variability within populations, plastic responses of leaf shape to the growth environment, and genetic differences among populations in plastic response to the environment. A second major goal of the research was to investigate the effects of differences in leaf dissection on the capacity for leaf temperatures to become uncoupled from air temperatures. Greenhouse experiments using clonal replicates of plants collected at different altitudes revealed that there were genetic differences among populations in the degree of dissection, and that other sources of phenotypic variation, such as plasticity, were also significant. Leaves from the highest altitude population had the most compact shape under all conditions, while those from lower altitude populations were always more open and highly dissected. In both simulations and actual measurements the dissected leaves of low altitude plants remained close to air temperatures, while the compact leaves of high altitude plants had the capacity to warm up substantially above air temperatures. THE RELATIONSHIP between climate and leaf size, shape and structure is among the earliest problems to have received the attention of plant geographers and ecologists (e.g., Warming, 1903; Schimper, 1909). Most work in this area has concentrated on comparisons between species with differing adaptations to environmental conditions (e.g., Givnish, 1986). In contrast, we know relatively little about the nature, extent and significance of variation in leaf form within and among populations within species (but see Lewis, 1969; Meinzner, Gold' Received for publication 20 July 1987; revision accepted 16 December 1987. This work was supported in part by a grant from the American Philosophical Society and by a summer faculty fellowship from the State University of New York at Stony Brook. I am grateful to the Carnegie Institute of Washington for use of their facilities, and especially to Frank Nicholson for logistical support, and to Dr. Malcolm Nobs for invaluable help in collecting plants from the original Carnegie sites. Goose Lake Prairie plants were collected with help from Charlie Fenster. Dr. Robert Pearcy and Dr. K. T. Paw U provided use of their laboratory facilities, and the project benefited greatly from their expertise and interest. I am indebted to Drs. J. A. Teeri, S. Vogel, and F. J. Rohlf for valuable discussion and to K. Fiala for computer consultation. The research would not have been possible without critical laboratory assistance from Dan Van Auker, Kathy Kretzschmar, Stella Bianco, Carol South, and Mark Treitel. The clarity of this paper benefited from comments by G. Cunningham and E. Taylor. This is Contribution No. 648 in Ecology and Evolution at the State University of New York at Stony Brook. stein, and Rundel, 1985). Leaf shape differs dramatically among populations of the Achillea millefolium complex along an altitudinal gradient in the Sierra Nevada of California (personal observation). The purposes of the present study were 1) to find an objective, repeatable measure that could statistically separate differences in appearance among highly dissected leaves with complex geometry, 2) to determine the extent to which phenotypic differences in leaf shape were due to genetic differences among populations (ecotypic differentiation) and within populations (genetic variability), due to plastic response of leaf shape to the growth environment, and due to differences in plastic response among populations, and 3) to investigate the potentially adaptive effects of differences in leaf shape on the capacity for leaf temperatures to become uncoupled from air temperatures. Achillea millefolium was one of the first species in which the existence of ecotypic differentiation was experimentally demonstrated (Clausen, Keck, and Hiesey, 1948). Although these workers noted differences in leaf shape among ecotypes, the only related trait to be studied quantitatively was leaf length. Leaves of North American Achilleas are highly dissected, as indicated by the species name (although they are actually simple, not compound leaves). The degree of leaf dissection appeared to differ along an altitudinal gradient, from