Strong Phenotypic Variation in Floral Design and Display Traits of an Annual Tarweed in Relation to Small-Scale Topographic Heterogeneity in Semiarid Chile

Abstract

Floral trait expression in wild populations varies in relation to environmental gradients. Variation can be observed among and within plant populations. We investigated the changes in floral phenotype within populations and the probability of plant pollinator visits in relation to small-scale variation in soil moisture and plant cover in a semiarid ecosystem. We measured the variability of floral phenotypes of three wild populations of Madia chilensis along a gradient constituted by three topographic positions (south-facing slope, north-facing slope, and ravine). Changes in soil moisture, plant density, leaf water content, and internode elongation were measured for one population over two study years. Pollinator visit probabilities were also estimated. Floral phenotypes were strongly segregated among topographic positions but less segregated among populations. Plants with the lowest water contents and the smallest or least-conspicuous flower heads grew in the drier north-facing slope, while plants with the highest water contents and showier flower design and display grew in the wetter ravine. Probabilities of pollinator visits were higher on the north-facing slope. Differences in floral phenotypes and probabilities of pollinator visits of M. chilensis were strongly connected to topographic variations at small spatial scale. Topographic heterogeneity at small spatial scale could thus have important consequences for floral trait evolution and pollination ecology.