A Hollywood palm icon unmasked: clinal variation in Washingtonia (Arecaceae) of Peninsular California

摘要

Washingtonia is an American genus of palms currently composed of two species, W. filifera and W. robusta. Poor understanding of their morphology and distribution led initially to the description of numerous new species and, later, to a simplification resulting in the current understanding of the genus. In the wild, both distinct forms occur in the extremes of a 1300-km gradient, linked by morphological intermediates, suggesting that there is continuous morphological variation of the two forms instead of the long-held idea of two species. We addressed this hypothesis by taking measurements of morphological traits of stems and functional traits of leaves at 18 sites spanning the distribution range of the genus in Mexico and the USA. We examined the relationship between the traits of the plants, individually and all traits combined, and latitude using a principal component analysis. We compared a linear model against a non-linear model to test whether traits in Washingtonia change gradually along the latitudinal gradient or if there are clear geographical breaks between populations. We found strong statistical support for the hypothesis that there is a gradual change in vegetative morphological traits in Washingtonia along the latitudinal gradient of Peninsular California, instead of two clear morphologically distinguishable species with a hybrid zone. Our findings suggest that Washingtonia is composed of one highly variable species and that local differentiation of populations is related to environmental gradients.