2018 ESA Annual Meeting (August 5 -- 10)

COS 90-7 - Clinal variation of the genus Washingtonia (Arecaceae)

Thursday, August 9, 2018: 10:10 AM
355, New Orleans Ernest N. Morial Convention Center
Lorena Villanueva-Almanza1, Eric Focht1, Eleinis Ávila-Lovera1, Vanessa Perez1, Christian Silva-Bejarano2 and Exequiel Ezcurra3, (1)Department of Botany and Plant Sciences, University of California, Riverside, CA, (2)Centro de Investigaciones Biológicas del Noroeste, La Paz, Mexico, (3)UC Mexus, University of California, Riverside, Riverside, CA
Background/Question/Methods

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 an oversimplification of the genus resulting in the current two-species circumscription. In the wild, both distinct forms occur in the extremes of a 1400-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 8 morphometric traits of stems and leaves and 5 functional traits of leaves in 17 sites spanning the distribution range of the genus, from Joshua Tree National Park to the Cape Region in Baja California Sur, Mexico. Sampling included Kofa Canyon, in Arizona, and Nacapule Canyon, in Sonora, two populations outside of Peninsular California. We examined the relationship between the plants’ traits and latitude, individually, and between all traits (using a Principal Component Analysis) and latitude. We compared a linear model against a non-linear model to test whether traits of Washingtonia change gradually along the latitudinal gradient or if there are clear geographical breaks among populations.

Results/Conclusions

The linear model, as revealed by the model selection analysis, shows that, although palms of each canyon have distinct traits, there is a clear morphological continuum in Washingtonia, with two extreme forms. Northernmost palms have thick, blue, amphistomatic leaves and wide stems, while southernmost palms have thin, green, hypostomatic leaves and slender stems. These traits change gradually along the gradient.

Of all sites, the Arizona population is the most divergent of the latitudinal pattern, reflecting a different evolutionary path better suited for extreme drought conditions, as shown by 13C analysis.

Traits like diameter at breast height, which increases northwards,and leaf greenness, which decreases, seem to be adaptations to cold temperature in northern canyons, allowing the plants to avoid freezing and take advantage of favorable moisture during the coldest months of the year.

These results have consequences on the taxonomy and biogeography of Washingtonia. Our work suggests the genus is composed of only one species and that its current distribution is made up of relict populations of a once widespread ancestral habitat.