Distribution and regional ecology of Californian palm oases interpreted from Google Earth images.

Abstract

The native fan palms of the Baja California peninsula (Mexico) and neighboring southern California (USA) comprise two species each in Washingtonia and Brahea. While these are among the most familiar ornamental palms in the world, many features of their taxonomy, distribution, and environmental relationships remain unresolved. We utilized Google Earth Digital Globe imagery (and limited surveys by air and land) to develop the first map detailing the geographic distribution of palm populations in the Peninsular Range Province (PRP) and used these data to formulate new hypotheses on their biotic and abiotic relationships. We inventoried 15,216 populations covering an area of 8533 ha, containing an estimated 1.31 million trees. Tree populations estimated by manual tallies from Google Earth images closely matched direct counts published by previous workers. We documented significant extensions in the distributions of both Washingtonia species, as well as 66 hitherto unreported W. robusta oases in the Sierra La Asamblea. We also recorded thousands of upland populations of dwarf B. armata. Google Earth imagery shows that native fan palms are widespread in remote mountains of the entire peninsula, but in Baja California Sur W. robusta is mostly restricted to a few highly altered agricultural landscapes associated with extensive wetlands. It has long been assumed that the center of W. robusta distribution is in the tropical southern peninsula. Our observations suggest it may have been dispersed there by indigenous peoples before European contact, and its center of genetic diversity may instead lie in the Central Desert (ca. lat 28u–30uN).