Species Disjunctions in Larrea: Evidence From Morphology, Cytogenetics, Phenolic Compounds, and Seed Albumins

Abstract

Larrea divaricata, the creosote bush, is often regarded as the most drought tolerant higher plant in North America (Barbour, 1969; Morello, 1955). It is probably a single species having an enormous disjunct distribution. In North America it extends from the State of Nevada to the State of Hidalgo in Mexico, where it is called gobernadora on account of its dominance in the natural vegetation (Fig. 1). In South America it occurs in several isolated places in Peru (Ica, Nazca, Chuquibamba, Moquegua), in Bolivia (Tarija), and in Chile (Atacama, Concepcion). In Argentina it is called jarilla, and it extends from Salta in the North to North Patagonia (Chubut) (Fig. 2). As pointed out by Barbour the disjunction reaches 36 degrees latitude, that is, nearly 4,000 km (Barbour, 1969). The question that immediately arises, when considering such an extense distribution is whether we are dealing with a single species or at least two vicariant species: Larrea tridentata (North America) and L. divaricata (South America). How much have these populations diverged since they became separated? Typical Larrea divaricata was described from Mendoza, Argentina, South America, and the North American taxon has been named L. tridentata. Morphologically, they can easily be separated by the form of the stipules, the North American populations having acute while the South American ones have obtuse stipules. They could be distinguished, therefore, as different subspecies as done recently by Felger and Lowe (1970) who recognize L. divaricata Cav. subsp. divaricata for the South American subspecies and its ecotypes and subsp. tridentata (Sesse & Moc. ex DC.) Felger & Lowe for the North American subspecies and its ecotypes.