The Floristic Disjunctions Between the "Monte" in Argentina and the "Sonoran Desert" in Mexico and the United States

Abstract

One of the best known but most puzzling disjunctions is that between the temperate regions of South America and the temperate regions of North America. It is puzzling because it involves a large number of floristic elements, and because it is evident from the geological data available that the two regions have never been closer to each other than they are today (Dietz, 1961). Raven (1963, 1971) has addressed himself to this problem and has very accurately pointed out that we are dealing with more than one type of disjunction. There is the disjunction involving elements that grow in the cold regions of South and North America; the disjunctions involving plants that grow in areas with a Mediterranean type of climate in both continents; the disjunctions between areas with a mesic hydric and temperature regimes; and finally the disjunctions between the so-called In his reviews, Raven presents evidence that indicates that these disjunctions are relatively recent and that they probably have arisen almost exclusively by long-range dispersal. However, Raven did not discuss in detail the disjunction involving the desert areas. This is the best known one (Bray, 1898, 1900; Campbell, 1944) and probably the most controversial. It also involves the largest percentage of the flora. In this paper I want to present data that lead me to believe that we are dealing with patterns of distribution that have been established over a long period of time and that no one single explanation can encompass them all. In order to clarify the discussion, only plants that grow in the phytogeographical province of the Monte in Argentina and the Sonoran Desert in Mexico and the southwestern United States will be discussed.