Purshia: The Wild and Bitter Roses (2002)
James A. Young & Charlie D. Clements

How, in terms of geologic times scales and plant evolution, did antelope bitterbrush, desert bitterbrush, and cliffrose come to occupy the landscapes where they are found today? The family Rosaceae consists of about 110 genera and some 3,000 species.9 Its members occur worldwide but are more common in northern temperate regions, especially in western North America and eastern Asia.10 The family Rosaceae is one of 17 families in the order Rosales, which probably derived from the primitive Magnoliales fairly early in angiosperm history.11 Obviously, the evidence of the origins of the rose family is composed of incomplete bits and pieces obtained from the fossil plant and pollen records. The Rosaceae were well represented by the time of the Paleogene some 50 million years ago.12 The order Rosales was associated with the northern group of continents (Laurasia) when Gondwana began to break into roughly the continents we know today.13

The three shrubs of interest to us fall into the subfamily Rosideae and the tribe Dryadeae. The common haploid (x) chromosome number of members of this subfamily is 7 or 9, with x = 8 or 14 also occurring. Polyploidy has been reported in this subfamily. Representative genera of the temperate deserts of the American West and their haploid chromosome numbers are Purshia (9), Cowania (9), Cercocarpus (9), and Fallugia (14)14 The mountain mahogany species (Cercocarpus) occur sympatrically with the two bitterbrushes and cliffrose. Apache plume (Fallugia paradoxa), the only member of its genus, occurs in the Southwest from California to west Texas, northward to Utah and Colorado, and well south into Mexico (Map 1.3), generally in environments transitory from pinyon-juniper woodlands to more arid situations. Blackbrush (Coleogyne [haploid chromosome number = 8]) is found in a broad band across the southern Great Basin in areas in transition from sagebrush to warm desert ecosystems.

Map 1.3. Distribution of cliffrose and Apache plume in western North America.