LEAF ARCHITECTURAL ANALYSIS OF EXTANT AND EXTINCT SPECIES OF HOLODISCUS (ROSACEAE) A GENUS WITH ORIGINS IN THE TERTIARY OF WESTERN NORTH AMERICA

Extant taxa of Holodiscus are montane mesophytic to xerophytic shrubs inhabiting sub-humid to arid climates. Current range of the genus is from central British Columbia, south across the western United States, into Mexico, Central America and terminating in Colombia and Venezuela. Throughout its range it is often found in edge habitats and rocky outcrops. Leaf architectural analysis of the extant taxa revealed two species complexes, the dicolor complex (H. discolor, and H. microphyllus) which ranges from British Columbia to the northern edge of the Trans-Mexican Volcanic Belt (TMVB), and the argenteus complex (H. alpestris, H. argenteus, H. fissus, H. orizabae) which ranges from the TMVB south to Colombia and Venezuela. A new species from the Eocene Copper Basin Flora of north-eastern Nevada bears distinctive features of both complexes and is a plausible ancestral taxon for the genus. The species (H. lisii) with leaf architecture indicative of the discolor complex first appears in the Florissant Formation (Colorado). In the late Oligocene Creede Flora (Colorado) leaves of H. stevenii display architectural characteristics indicative of increasing aridity which are seen in the living taxa of H. microphyllus and H. discolor. Taxa of the discolor complex are deciduous and typically flower between April and July; whereas taxa of the argenteus complex are not deciduous and typically flower between October and February. Leaf architectural differences between these two species complexes reflect the deciduous and non-deciduous nature of the leaves; and the fossil record would suggest that the deciduous condition arose in response to decreasing mean annual temperatures between the Eocene and Oligocene. Interpreting the fossil record of the species has been increased substantially with a comprehensive study of all the extant taxa in the genus.