An Overview of the Origins of the Modern Vegetation and Flora of the Northern Rocky Mountains

Abstract

The present flora of the northern Rocky Mountains has diverse origins. The Late Cretaceous mesothermal to megathermal evergreen vegetation of this region had few taxa, even at the familial level, that live in the northern Rocky Mountains today. The terminal Cretaceous event resulted in broadleaved deciduous taxa occupying mesothermal climate in the northern Rocky Mountain region, where they underwent major diversification. The early Eocene thermal maximum severely restricted the areas of microthermal climate and created a climatic/geographic bottleneck for microthermal lineages. By the early middle Eocene, volcanic uplands that supported microthermal coniferous forests of Pinaceae and Cupressaceae had developed in parts of the northern Rocky Mountain region. These Eocene coniferous forests contained a diverse broad-leaved subsidiary element, most of which represented adaptive radiations of mesothermal clades into newly created microthermal climate. During the later Eocene, microthermal vegetation was progressively enriched by internal diversification and by adaptation of mesothermal groups to microthermal climate; extinction also was a major factor in the changing composition of the microthermal flora. The Eocene volcanic uplands of western North America played a major role in the diversification of arcto-tertiary clades; dispersals between western North America and Eurasia of members of these clades was readily accomplished during the later Eocene by way of Beringia. Some of the Eocene microthermal lineages survive relatively unchanged in the northern Rocky Mountains today, and other extant lineages, although ultimately derived from taxa in the Eocene uplands, represent morphological types that originated and diverged in Eurasia, arriving in North America by migration. Oligocene and Neogene floras are largely unknown in the northern Rocky Mountains, although the Columbia Plateaus to the west contain many assemblages of these ages. These assemblages represent microthermal broad-leaved deciduous or coniferous forests, and most constituent taxa represent lineages that were derived from Eocene upland vegetation following the terminal Eocene temperature deterioration. With the presence of many extant northern Rocky Mountain lineages, the Columbia Plateaus assemblages provide evidence that the northern Rocky Mountain region was probably occupied primarily by coniferous forest during the Oligocene and most of the Neogene. During the mid-Miocene thermal maximum, numerous microthermal lineages were exchanged between western North America and Eurasia. Other extant northern Rocky Mountain lineages are known during the Miocene only in areas such as Alaska and probably represent migrants into the northern Rocky Mountains during the late Neogene. The present flora of the northern Rocky Mountains therefore clearly represents a complex overlay of numerous historical biogeographic patterns. The modem vegetation of the northern Rocky Mountain region is primarily steppe at low altitudes and coniferous forest at higher altitudes (Habeck, 1987). Temperatures are entirely microthermal.3 The lower and drier part of the coniferous forest belongs to the Pinus ponderosa zone, whereas most of the mesic coniferous forest at higher altitudes belongs to the Abies grandis zone (Franklin & Dyrness, 1969). The vegetational/climatic types that have occupied the northern Rocky Mountain region are documented by successive microfossil and megafossil plant assemblages. In some instances, lack of assemblages that represent particular intervals makes it necessary to infer climate and vegetation from the fossil record of adjacent regions. This report addresses vegetational (physiognomic) types that have occupied the northern Rocky Mountain region in the past and the history of the lineages that comprise the modern flora. Throughout, the focus is primarily on the woody