R.T. Ogilvie. 1998. Vascular Plants in Smith, I.M., and G.G.E. Scudder, eds. Assessment of species diversity in the Montane Cordillera Ecozone. Burlington: Ecological Monitoring and Assessment Network, 1998.

VASCULAR PLANTS

R.T. Ogilvie
Department of Biology
University of Victoria
P.O. Box 1700, MS 7094
Victoria, B.C.
V8W 2Y2

1. INTRODUCTION

1.1 General Features of the Montane Cordillera

The Montane Cordilleran Ecozone is both vast and variable. It covers an area of approximately 405,000 square kilometers; extending from the foothills and eastern slopes of the Rocky Mountains in Alberta to the eastern crest of the Pacific Coast Mountains in British Columbia. It extends over eight degrees latitude (between 49 to 57 degrees N. latitude), and averages in width ca. eight degrees longitude (between 114 to 121 degrees W. longitude in the south, and between 123 to 131 degrees W. longitude in the north).

These gross geographic features result in extremes of a mild temperate climate in the south and a pronounced cold boreal climate in the north; and a contrasting oceanic climate in the west and a pronounced continental climate in the east. Superimposed on this variability is the extreme range in topography and altitude. The western cordillera region has been described as a sea of mountains: a sequence of mountain ranges like wave-crests alternating with a series of valleys and plateaus like the troughs of waves. From east to west the sequence of mountain systems is: the Rocky Mountain System, the Columbia Mountain System, the Central Plateau and Mountain system, the Cascade Mountains, and the Coast Mountain System. Each of these mountain systems is made up of several mountain ranges, approximately thirty in total.

Elevations range from ca. 300 m. in the lowest valley bottoms to maximum altitudes of 3,954 m. in the Rocky Mountains, 3,500 m. in the Columbia Mountains, 2317 m. in the Okanagon Highlands, 2628 m. in the Cascade Mountains, and 3240 m. in the eastern Coast Mountains. The western windward sides of the mountain ranges are wet, having high rainfall and deep snowpack, the eastern lee-side of the mountain ranges show the rain-shadow effect with lower precipitation and arid conditions. Altitudinal effects are pronounced, with increasing precipitation and decreasing temperatures with higher elevation, resulting in pronounced arid conditions in the southern interior valleys and cold temperatures and deep snow conditions at high elevations, with glaciers and icefields on the highest peaks and ridges in all the main mountain systems. Correlated with this is a distinctive altitudinal zonation of the vegetation, for example: at valley bottom semi-desert sagebrush and bunchgrass vegetation, above which is ponderosa pine and Douglas-fir savanna, and with increasing altitude closed coniferous forest of Douglas-fir, red cedar, and hemlock, rising up to the subalpine forest of spruce and fir, and on the highest vegetated mountain summits alpine heath and meadow vegetation.

Floristic diversity in different parts of Canada has been explained by the simple fact that Canada borders on the U.S. and consequently many southern species, including rare taxa, reach their northern range limit in Canada. This is only a partial explanation for areas of different floristic diversity. Regions such as British Columbia with high diversity in geological substrates ranging from basic sedimentary strata to acidic metamorphic and granitic strata, and specialised serpentine conditions; with the great physiographic variability described above, resulting in major diversity in patterns of climate; and also a highly diverse geological history involving mountain uplift, volcanism, and several sequences of glacial advances and recessions: all these factors have acted as evolutionary selection forces on the potential floras originating not only from the south, but also the west (amphipacific), the north (circumpolar), and the east (the central plains); which have given rise to the floristic diversity that exists now.

1.2 Geological and Phytogeographic History

The Tertiary geological and phytogeographical history of the montane cordilleran area are discussed by Daubenmire (1975, 1977, 1978, 1982), Wolfe (1969, 1987), and Leopold and Denton (1987). During the Eocene the North American landscape was of low relief: a plain with scattered small mountains, the climate was much warmer, frost-free, and geographically very uniform; the major vegetation was quasi-tropical broadleaved evergreen forest. On the low isolated hills there was frost-tolerant forest of evergreen conifers (e.g. Pinus, Taxus) and deciduous broadleafed trees (e.g. Betula, Acer). Climatic cooling occurred in late Eocene and early Oligocene (40 MA. BP), resulting in a decrease in the tropical broadleafed evergreen species and increase in the cold-tolerant temperate species (e.g. Pseudotsuga, Tsuga, Fraxinus) migrating from the north and from the low hills. In late Oligocene and Miocene (25 MA. BP.) uplift of the Rocky Mountains occurred, resulting in interception of moisture from the Pacific Ocean, and producing a moist climate on the west slope of the mountains and an arid rain shadow on the east flank in the Great Plains. The latter area had thinning of the forest, bare hilltops, and immigration of grass species (e.g. Stipa, Panicum, Setaria), and increase in numerous ungulate grazing animals. On the west side of the Rocky Mountains the abundant moisture supported a rich mixed coniferous-deciduous forest which became isolated, by the arid prairie grasslands, from the eastern decidous forest. Continuation of the Rocky Mountain uplift lead to migration from the boreal region of microthermal species which occupied the upper mountains slopes: a subalpine coniferous forest of Picea, Abies, Pinus contorta, Tsuga mertensiana, and many cold-tolerant heath species. The Cascade Mountains were uplifted in mid-Pleiocene (3.5 MA. BP.), resulting in rain-shadow and arid conditions on the east slope of the Cascade Mountains and extending eastward across the trough to the Rocky Mountains. In this arid area the forest became thinned and was composed of drought-tolerant species of the boreal flora (e.g. Artemisia tridentata, Agropyron spicatum, Festuca idahoensis, Poa secunda), and also of xerophytic immigrants from the southern intermountain area (e.g. Pinus ponderosa, Purshia tridentata, Chrysothamnus, Eriogonum). On the moist western slope of the Rocky Mountains there was temperate evergreen coniferous forest, but elimination of most of the ancient deciduous forest trees which were less cold-tolerant than the evergreen conifers.

Thus, the major floristic patterns of western North America have an ancient geological origin: separation of the western coniferous forest from the eastern deciduous forest, and formation of the central plains grassland in early Miocene following the Rocky Mountain uplift; and later development of the Interior intermountain flora and vegetation in the mid-Pleiocene following uplift of the Cascade Mountains. Following these ancient geological events the more recent Pleistocene glaciations resulted in a series of migrations, extinctions, and immigrations, leading to the present floristic composition and vegetation patterns.

The Quaternary glacial geology is reviewed and summarised in the Geological Survey of Canada compendium edited by Fulton (1989), more specifically in the contributions therein by Clague, Ryder, Mathews, Rutter, Hughes, Schweger; and the reports by Mathews (1980, 1986). Pleistocene glaciation in the cordillera region developed from expansion of alpine glaciers and icefields and coalescence with piedmont and valley glaciers. At its maximum development this Cordilleran Glacial Complex (Cordilleran Ice Sheet) was a 2,500 m. thick ice dome which extended down the west flank of the Coast Range into the Pacific Ocean and eastward across the interior plateaus and mountain ranges and down the east slope of the Rocky Mountains to the Interior Plains where it approached or met with the Laurentide Ice Sheet. The highest mountain peaks (above 2,500 m.) protruded from the ice sheet as nunataks. The Coast Mountains were a major ice-divide, deflecting glacial flow both westward and eastward, but several of the Interior mountain ranges also served as ice-divides deflecting glacial flow off their flanks. Glaciation in the Rocky Mountains developed partly independent from the main cordilleran ice sheet. Deglaciation occurred by frontal retreat along the peripheral areas, and followed the reverse sequence of glacial growth: mountain peaks and upland areas first became ice-free, and the last areas to become free of ice were the lower valley sides and valley bottoms. Dated glacial deposits have shown that during the Quaternary there were three or four glaciations separated by interglacial (non-glacial) intervals. The most recent Fraser glaciation of the late Wisconsinon (ca. 26 KA) is the best documented, because the deposits of the earlier glaciations often have been destroyed, overrun, or intermixed with later glacial deposits; and moreover, the older deposits may exceed the limits of radiocarbon dating.

The relationship of the Cordilleran and Laurentide ice sheets are complex; there is evidence that at certain periods and places the two ice sheets were not synchronised in their advances and retreats, and at other periods and places they were synchronised. The non-synchroneity is explained by the Cordilleran glaciers having a much shorter travel distance from their sources, allowing them to advance beyond the mountain front before the more massive Laurentide ice sheet reached the area. This has a direct bearing on the “ice-free corridor” concept which postulates that at times when the Cordilleran and Laurentide ice sheets did not meet, an ice-free corridor existed from the non-glaciated southern region to the ice-free Yukon Valley leading into the Bering land-bridge linking North America and Siberia, and providing a migration route for humans, animals, and plants (Rutter, 1978, 1980, 1984), Rutter & Schweger (1980), Schweger (1989), Bobrowsky & Rutter (1990). The research of W.H. Mathews (1980, 1986) in northeastern British Columbia and adjacent Alberta showed that there was interbedding and overlap of Cordilleran and Laurentide tills and glacial erratics from mid to late Wisconsinan (ca. 27 to 11 KA. BP) along the eastern front of the Rocky Mountains from the Yukon border southward to the Athabasca River Valley, and thus the northern two-thirds of the corridor was closed throughout the peak of the last glacial advance. Schweger (1989) summarised the geological and paleoecological data for the proposed corridor area, and confirmed that in the area south from the Athabasca River Valley to the unglaciated plains along the U.S. border, from mid to late Wisconsinan (24 to 11 KA. BP.) there was cold, dry tundra, and beginning at 15.5 KA. there was the start of deglaciation of the perimeter ice. Thus, during the last Wisconsinan glaciation the ice-free corridor, at most was a short cul-de-sac occurring from the southern non-glaciated plains northward only approximately 480 km.

 

TABLE OF CONTENTS