Ministry of Environment Ecology

PART I
Ecoregion Classification Background

British Columbia's Environment - The Setting

British Columbia has many ecosystems due to its varied physiography and climates. It is located at mid- to northerly latitudes and is bounded on the west by oceanic influences of the northern Pacific Ocean and on the east by continental climates of the Interior Plains and Rocky Mountains. Its varied geological history has resulted in a complex topography. In addition, the province has had a complex climatic history. Current climatic patterns are varied but, most typically, the province is dominated by moist, cool to cold, temperate climates in a mountainous setting, most of which is higher than 1,000 m above sea level. The plants and animals of the province are affected by that environment and also by historic factors such as position of glacial refugia or barriers to dispersal and migration.

The combined influence of differential heating of land and ocean masses and the spin of the earth has created a general circulation in the earth's atmosphere (Thompson 1981). That circulation is separated into a number of latitudinal belts, one being the prevailing westerly winds that influence British Columbia and most of southern Canada and the northern contiguous United States (Marsh 1988). Because of the prevailing winds, the general movement of the upper air is from west to east. Low and high pressure areas move across north-central North America embedded in the westerlies stream. Their movement is also associated with an interaction between southward flowing cold air and northward flowing warm air. In winter, cold, high pressure areas dominate the interior of the continent and relatively warm, low pressure areas dominated the coastal areas. In summer the pattern is reversed with large semi-permanent high pressure area over the northeastern Pacific dominating the general circulation in western Canada. These circulating patterns create 14 distinct regional climates.

The rugged relief of the western cordillera has a great effect on the climate of western Canada, northern Washington, and the panhandle of Alaska. The Coast Mountains limit the mild, humid Pacific air to a narrow band along the coast. As the prevailing eastward-moving air is forced to rise over successive mountain ranges, precipitation occurs on windward slopes. The Rocky Mountains commonly block westward-moving outbreaks of cold Arctic air. Southward-moving Arctic air from the Yukon and northern British Columbia is impeded by the Coast Mountains and so flows into the interior of the province. During warm months, hot, dry air from the Great Basin of the United States occasionally moves into the southern interior plateau area from the southeast, bringing clear skies and hot temperatures.

Most of British Columbia is comprised of a series of land masses (terranes) that have collided with western North America during the past 190 million years (Gabrielse et al. 1991; Yorath 1990). The two major accretion events that have occurred have caused uplift and distortion of the original continental margin. These events are still happening as the last portion of the ocean plate disappears under the continent. By using these geological events, the province can be subdivided into several major physiographic units. The oldest portion of the North American continent in the province, the northeastern plain, is a relatively flat plateau - the remnant of a great inland sea. West and southward, the former continental margin has been uplifted and distorted to form the Rocky and eastern Columbia mountains. In the centre of the province, the interior mountains and plateaus are composed of four large terranes and several smaller ones that form a superterrane that docked against the continent distorting the continental margin into the Rocky and Columbia mountains. The coastal mountains, islands, and continental shelves are composed of two large terranes and several smaller ones that docked as a second super terrane against the first. Intensive heating of basement rocks has resulted in abelt of extensive granitic intrusion. The western-most portion of British Columbia is a deep-water, oceanicsea - the continental rise. That physiographic area consists of gentle slopes overlain by an apron of thick sediments.

Within the past two million years, five successive periods of continental glaciation, followed by warm periods, have occurred over British Columbia. The result has been a reshaping of the landforms and deposition of surface materials. Since the waning of the continental ice sheets 12,000 to 15,000 years ago, there have been several climatic fluctuations in the province (Clague 1981, 1989; Fulton 1989; Pielou 1991). As recently as a few hundred years ago, there was a short period of cordilleran ice build-up; however, the current climatic trend in British Columbia as we enter an era of global warming caused by the build up of "green house" gasses possibly is for warmer and widely fluctuating climates.

The vegetation of British Columbia reflects the climatic and physiographic differences both provincially and regionally. Marked vegetation belts are a striking feature of regional vegetation (Meidinger and Pojar 1991). On the coast, the natural vegetation is needle-leaf forests of Douglas-fir, western redcedar, western hemlock, Sitka spruce, amabilis fir and yellow-cedar. These are some of the densest of all coniferous forests and hold some of the world's largest trees. On the upper mountain slopes lies a narrow subalpine belt of mountain hemlock and amabilis fir forests. Rugged, moist alpine is common at the higher elevations and relict glaciers dominate much of the high Pacific and Boundary ranges. In the Southern Interior, steppe vegetation dominates the major valleys and basins. Sagebrush, ponderosa pine, or Douglas-fir are common throughout. However, an increase in elevation prolongs the winter season enabling Douglas-fir, lodgepole pine, Engelmann spruce, and subalpine fir forests to become established. In the central interior and southeastern mountains at the lower levels is a montane belt of Douglas-fir and lodgepole pine forests, whereas western hemlock and western red cedar are characteristic where moisture is greatly increased. Above is the subalpine belt, dominated in most places by Engelmann spruce and subalpine fir or lodgepole pine forests. The uppermost vegetation belt is the alpine; quite often relict glaciers are present on the highest portions. In the Sub-Boreal Interior, the Sub-Boreal Pine-Spruce zone has forests of lodgepole pine with some white spruce, which are characteristic in the southern portion. In the northern portions, the Sub- Boreal Spruce zone forests of lodgepole pine, hybrid spruce, and subalpine fir are common. In the cold northern mountains, vertical vegetation is characterized by muskeg and black spruce in the low lying, poorly drained areas, or willow-birch shrublands in the low valleys where cold air frequently pools. White spruce, lodgepole pine, subalpine fir, and occasionally trembling aspen occur on the midslopes. Alpine grasslands dominate most rounded summits while barren rock or mat-vegetation occur on the highest summits. On the Alberta Plateau, white and black spruce and lodgepole pine forests are dominant. Some poorly drained areas have muskeg with black spruce and tamarack, the lower basins and riverbreak areas have shrub rich grasslands intermixed with aspen. In the Fort Nelson Lowlands, extensive wetlands and muskeg occur over much of the area because it is so poorly drained, although the better drained upland areas have well developed boreal forests.

British Columbia's oceanic environment is dominated by the Subarctic Current which has moved eastward across the breadth of the northern Pacific Ocean. In the Subarctic Pacific Region that current divides into a counterclockwise flowing Alaska Current and a clockwise flowing California Current. Those currents meet the continental shelf currents and the freshwater discharges of British Columbia's rivers to form estuarine conditions for most of the coastal areas. Inland many fjords and glacial scoured inter-island channels dominate the marine environment (Thomson 1981)."British Columbia's oceanic environment is dominated by the Subarctic Current which has moved eastward across the breadth of the northern Pacific Ocean. In the Subarctic Pacific Region that current divides into a counterclockwise flowing Alaska Current and a clockwise flowing California Current. Those currents meet the continental shelf currents and the freshwater discharges of British Columbia's rivers to form estuarine conditions for most of the coastal areas. Inland many fjords and glacial scoured inter-island channels dominate the marine environment" (Thomson 1981).

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