Ministry of Environment

Soils

1.4 Vegetation

R.K. Jones and R. Annas

Introduction

There is a wide range of vegetation in British Columbia. Along the humid west coast, mid- to high-latitude coniferous "rain forests" occur, whereas many dry interior valleys, in the lee of mountain ranges, have open woodlands and steppe (grassland) communities. In the north, with its cold continental climate, are high latitude northern boreal forests, characteristic of Continental North America and Euro-Siberia. Small areas in south- western British Columbia in the rainshadows of the Olympic Peninsula and Vancouver Island, have a mild, mediterranean climate which supports open grassy deciduous forests of Garry oak. The mountains provide environments that range from relatively favourable for plant growth at low elevations to most severe and hostile at high elevations. As implied by these few examples the vegetation pattern varies vertically as well as horizontally across the province. Some specific examples of such variations are shown in Figure 1.4.1 and the province-wide pattern of vegetation is shown in Figure 1.4.2.

Although vegetation changes spatially in response to differing environmental and historical conditions, it is possible to discern areas of vegetation which are relatively homogeneous in species composition and physical structure. Such homogeneous areas, which are called plant communities, can be classified and mapped in a fashion similar to soils. On a macro-scale, distribution patterns of plant communities can also be distinguished. It is useful to discuss these vegetation patterns within the framework of some classification system.

Several classification schemes have been developed for the vegetation of British Columbia, each one suiting a particular purpose. This section will use the Biogeoclimatic Zone classification as developed by Dr. V.J. Krajina and his colleagues and students since the early 1950's in the Department of Botany, University of British Columbia.

The Concept of Biogeoclimatic Zonation

A biogeoclimatic zone is a geographic area having characteristic vegetation with associated animals, soils and climate. Each forested zone is under a broadly homogeneous macro-climate and is characterized and usually named by one or more of the dominant shade-tolerant tree species which are often capable of self regeneration on most of the habitats. Generally, each zone can be broken into subzones which are distinguished by the climax plant association (groupings of several similar plant communities) on a mesic site. This site is the mid-point of the nutrient and moisture range occurring within the subzone; it is therefore not subject to excessive drainage nor to seepage waters and hence the vegetation best reflects the macro-climate.

Each subzone will have a series of communities developed in habitats which are strongly influenced by factors other than macro-climate. These factors may be the result of different soils, drainage, moisture regimes, slope, aspect, and micro-climate to name a few. Thus a subzone may include sites from nutrient-poor ones on sand dunes, to nutrient-rich seepage sites on alluvial floodplains. Additionally, plant communities will often be kept from their climatic or other types of climaxes (edaphic) by such factors as fire, logging, grazing, and flooding. All these factors result in a range of vegetation for subzones and zones. The following discussion is, however, limited to the broader characteristics of the twelve biogeoclimatic zones as proposed by Krajina (Figure 1.4.2).

The Biogeoclimatic Zones of British Columbia

The Alpine Tundra Zone (AT)

The AT zone consists of treeless meadows, slopes, windswept ridges, snowfields, and icefields at high elevations in mountainous terrain. This zone starts at an elevation of 2250 m in southeastern British Columbia but in the cool, moist climate of the northwest, it occurs as low as 900 m. The harsh climate in most alpine environments is a response to increasing elevation or increasing latitude. Thus, alpine and arctic vegetation are very similar.

The alpine is a cold, windy, snowy environment with a very short frost-free period. Many alpine species show specific adaptations for these conditions. Abrasion from wind-blown snow particles damages vegetation by breaking leaf or stem surfaces which compounds the desiccating effect of wind. These are two of the main factors which limit the distribution of trees at treeline. Occasional straggling, prostrate trees, of krummholz form, can be found in relatively protected areas high above treeline.

Adaptations for abrasion and desiccation vary from dwarf forms of woody plants such as the arctic and alpine willows, which are only a few centimetres high, to the development of cushion or mat-like forms of growth which protect most of the plant colony from the wind. Many species on wind-swept slopes and ridges such as moss campion (Silene acaulis) and entire-leaved white mountain-avens (Dryas integrifolia) grow in cushion or mat form.

Alpine "meadows" occur under moister conditions on flat or gently sloping topography where snow cover lasts the longest. Their vegetative season is short and colourful. A few conspicuous alpine meadow species are Indian paintbrush (Castilleja spp.), lupines (Lupinus spp.), Sitka valerian (Valeriana sitchensis), anemones (Anemone spp.), and arnicas (Arnica spp.). Meadows, especially where dense grass and sedge communities are present, give a very turfy top to the soils.
Figure 1.4.1

Alpine heath communities, composed of "mountain heathers" (Phyllodoce spp. and Cassiope spp.) and other members of the heath family such as alpine-azalea (Loiseleuria procumbens) and crowberry (Empetrum nigrum) are very common and sometimes occupy extensive areas.

Under extreme conditions at the highest elevations, only lichens and a few mosses can survive. In certain lower elevation environments of the zone, vascular plants may have difficulty becoming established due to the frequent churning of soils by solifluction.

Mountain Hemlock Zone (MH)

The MH zone occurs along the Pacific Coast Mountains in a climate of cool, short summers and cool, long, wet winters, with snow cover for several months. At low elevations, up to 900-1100 m in south coastal regions and up to 300 m further north, the forests are characteristically dense and productive. Tree growth becomes progressively poorer with elevation due to the shorter growing season, increased duration of snow cover, and cooler temperatures. The subalpine forests become more open with increasing elevation resulting in discontinuous parkland forests adjacent to the AT zone.

In the lower subzone of the MH zone, mesic habitats are on benches and upper slopes which receive temporary seepage during the early summer. The generally high moisture content and low temperature of the soils result in slow decomposition of litter and a relatively high proportion of organic matter in most soils. Mountain hemlock (Tsuga mertensiana), the most common tree, is particularly well adapted for such conditions, since its roots are shallow and occur principally in organic soil horizons. Pacific silver fir (amabilis fir) (Abies amabilis) and yellow cedar (Chamaecyparis nootkatensis) are the other major trees of the zone, with the yellow cedar generally confined to habitats with abundant moisture.

Dominant shrubs on mesic sites are the blueberries (Vaccinium alaskaense and V. membranaceum) and Pacific menziesia (false azalea) (Menziesia ferruginea). The herb layer is usually not well developed with only five leaved creeping raspberry (Rubus pedatus) and blue-bead Clintonia (Queen's cup) (Clintonia uniflora) being common. Dicranum spp. and Rhytidiopsis robusta dominate the usually well developed moss layer of mesic sites. On lower and middle slope positions, seepage water is rather plentiful as expressed by a robust understory of small twisted-stalk (Streptopus roseus), deer fern (Blechnum spicant), false hellebore (Veratrum eschscholtzii) and unifoliate-leaved foamflower (mitrewort) (Tiarella unifoliata).

In the parkland subzone, a mosaic of plant communities occurs which grades into the alpine. In basins and depressions, where snow cover remains the longest, a dominant sedge community of Carex nigricans occurs on soils with characteristically turfy surface horizons. Heath communities, typical of the alpine, are also extensive.

The Subalpine Engelmann Spruce - Subalpine Fir Zone (ESSF)

The ESSF zone lies between the alpine tundra and the lowland forests throughout much of the interior of British Columbia. It is the most widely distributed zone in the province, occurring between latitudes 49 to 57 N and at elevations ranging from 950-1550 m in the northwest to 1260-2250 m in the southeast. The climate is colder, drier, and more continental than that of the MH zone. Trees occurring in the ESSF zone must be able to tolerate relatively severe winters with frozen ground.

Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) which is also known as alpine fir, are characteristically the dominant climax trees throughout the entire zone. Seral species, such as lodgepole pine (Pinus contorta var. latifolia), are common over large areas after fires, especially in drier regions.

Northern regions of the ESSF zone have boreal and sub-boreal influences; eastern regions have Cordilleran qualities, western portions have coastal transitions, and southern localities have dry continental variants. Not unlike the MH zone, the ESSF zone has elevational gradients in forest cover distribution whereby there are closed forest stands at lower elevations and tree "islands" or parklands in the upper limits of the zone. Unlike the MH zone, it is believed that the clumped tree distribution in the interior subalpine parkland, at least in the drier areas results from heavy snow accumulation which provides favorable moisture for tree growth. Areas between the tree clumps are open and exposed resulting in dry grassland communities.

In the lower elevations of the wetter subzones of the ESSF zone, forests have closed canopies. Even though precipitation is sufficiently high for Podzolic soils (see Part 2.4), productive seepage sites are very frequent.

Figure 1.4.2

These have plant species typical of soils containing high moisture and nutrients (e.g. mountain alder (Alnus incana), oak fern (Gymnocarium dryopteris), Sitka valerian and false hellebore).

In the drier, less productive parts of the zone where Brunisolic soils are common, many species characteristic of dry conditions occur frequently (e.g. kinnikinnick (Arctostaphylos uva-ursi), pine grass (Calamagrostis rubescens), and showy aster (Aster conspicuus)).

The Spruce - Willow - Birch Zone (SWB)

This is the third and most northerly subalpine zone in the province. It extends from a latitude of roughly 57º N and stretches well into the Yukon Territory and Mackenzie District of the Northwest Territories. Subalpine and subarctic climates (at higher latitudes) are characterized by severe winters. Moderate snow cover occurs over frozen ground in the southern part of this zone and over ground with permafrost in the northern part. The elevational ranges of the zone in the southeast occur between 1100 and 1700 m and in the northwest between 900 and 1500 m.

The plant communities found in this zone have yet to be studied in British Columbia. Detailed studies in the Yukon region (Ogilvie Mountains) commonly found white spruce (Picea glauca) on rich, well drained soils and black spruce (Picea mariana) on poor, imperfectly drained soils in the lower elevations of the zone. Persistent frosts, common at elevations above 1200 m, strongly affect the continuance of the coniferous trees. In their place, deciduous willow and birch shrubs predominate. The willows, mainly Salix barclayi, S. planifolia, S. pulchra, and S. glauca, prefer habitats with favourable nutrient regimes; habitats with poor drainage and less nutrients are characterized by bog glandular birch (Betula glandulosa). In some valleys subject to cold air drainage from adjacent mountains, the shrub willows and birches will occupy the valley floor as well as the upper elevations of the zone. The spruce forests are therefore bounded both above and below by a shrub subzone.

The Boreal White and Black Spruce Zone (BWBS)

The largest occurrence of the boreal forest occurs on an extension of the Great Plains into northeastern British Columbia. It occurs also at the lower elevations of the main valleys west of the Rocky Mountains. The northern continental climate, with its frequent arctic air masses, produces short growing seasons and long cold winters. Discontinuous permafrost is common in northern parts of this zone. This area is situated approximately north of latitude 54º N and at elevations between 165 and 850 m. Forest fires are frequent throughout the zone, maintaining nearly all the forests in an early successional stage.

White spruce, black spruce, lodgepole pine, tamarack (Larix laricina), balsam poplar (Populus balsamifera), common paper birch (Betula papyrifera), and Alaska paper birch (Betula neoalaskana) are the major tree species in the forested regions of the zone. Grassland communities occur along the rivers in the southern Peace River area, and on the steep slopes above the Stikine River in the lee of the coastal mountains. Other than these drier grasslands, the remaining boreal forest is a mosaic of forest and bog communities.

In the gentle topography of the Fort Nelson Lowland of the Alberta Plateau, productive forests of trembling aspen (Populus tremuloides) and to a lesser extent white spruce characterize the mesic habitats as a "fire climax". It is believed that biogenic cycling of calcium by trembling aspen helps to maintain the Luvisolic soils of the area in a relatively high productive state by retarding acid leaching of the soils. Black spruce, a theoretical climatic climax species, produces acid litter which has the opposite effect.

Relatively open pine-lichen forests occur on the driest sites which are generally situated on rapidly drained outwash deposits, but dense black spruce-moss communities develop on imperfectly drained habitats. The most productive forests of this zone occur on rich alluvial sites where white spruce and balsam poplar may reach heights of over 50 m. These sites are among the most productive forests in the Canadian boreal forest.

Northeastern bog communities develop on poorly drained organic soils, characteristic of the gentle topography of the Alberta Plateau. The buildup of organic matter from the bog vegetation tends to insulate ground frozen in the winter, which results in discontinuous permafrost being characteristic of the northerly areas of the zone. Sphagnum fuscum is the dominant peat moss in this terrain.

Black spruce and occasionally tamarack are the main trees occurring on organic terrain, however, they are much reduced in size. Tamarack forms pure stands only in nutritionally rich fen types of environments.

The Sub-Boreal Spruce Zone (SBS)

The SBS zone is located roughly in the centre of British Columbia where the climate is slightly less continental than that of the BWBS zone. The winters are shorter in the SBS zone, giving a slightly longer growing season. The elevational range of the zone is between 330 and 950 m in the northwest and up to 1100 m in the northeast.

Upland coniferous forests, composed mainly of white spruce and lodgepole pine, as well as deciduous forests of trembling aspen, dominate the landscape. Although white spruce is the dominant mesic climax species, fire and other disturbances result in a variety of vegetation in circum-mesic sites, including overstocked stands of lodgepole pine or stands of trembling aspen. Subalpine fir (alpine fir) is common in the understory, particularly in the shrub layer.

Lodgepole pine-lichen sites are common on dry, well drained outwash materials and these sometimes cover extensive areas. Subalpine fir and white spruce grow best on seepage and alluvial sites. Black spruce bogs, although fairly common, are never as extensive as they are in the boreal forest.

The Cariboo Aspen - Lodgepole Pine Zone (CALP)

The CALP zone contains a wide variety of habitats from treeless grasslands along the Fraser River to fairly dense forests in upland areas of the Fraser Plateau. With a climate of cold winters and warm dry summers, the CALP zone occurs at elevations between 510 and 1070 m.

The driest habitats of the zone occur as belts along the Fraser and Chilcotin Rivers where grasses, with their fibrous root system, are able to compete successfully with trees for moisture, resulting in extensive grasslands along these rivers. Adjacent to the grasslands, open forests are frequent, mainly of Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca). The remainder of the zone is characterized by more closed forests of Douglas-fir, white spruce, lodgepole pine, and trembling aspen.

The open range, on the slopes along the Fraser and Chilcotin Rivers, has bluebunch wheat grass (Agropyron spicatum) as the dominant climax grass species. In many areas, bluebunch wheat grass is reduced through overgrazing and is replaced by less desirable species. At the lower elevations big basin sagebrush (Artemisia tridentata) is especially favoured by overgrazing. The addition of organic matter to the upper soil horizons, through the decay of grass roots, is a major factor in the development of the Chernozemic soils characteristic of these grasslands.

Much of the zone, where Douglas-fir is the climax species, has a well developed herb layer dominated by pine grass which is a major forage species for grazing. Although Douglas-fir is a climax species in the southern part of the CALP zone, extensive areas are dominated by stands of lodgepole pine and trembling aspen which have originated from fire. In the northern part of the zone white spruce becomes the climatic climax species, indicating characteristics very similar to the SBS zone.

The Interior Western Hemlock Zone (IWH)

This zone occurs mostly in southern British Columbia between the Monashee and Rocky Mountains at elevations between 360 and 1260 m. It is often referred to as the "interior wet belt" since it experiences the highest precipitation of all the interior regions. Its climate makes it the most productive forest zone in the interior. Western hemlock (Tsuga heterophylla), with lesser amounts of western red cedar (Thuja plicata), dominate the few remaining mature forests.

The most common forest association in the IWH zone is a western hemlock-moss association, this also being the mesic or climatic climax association for the zone. In the southern and drier part of the zone, Douglas-fir and, to a lesser extent, western white pine (Pinus monticola) are frequently dominant after fires. On sites subject to permanent seepage waters, western red cedar-western hemlock stands are very common, sometimes occupying large areas. Where the seepage water is close to the surface, cedar-hemlock-devil's club communities are present. Generally Podzolic soils dominate the landscape of the zone. In many respects, the IWH zone is similar to the Coastal Western Hemlock (CWH) zone.

The Interior Douglas-Fir Zone (IDF)

The IDF zone occurs in the southern third of the province in the rainshadow of the Coast, Cascade, and Columbia Mountains. Typically, this zone occupies the middle position in the vertical sequence of zones common to dry interior valleys - the Ponderosa pine-Bunchgrass (PPBG) zone occurring at lower and the ESSF zone at higher elevations. The IDF zone lies between 300 and 1350 m in the southeast and 450 and 900 m in the north.

Both open and closed forests characterize the zone. Ponderosa pine (Pinus ponderosa), a seral species, and Douglas-fir, a climax species, form open forests at lower elevations. Here bluebunch wheat grass is one of the most important range species in the steppe-like understory vegetation. Many species characteristic of the lower grassland and arid shrub communities of the PPBG zone occur in these open forests. Grazing activity and fire history tend to determine the composition and pattern of present day vegetation. In general, it is believed that grassland communities are promoted by fire and shrub communities by intensive grazing.

At higher elevations of the zone, the climate is moister and forests become more closed. Pine grass becomes a dominant understory species, replacing bluebunch wheat grass. Closed forest species such as Oregon boxwood (Paxistima myrsinites) and common pipsissewa (Chimaphila umbellata) are more frequent in the wetter parts of the zone with the abundance of pine grass being reduced. Western larch (Larix occidentalis), western red cedar, white spruce, Engelmann spruce, and lodgepole pine are other species frequently found in the zone. Brunisols and Luvisols comprise the major soils of the zone.

The Ponderosa Pine - Bunchgrass Zone (PPBG)

This is the driest and warmest zone in British Columbia. (In certain locations it is semiarid.) Located in the southern third of the province, the PPBG zone is distributed along valley floors, the lower slopes of many of the dry interior valleys and in local areas scattered across the interior plateau. The elevational range of the zone varies with aspect but generally occurs between 270 and 750 m. It occupies the lowest position in the vertical sequence of zones common to dry interior valleys (see Figure 1.4.1).

The PPBG zone is believed to be the northernmost extension of the Palouse Prairie found in the intermontane regions of the northwestern United States. Like the IDF zone, the influence of fire and grazing is expressed in the present day vegetation patterns. The vegetative cover consists of a mosaic of treeless bunchgrass steppe and parkland forest.

The high frequency of ground fire in the dry subzone helps to maintain a grassland vegetation including bluebunch wheat grass, needle-and-thread grass (Stipa comata), sand dropseed (Sporobolus cryptandrus) and rough fescue (Festuca scabrella). Most of these species provide good forage for cattle and other livestock. Overgrazing frequently causes a shift from productive grassland communities to communities dominated by sagebrush and other less palatable weed species such as common rabbit brush (Chrysothamnus nauseosus), low pussytoes (Antennaria dimorpha), and brittle prickly-pear cactus (Opuntia fragilis).

The ponderosa pine parkland, the wet subzone, is a discontinuous belt of open forest interspersed with the open grasslands. Ponderosa pine and, to a lesser extent, Douglas-fir are the dominant trees in the subzone. For the most part plant species characteristic of the open grassland form the dominant ground cover.

Alluvial communities along streamsides and on floodplains throughout the PPBG zone are characterized by black cottonwood (Populus balsamifera spp. trichocarpa) and various willows.

The Coastal Douglas-Fir Zone (CDF)

The CDF zone is situated along the south coastal region in the rainshadow of the Olympic Peninsula and Vancouver Island mountains. It ranges in elevation from sea level to 450 m on the east coast of Vancouver Island and from sea level to 150 m on the mainland and Gulf Islands. The zone has a relatively humid, mediterranean climate with warm, dry summers, and moderate, wet winters. Coast Douglas-fir (Pseudotsuga menziesii var. menziesii) is the main climatic climax species on mesic sites.

Under the most extreme coastal rainshadow effects, a dry subzone occurs where Garry oak (Quercus garryana) communities are common on rich sites with Pacific madrone (Arbutus menziesii) communities being common on poor, rocky, dry sites. The open and colourful grasslands of the Garry oak subzone have interesting soils as a result of the climate and vegetation.

Most of the CDF zone belongs to the wet subzone where Douglas-fir is the most common species, especially on mesic sites. However, many other coniferous species are present. On mesic sites, salal (Gaultheria shallon) dominates the shrub layer. The most characteristic soils of mesic sites of the zone are Dystric Brunisols and Humo-Ferric Podzols.

The Coastal Western Hemlock Zone (CWH)

The CWH zone is the wettest and most productive forest zone in British Columbia. The humid conditions prevalent in this coastal zone are primarily the result of the continual eastward movement of Pacific air masses. Depending upon the occurrence of a coastal rainshadow, the CWH zone can occupy either the middle or lowest position in the vertical sequence of zones common to coastal British Columbia. The MH zone occurs at upper elevations and the CDF (if it is present) at lower elevations. The zone ranges up to 900 m on windward slopes in the south, up to 1050 m on leeward slopes in the south, and up to 300 m in the north coastal region.

The most characteristic plant associations are hemlock-moss communities. In the drier subzone, coast Douglas-fir is a common successional tree species to the climax western hemlock. In the wetter subzone, Pacific silver fir (amabilis fir) is very common on mesic sites along with the dominant western hemlock.

Plant communities within the CWH zone vary with local topography and slope position. Rock outcrops and areas with well drained, shallow soils, particularly those with southerly aspects, commonly support Douglas-fir, salal, and a carpet of mosses and lichens. Depressional areas and lower slopes, where there is permanent seepage water, maintain a mixed forest of western hemlock and western red cedar with a vigorous understory of ferns.

One of the characteristics of the ecosystems of this zone is the buildup of organic matter in the soils, especially in the wetter subzone. In north coastal regions and on the Queen Charlotte Islands, there is a net accumulation of organic matter resulting from the dampness of the soils and the cool temperatures. This often leads to the development of organic soils and ultimately bogs, even on mesic sites. These bog ecosystems are characterized by poorly growing western hemlock, lodgepole pine, red cedar, and yellow cedar.

Further Reading

  • Biel, Charles E., Roy L. Taylor, and Geraldine A. Guppy, 1976. The Biogeoclimatic Zones of British Columbia. Davidsonii, 7 (4), 45-55.

    An account of the biogeoclimatic classification of V.J. Krajina and a short description of each zone.

  • Krajina, V.J., 1969. Ecology of Forest Trees in British Columbia. Ecology of Western North America, 2 (1), 1-146.

    The biogeoclimatic classification with technical descriptions and definitions of each zone and subzone.

  • Rowe, J.S., 1959. Forest Regions of Canada. Dept. Northern Affairs and National Resources, Forestry Branch, Ottawa. Bull. 123, 71 pp.

    A descriptive classification of forests across the whole of Canada.

  • Taylor, Roy L. and Bruce MacBryde, 1977. Vascular Plants of British Columbia. Botanical Garden, University of British Columbia, Tech. Bull. 4, University of British Columbia Press, Vancouver, 754 pp.

    An inventory which has been followed for plant names in this publication.