Vegetation in British Columbia’s rangelands has a high level of diversity due to the wide variations in climate, soils and topography throughout the province. Biogeoclimatic zones, defined as the broadest vegetation complexes that reflect the same regional climate, are used for range classification and management. The zones are generally named after one or two dominant plant species that are present over a wide range of conditions. Biogeoclimatic zones can be further broken down into subzones, which have less climatic variability and narrower geographic distribution than zones. Biogeoclimatic phases are areas within a zone that contain non-zonal ecosystems. In the Southern Interior, the most important zones in terms of area and forage production potential for livestock and wildlife are Bunchgrass, Ponderosa Pine, Interior Douglas-fir and Montane Spruce.
The Bunchgrass Zone is located in the hot, dry valley bottoms of southern BC. It is found in the Okanagan Valley south of Summerland, the Similkameen River Valley to Keremeos, the Thompson River Valley from Spences Bridge to Pritchard, the Nicola Valley, and the Middle Fraser and Lower Chilcotin River valleys from the Farwell Canyon to Big Bar. In BC, this biogeoclimatic zone usually occurs below the Ponderosa Pine Zone, but it directly borders with the Interior Douglas-fir Zone in some areas. In BC, grasslands cover about 1.2 million hectares but only 300,000 hectares are classified as bunchgrass.
The climate of the Bunchgrass Zone is characterized by hot, dry summers and moderately cold winters often with little snowfall. Toward the north, the winters are colder, and in the more western parts of the zone, have the highest levels of precipitation occurring in May-June and December-January.
Bluebunch wheatgrass is the dominant species found throughout the zone, and well-developed lichen crusts are also common. Other common plants include prairie sagewort, rabbit-brush, big sagebrush, junegrass, Sandberg’s bluegrass, needle-and-thread grass and yarrow.
The Bunchgrass Zone is found in the Southern Interior Forest Region, with the majority occurring on private land. The zone is small and most of it is used for cultivated crops or non-agricultural purposes. The remaining grasslands are critical to the livestock industry in BC because they provide spring, fall and winter range. The zone also supports a high density and diversity of wildlife including reptiles, birds, small mammals, and large ungulates.
The Ponderosa Pine Zone is characterized by open, savannah-like stands of ponderosa pine with a well developed ground cover of grasses and forbs. This zone occupies 0.3% of the province and occurs above the Bunchgrass Zone and below the Douglas-fir Zone on the lower slopes and terraces of the driest, warmest valleys of the Southern Interior.
This zone is the warmest and driest forest zone in the province during the growing season.
Ponderosa pine is the dominant tree throughout the zone but Douglas-fir can also be found in the moist areas and gullies, as well as on drier sites in cooler northern areas. The zone most frequently exists as a mixture of open forest and grassland. Plants found under the pine canopy are similar to those found in the Bunchgrass zone. Few shrubs are present in the Ponderosa Pine Zone except for big sagebrush and rabbit-brush, and scattered plants of wild rose and Saskatoon. Shrubs such as antelope brush, Saskatoon, common choke cherry and rose contribute to the understory in the more southerly portions of the zone, especially in the East Kootenay region.
The Ponderosa Pine Zone is an important source of early spring and late-fall range for cattle in the Southern Interior Forest Region. In most years there is sufficient fall regrowth to provide supplemental forage.
The Ponderosa Pine Zone has relatively light snowfall and short winters, so it provides winter and spring forage for deer, big-horn sheep and Rocky Mountain Elk. The conifers also provide food and cover for many birds and small mammals. Rugged cliffs and talus slopes, which are common to the zone, can provide lambing grounds for bighorn sheep.
The Interior Douglas-fir Zone occupies most of the low- to mid-elevation terrain in the Southern Interior plateau, the Southern Rocky Mountain trench and portions of the lee side of the Coast Mountains. Topography varies from nearly level to rolling and steep slopes.
The zone occurs over a wide range of elevations from 350m in some valley bottoms to over 1450m where it often joins with the Montane Spruce Zone.
Cool winters, warm dry summers and a fairly long growing season characterize the climate of the Interior Douglas-fir Zone. Both moisture deficits and deep frosts during the growing season can limit plant growth.
The landscape of the Interior Douglas-fir Zone includes grasslands, savannahs with open canopies and grass understories, as well as closed forests with a ground cover consisting of a mix of shrubs, forbs and grasses.
Douglas-fir is the dominant tree species in the forested parts of this zone. Lodgepole pine commonly occurs as a seral species at higher elevations where it forms even-aged stands after disturbance such as fire. Trembling aspen also occurs as a common seral species, especially on deep, rich soils. Generally the understory is well developed in Douglas-fir forests but few plants other than Saskatoon, birch leaved spirea, pinegrass, and feathermoss occur constantly across all subzones. Common shrubs and forbs include rose, willow, timber milk-vetch, creamy peavine and lupine.
Three large grassland phases in the Interior Douglas-fir Zone are important forage resources for livestock and wildlife. These grasslands occur due to a combination of soil, topographic conditions and fire. Grassland plant communities in the Interior Douglas-fir Zone share common species with the lower elevation grasslands in the Bunchgrass and Ponderosa Pine zones. In comparison with the lower grasslands however, they are characterized by the absence of big sagebrush and by having more forbs, taller grass growth, and denser plant cover.
Bluebunch wheatgrass can be the sole dominant species on dry grassland sites throughout the zone, but with different moisture conditions it often co-dominates with rough fescue, Idaho fescue, or needlegrasses on grasslands at higher stages of plant succession.
Many of these grasslands are like those found at lower elevations and contain plant communities altered by past grazing practices. Species composition on these sites generally contain less bluebunch and rough fescue than on climax sites, and more Kentucky bluegrass and the smaller bunchgrasses such as needle-and-thread, junegrass, and Columbia needlegrass.
Idaho fescue co-dominates with bluebunch wheatgrass in the phase of the Very Dry Hot subzone which occurs in the Similkameen Valley, Okanagan Valley, Kettle Valley and southern Rocky Mountain Trench. Typical forbs include arrow-leaved balsamroot, silky lupine and parsniped flowered buckwheat.
Rough fescue replaces Idaho fescue in this phase which occurs on the Douglas Plateau between Nicola and Chapperon lakes, along Quilchena Creek and in the Kamloops area. Common forbs include arrow-leaved balsamroot, timber milkvetch and yarrow.
This is the wettest and smallest phase in the Interior Douglas-fir Zone and has a limited distribution. Kentucky bluegrass, Rough fescue and bluebunch wheatgrass co-dominate the plant communities, although their relative importance depends on effective moisture and range condition. Forbs such a balsamroot, timber milk-vetch, wild onion and low pussytoes are common.
The Interior Douglas-fir Zone is one of the most important zones for cattle and wildlife in BC. In the Kamloops area the Very Dry Hot, Dry Cool, and Moist Warm subzones provide important range for livestock grazing. The very warm subzone supplies valuable grassland range. The Dry Cool, and Moist Warm subzones provides both open range and extensive grazing in open forests and cut blocks in the Cariboo.
Photo:
Larry Halverson (Ponderosa Pine)Kristi Iverson (Blue bunch wheatgrass)
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