Aspen parkland refers to a very large area of transitional biome between prairie and boreal forest in two sections, namely the Peace River Country of northwestern Alberta crossing the border into British Columbia, and a much larger area stretching from central Alberta, all across central Saskatchewan to south central Manitoba and continuing into small parts of the US states of Minnesota and North Dakota. Aspen parkland consists of groves of aspen, poplar and spruce, interspersed with areas of prairie grasslands, also intersected by large stream and river valleys lined with aspen-spruce forests and dense shrubbery. This is the largest boreal-grassland transition zone in the world and is a zone of constant competition and tension as prairie and woodlands struggle to overtake each other within the parkland.
This article focuses on this biome in North America. Similar biomes also exist in Russia north of the steppes (forest steppe).
Definitions According to the Ecological Framework of Canada that was published in 1999, the Aspen Parkland ecoregion (#156) is the largest and northernmost section of Prairies Ecozone. This definition is the arc-shaped region (i.e. including the WWF's central and foothills parkland but excluding the Peace River region).
Partly defined by climate, it had a mean annual temperature of approximately 1.5 °C circa 1999, and rainfall varied from 400 to 500 mm/annum. It includes the communities of Red Deer and Edmonton in Alberta; Lloydminster on the Alberta–Saskatchewan border; North Battleford, Saskatoon, Humboldt, and Yorkton in Saskatchewan; and Brandon, Manitoba as its major population centres and have a total population of 1.689 million. By this definition, there are approximately of this ecoregion in the province of Alberta.
According to the World Wide Fund for Nature the Canadian Aspen forests and parklands (NA0802) encompass eight ecoregions as used in the Ecological Framework of Canada. They include the Peace Lowland, Western Boreal, Boreal Transition, Interlake Plain, Aspen Parkland, and Southwest Manitoba Uplands (TEC 138, 143, 149, 155, 156, 161, 163, and 164). These ecoregions lie in both the Boreal Plains Ecozone and the Prairies Ecozone (Ecological Stratification Working Group 1995). The Boreal sections are Manitoba Lowlands, Aspen-Oak, Aspen Grove, Mixedwood, and Lower Foothills (15-17, 18a and 19a).
Setting The aspen parkland biome runs in a thin band no wider than 500 km through the Prairie Provinces, although it gets broader to the west, especially in Alberta. This is a hilly landscape with many small lakes and ponds. The cities of Edmonton and Saskatoon are the largest cities completely in this biome while Winnipeg is bordered by tallgrass prairie to the west and south and the aspen parkland to the northeast, and Calgary is bordered by prairie to the east and the Foothills Parkland to the west.
There are three main sections of aspen parkland: Peace River, Central, and Foothills. The Central Parkland is the largest section and is part of main band of aspen parkland extending across Alberta, Saskatchewan and Manitoba, bordered by prairie to the south and the boreal forest to the north. The Peace River Country is located along the Peace River region of the province, extending across the border into northeastern British Columbia, and is completely surrounded by boreal forest, cutting it off from the Central Parkland, it extends as far north as Fort Vermilion at 58°N 116°W. The Foothills parkland covers the Foothills of the Rocky Mountains as far south as Waterton Lakes National Park.
Climate The region has a humid continental climate accompanied by a subhumid low boreal transitional grassland ecoclimate. Summers are warm and short and winters can be long and cold. The mean annual temperatures range from , with summers ranging , and winters ranging . The Peace River Country in northwestern Alberta and northeastern areas of the North Interior in British Columbia has the coolest climate, but still supports extensive farmland. Southwest Manitoba sees the warmest. Annual precipitation is usually between . Chinook winds off the foothills also occur in winter, mainly affecting Alberta.
Flora
Four significantly different habitats are common in the aspen parklands: The fescue prairie, the woodlands, the ravines and the wetlands and lakes. A rarer habitat type, tallgrass aspen parkland, occurs only in the extreme southeastern corner of the aspen parklands biome (southeastern Manitoba/northwestern Minnesota). The fescue prairie is a meadowland rich in vegetation variety which forms the cover for the development of the richer soils that underlie the parklands. The close association with woodlands and wetlands makes this a choice location for many plants and a preferred range or home site for a wide diversity of wildlife. The richer soil and increased precipitation favours the natural growth of fescue grass, but varying conditions such as moisture level and grazing pressures allow for the invasion of secondary plant species.
There are numerous grasses and sedges in the fescue prairie. Gravelly and rocky terrain is a good location for parry oat grass. Dry areas favour June, porcupine and spear grass. Wet areas are often covered with slender wheat grass and timber oat grass. Prairie rose and snowberry are common shrubs found in these grasslands. Woody plant encroachment is widespread in the parklands.
The forested, or woodlands area is dominated by trembling aspen (Populus tremuloides), balsam poplar (Populus balsamifera), other poplars and spruces, although other species of trees including paper birch (Betula papyrifera) do occur. Pines, mostly jack pine and lodgepole pine will often grow in areas that have sandy soil conditions. Other native species may include box elder, tamarack and willow, while the foothills area in the southeast of the region, such as Turtle Mountain or Spruce Woods Provincial Park, have woodland of white spruce and balsam fir but quaking aspen will dominate where the woodland has been cleared by fire. The proportion of forests to grasslands has increased somewhat over the prairie in areas not affected by agriculture in the last 100 years. This increase is partly due to the reduction of prairie fires which used to destroy the new saplings on the fringes of the aspen groves. Also, it was a common practice for farmers to plant stands of trees as windbreaks.
Aspen woodlands support an extensive understory consisting of mid-sized and small shrubs, some herbs and ground cover. Spruce-dominated woodlands usually do not support a dense understory due to more acidic and nutrient-poor soils and a denser canopy, which reduces sunlight reaching the forest floor below. However, in areas where a mixture of aspen and spruce occur, a fairly dense understory can still thrive. The mixed wood understory, as it is called, supports the greatest diversity of forest wildlife in the aspen parkland.
Large shrubs such as red-osier dogwood, beaked willow, saskatoon, chokecherry and pincherry, along with the smaller shrubs including prickly rose, snowberry, beaked hazelnut and high bush cranberry, form a dense entangled understory. Dense shrubbery is a typical feature in aspen-dominated forests. Common herbs found in the woodlands include: Lindley's aster (Aster ciliolatus), northern bedstraw (Galium boreale), pea vine, Western Canada violet (Viola canadensis), dewberry and bunchberry. Mosses appear at the base of trees and on the ground.
Wetlands are very common in this biome, including lakes, shallow open water, marshes, and grassy wetlands. Glacial erosion has contributed to such features by creating depressions in which standing water can collect. In the larger depressions, permanent lakes or ponds of water remain. Many of the lakes have a saline character, thus most shore vegetation has a high tolerance of salty soils. These lakes are known as alkali lakes. Wet meadows are flooded in the spring and dry by fall. They contain rushes, sedges and grasses and provide excellent opportunities to study the similarities and differences of these forms of vegetation.
Rivers and streams erode valleys throughout the parkland ecoregion. Steep hills and ravines result in a unique topography. Southwest slopes with increased exposure to the sun are dry and often more grass covered, while the shaded north and east exposures retain more moisture and tend to have greater forest cover. Some forms of vegetation unique to the ravines include: poplar, spruce, birch, willow, and river alder.
Wildflowers are an important component of the grassland association of the parkland. Look for common yarrow, cut-leaf anemone, rock cress, creeping white prairie aster, milk vetch, late yellow loco weed, goldenrod, wild prairie rose, prairie crocus, and tiger lily.
Aspen understory There are three main factors which influence the understory vegetation in the aspen stands of the mixed wood forest.
Good sun exposure encourages a dense vegetation growth below the canopy. This is of particular importance in the early spring before the trees are in leaf. Warm soil and air temperature at the base level result in rapid melting process in spring which favours the growth of shrubs. A large percentage of precipitation passes through the canopy. This provides a protective snow cover in winter and in warm seasons precipitation percolates through the leaf cover to nourish plants which require surface soil moisture.
The result of the above factors is an extensive understory of vegetation in the aspen forest. Common shrubs and herbs are: saskatoon, red-osier dogwood, raspberry, wild rose, currants and bracted honeysuckle, wild sarsaparilla, hairy lungwort, asters, and peavine. Twinflower, strawberries, bunchberries, horsetails and wintergreen form an attractive grown cover.
The mineral soil is covered by a decaying cover of organic matter. Numerous consumers and decomposers create humus materials. Burrowing animals mix the new fertile materials with the soil to form a rich rooting compound.
Spruce forest understory Factors which influence the understory vegetation of spruce stands in the boreal forest association include:
1. Year round reduced sun exposure below the canopy restricts the forest undergrowth to shade tolerant species.
2. A large percentage of the precipitation is trapped in the upper tree boughs of the spruce forest and is released through evaporation. The ground cover of feather moss quickly absorbs most of the moisture which does penetrate the canopy. These factors combine to cause drier conditions in the underlying mineral soils.
3. The fallen acidic spruce needles are not fully decomposed and combine with the moss base. Water held in the moss carries the acid from the spruce needles into the mineral soil and leaches out soil nutrients – leaving a highly acidic, low nutrient soil base which is unsuitable for most boreal vegetation.
As a result of the above factors the forest floor ranges from nearly devoid of vegetation to a dense carpet of feather moss. A sparse community of shade tolerant shrubs exists in this environment. Some species of plants in the understory are Green Alder, low bush cranberry, prickly rose, bunchberry, twinflower, wild lily-of-the-valley, northern Comandra and wintergreens.
Mixedwood stands understory
There are several factors which influence the mixedwood stands in the boreal forest.
Where there are stands of aspen and spruce forests in close association with each other, a mixed wood forest occurs. Each group forms its own microassociation as described previously.