Ecology - Stop 3

Walking south along Timber Creek towards the distant outlying stands of small spruce trees of the Old Crow Flats would soon become an ordeal due to the hummocky nature of the ground. To understand these frustrating impediments to travel, it is necessary to look at the nature of the ground and the underlying cause of many of the landforms we see around us.

The presence of permanently frozen ground beneath the surface even in summer affects the terrain and the vegetation around us in several ways. Within permafrost, most of the soil moisture occurs in the form of ice. This ice exists in a variety of forms, ranging from tiny crystals and veins to large bodies of more or less pure ice meters or more in size. The permafrost forms a layer near the ground surface through which water cannot pass. Moisture from rain, melting snow, or thawing ground ice in the active layer, is held near the surface, keeping the ground moist or wet. In areas with very low annual rain or snowfall, this surface moisture supports far more vegetation growth than could otherwise survive.

Many of the distinctive landscape features in the northern parts of the Old Crow Basin are typical of arctic regions and result from the freeze-thaw action in the active layer as summer warmth or winter cold penetrates into the underlying permafrost. Landscape features range from frost boils less than 1 m in diameter to icewedge polygons 30 to 50 m across (more in Geomorphology). Tundra polygons are produced by successive annual additions to a network of ice wedges formed by moisture entering frost cracks in the soil. Polygons are usually separated by furrows, often about 50 cm deep, that develop above the ice wedges. These polygons and the surrounding frost cracks may provide miniature aquatic habitats if they become filled with water. The warmer "micro" environments allow invertebrates such as aquatic insects to live here, in turn providing food for other species.

The floor of the Old Crow Flats is underlain by lake- and river-deposited sediments. The wetland is sustained by the streams flowing in from the surrounding mountains and hills and the restricted drainage to the south. The wetlands consist of numerous shallow lakes of many sizes and are cut by meandering streams and rivers. The water table remains near or above the ground surface for prolonged periods, promoting the buildup of sedge and moss debris.

In the Old Crow Basin area, hummocks or tussocks are a wide-spread landform feature. Their structure is an interesting link between geomorphology and plant growth. Important tussock-forming species are cottongrass and water sedge. The surface of a cottongrass hummock is a mat of stems and leaves on top of a mass of tightly packed roots that penetrate a low mound of soil (Bird 1967). In spring the ground between the tussocks may be covered with standing water, but this soon dries out and birch and willow shrubs may grow in this space. The tussocks often contain ice crystals which last until late in the summer. Although the hummock form is partly the shape of the cottongrass, contorted soil in the core shows that soil movement is also involved in the forming of tussocks. The active layer under a tussock is deeper than beneath the depressions, and it has been suggested that there is both compression and vertical soil movement in the unfrozen ground with the onset of winter (Bird 1967). (A discussion of hummocks as a form of frost mound and their structure is found in "Geomorphology". More information on tussock vegetation is found in Vegetation.)

Of some importance in tussock formation is the ability of some plant species to take up nutrients in soils with reduced temperatures, low nutrient levels, shallow active layers, and low oxygen concentrations (Hettinger et al. 1973). Cottongrass is a dominant species where soils are poorly drained. Where there is a decreased soil volume, it can extend its roots into the freezing-melting zone of the permfrost surface. Cottongrass is efficient in sending nutrients back from the leaves into the rhizomes and roots, before the annual die-back of leaves, thus conserving nutrients (Hettinger et al. 1973). The cottongrass tussock is a self-sustaining "island" because of the ability of the plant to retain nutrients.

The role of humans in the ecosystems of the northern Yukon has been a varied one since the first humans occupied the area some 25,000 years ago. Evidence suggests that most inhabitants have had little impact on the environment of Vuntut National Park.

One of the most obvious traditional land uses in Vuntut National Park is the construction of caribou fences. The cutting of many hundreds of spruce trees and the use of dead trees and the building of the surround and the long wings of the fence have made a culturally important visual impact on the Park landscape (Figure 1.3). Unfortunately these constructions are quickly becoming less obvious through the natural processes of fire and decomposition.

Figure 1.3. Click figure to enlarge and see other photos.

As well as the caribou fences along the valley, there are older archaeological sites on the ridge above Timber Creek. The ridge is named Kyikavichik Ridge after Joe (Kay) Kyikavichik, a former chief at Old Crow who first led archaeologists to the site. Among many hundreds of artifacts of stone, archaeologists have found a very few artifacts of a style known as fluted points indicating an age of about 9,000 years ago (see "Archaeology"). Little is known of the people who lived here

As we move downstream along Timber creek past the heritage sites we see changes in the vegetation patterns. In general, the vegetation of the Old Crow Flats Ecoregion is a complex mixture of wetland, river, streams, and aquatic community types. Vegetation in upland sites within this Timber Creek ecodistrict consists of tussock tundra and shrub tundra communities with sheathed cottongrass, Labrador tea, shrub birch, and sphagnum moss as common plants (Wiken et al. 1981). Most rivers and streams are lined with white spruce, feltleaf willow, balsam poplar, and sometimes paper birch. Dense thickets of feltleaf and other willows characterize the more recently deposited terraces of meandering streams and rivers.

Although these willow thickets seem to be little more than an impediment to travel, they, like all other aspects of the environment, play a complex role in the cycle of life. For example, in the years following a crash in the population of snowshoe hares, when lynx must turn to other sources of prey, these dense willow thickets of the Old Crow Flats provide ambush spots for lynx hunting caribou calves (Russell pers. comm. 1999).

While heading south, the most notable change in vegetation is the gradual increase in the number and size of the trees as we approach the treeline. The approximate northern limit of trees (known as the treeline) crosses in an east-west line north of Old Crow Flats, roughly bisecting Vuntut National Park and the northern part of the Old Crow Basin Ecoregion (Wiken et al. 1981). To the north, sedge tussocks and low to medium shrubs are the dominant vegetation. To the south, tall shrubs and open stands of white spruce are more common.

Among the many ecological definitions of the arctic is one that states simply that the arctic is the area north of the treeline. By this and many other definitions, Vuntut National Park and the Old Crow Special Management Area have components of both arctic and sub-arctic ecosystems.

During summer, the climate of the Old Crow Basin has a warmer climate than the Northern Coastal Plain or the British-Richardson Mountains, since the mountains form a barrier to the cold fronts which develop over the Beaufort Sea. It is warm enough to support the growth of trees, but still cold enough that there does not appear to be any northward movement of the area of transition between tundra and forest.

The position of the treeline has been used extensively in monitoring past climate change and predicting the impacts of possible future climatic conditions. Research on past climates, traditional knowledge, and projections for the future all indicate that the treeline is not static, but moves north or south with changes in climate of the area (more in Vegetation).

Winter Roads

Other than the treeline, the most obvious feature in this area is a man-made one, the "winter road" that is marked on the National Topographical Sheet maps. This "road" is an important example of a common impact of modern technology on the ecosystem of the northern Yukon; terrain damage caused by vehicles associated with Dew line re-supply and seismic exploration. Running across Vuntut National Park north of the Old Crow Flats, this track is thought to have been made in 1956 by the Alaskan Freightways "Sno Monster" as it travelled from Alaska to the north coast Dew line site (Green 1972). The "winter road" which traverses Vuntut National Park very likely provides examples of a wide range of disturbance to hummocky terrain as it crosses a number of different terrains with different sensitivities.

Winter seismic trails also cross the Blow River Pass into Old Crow Flats from the northeast from the Arctic slope east of the Barn Mountains.

An example of the impact a seismic road may have on the ecology of the Old Crow Flats is the affect on caribou movements witnessed by archaeologist Dick Morlan in the early 1970s. Normally the Old Crow Flats appear to act as a sort of spreading ground which disperses the southward moving caribou herds in the fall, and causes them to subdivide into smaller groups. Morlan (1973) observed that the caribou, which usually forage over a very broad area as they cross the Flats, badly overgrazed a swath about 30 km wide centered on the bed of a winter road which was cut across the Flats in the winter of 1969. The road apparently channelled their movements with the noticeable effect of overgrazing.

Stop # 3. Timber Creek, the Middle Reaches

Permafrost and Patterned Ground

Tussocks

Traditional Land Use

Vegetation Patterns

Treeline

Winter Roads