Plant Communities and their Classification

Vuntut National Park and the Old Crow Flats Special Management Area straddle the "boundary" between the arctic and the subarctic. Since the treeline is often used to represent the boundary between the arctic and subarctic regions, the following paragraphs discuss the concept of treeline.

Treeline

The boundary between the arctic and subarctic is in fact a gradual change occurring as a wooded tundra (Bone 1992). Although the treeline is depicted on many standard maps as a dashed line (Figure 10.2), the treeline is actually a transition zone that may be distinct in some areas, but in others is many kilometres in width. The treeline also undoubtedly varies in location over time. The current treeline closely corresponds with the line (isotherm) which connects places that have a mean temperature for July of 10^oC. Depending on many natural factors, tundra may occur south of the treeline and patches of trees may occur well to the north. These factors include depth of the permafrost, protection from wind, south slope radiation, and drainage patterns. The proportion of tundra to forest varies on both sides of the treeline. In the northern Yukon, stands of trees occur as far north as the Firth River valley in Ivvavik National Park (69^o north), only 50 km from the Arctic Ocean. In both Vuntut and Ivvavik National Parks the location of the treeline dividing arctic from subarctic is complicated by the presence of another concept, the alpine treeline that separates the higher treeless alpine regions from the lower altitude forested regions.

It has long been recognized that the location of both treelines may have changed significantly over time. Elders among the Vuntut Gwitchin of Old Crow reported to Balikci (1963) that "important changes in vegetation have taken place within a century," and that they "remember their ancestors saying that no large trees grew in ancient times, and that the vegetation consisted mainly of shrubs, grasses, lichens and very little trees." Charlie Peter Charlie Sr. referred to this recently in an interview with Fafard (1999); "Well, many year ago maybe this, you could maybe you could see quite a ways."

Fossil evidence from mid Pleistocene time (130,000 to 80,000 years ago) suggests the treeline was located further north, the climate was much warmer than present, and the forest more dense than at present. Trees may have extended onto the coastal plain of the northern Yukon (see Palaeontology for details). During the time from 65,000 to 23,000 years ago there is evidence of cooling, with a climate similar or slightly cooler than today. Between 30,000 and 14,000 years ago, the plant communities existing northeast of the Old Crow Basin in the Richardson Mountains were of the arctic-alpine type and fossil evidence shows a climate definitely colder and drier than today. The northern Yukon (eastern Beringia) would have seen a return to tundra conditions and a lack of boreal forest during this period. By 12 thousand years ago the climate began to warm again with the re-appearance of forest species. This was also the time of extinction of many mammals, among them the woolly mammoth, giant beaver, and horse, possibly due to the change in vegetation types.

Current theories on climate change suggest that if predicted climate change patterns (including human-induced global warming) are accurate, within the next 100 years the treeline will move between 200 and 300 km north (Parks Canada 2000). This would allow willow and spruce to become more dominant in the regions surrounding Old Crow Flats.

Plant Communities

The classification of plant communities for the Old Crow Basin is a process that has been ongoing for many years. Early classifications were based on field studies and air photo interpretation. Newer studies use satellite imagery including computer enhancement of colour images to distinguish meaningful differences between vegetation types. In this section, several different classification systems are described in an effort to provide some understanding of the complexity of the vegetation and plant communities of the Old Crow Basin, and at the same time demonstrate the difficulties and the non-static nature of classifying vegetation.

The following general descriptions of plant communities are based on the work of Welsh and Rigby (1971). The six groupings used are: Aquatic Vegetation, Lake Margins, Streamsides and Bars Vegetation, Tussock Tundra, Alpine Tundra, and Boreal Forest. After the description of each of these general types, more information on communities within these units is presented from two other more recent studies.

Figure 10.3 Enhanced Satellite Image. Click on photo to enlarge.

The first is a new land cover classification system for the Old Crow Flats developed by Canadian Wildlife Service biologists, based on enhanced satellite and Landsat 5 Thematic Mapper images (Hawkings and Hughes 1995, Hawkings pers. comm. 1999) and divides the vegetation of the area into19 classifications, including clear and turbid water, four types of fens or emergent vegetation types, shrub thicket, open spruce forest, and wet and dry barrens. The 19 main classification are shown in bold [i.e., "Open Shrub (11)"] and the numbers in brackets refer to the keys to the vegetation types with brief descriptions that are shown in Figure 10.3 (satellite images) and Figure 10.4(photographs) . (Figure 10.3, Figure 10.4 from website http://www.taiga.net/wetlands/oldcrow/vegclass.html).

The second study is a preliminary habitat classification, which refers to previously established classification systems, but was largely based on 74 field plots done in the Thomas Creek valley of Vuntut National Park in 1999 (MacHutchon 2000). Determination of the habitats followed computer sorting of site characteristic and vegetation cover data. The identified habitats reflected common characteristics of landform, slope, aspect, vegetation cover, and moisture. Twenty-two vegetated habitats and three non-vegetated habitats within 9 landscape associations for the Thomas Creek valley were described (Table 10.2 from MacHutchon 2000, Table 10.1). This table also lists eleven satellite cover classes that it may be possible to differentiate in the Thomas Creek Valley based on the identified habitats.

Photographs illustrating some of the main vegetation types described in the following sections are shown in Figure 10.4.

A representation of the relationships between various plant communities in cross-section from lowland ponds to a mountain top is shown in Figure 10.5. Although this figure is from Ivvavik National Park it provides an indication of the similar situation in Vuntut National Park.

At the margins of the lakes and ponds there is a variety of submergent and emergent species. Submergent vegetation is defined as plants which are totally under water, and emergent means that stems, leaves, and flowers may be above the water level, while roots are below. Among the submerged vegetation types are pondweed (Potomogeton spp.) and milfoil (Myriophyllum spp.) (Simpson et al. 1989). The primary emergent species are the yellow pond lily, Tsee zhiI (Nuphar polysepalum), mare's tail (Hippuris vulgaris), and several types of sedges (Carex spp.) and cottongrass (Eriophorum spp.).

In the Old Crow Flats some lakes are filled with yellow pond lily which grows in a circular pattern some distance out from the edge of the pond in deeper water (Welsh and Rigby 1971). The margin of these ponds is occupied by species of sedge (Carex), arrow-grass (Triglochin maritima), bladderwort (Utricularia intermedia), bog-bean (Menyanthes trifoliata), and buttercup (Ranunculus hyperboreus). The northern pondweed (Potamogeton alpinus) is attached and submerged in some ponds. Emergent vegetation in the lakes consists of bladderwort, bog-bean, and species of sedge.

Several species of sedge, bullrush (Scirpus caespitosus), and butterwort (Pinguicula vulgaris) grow in moist sites in the heath-spruce lands in Old Crow Flats (Welsh and Rigby 1971).

Hawkings (1999) divides aquatic vegetation into Submergent vegetation, Floating-leaved Vegetation and four types of Emergent Vegetation/Fen. The following descriptions of aquatic vegetation are quoted directly from Hawkings (1999). The numbers in brackets refer to the key to the colour satellite images shown in Figure 10.3(see also photographs in Aquatic Biology, Figure 9.4.

Submergent Vegetation (3) is described as communities of pondweed Potamogeton spp.

Floating-leaved Vegetation (4) is dense communities of bur-reed (Sparganium) or yellow pond lily.

Emergent Vegetation, Fen (5a, 5b) Fibrous floating mats dominated by graminoids (usually water sedge, Carex aquatilis) or dense meadows of water sedge or Arctophila fulva in recently drained lake basins. Or (b) communities of water sedge (Carex aquatilis) and bog-bean (Menyanthes trifoliata).

Low-center Polygon Fen (5c) is very wet ploygons underlain by frozen peat and found in old lake basins and depressions. The vegetation is usually a sparse coverage of emergent graminoids (Carex aquatilis or Carex chordorhiza). It is a common feature of tundra further north on the Yukon Coastal Plain.

Sphagnum Mat (6) is mats of various Sphagnum mosses underlain by saturated peat.

In the tundra area, the lakes are generally small and commonly shallow. The margins are frequently occupied by emergent plants, principally water sedge and the grass, Arctophila fulva. The wet meadows at the water's edge are dominated by the cottongrass, Eriophorum angustifolium. Back from the water's edge a thicket of low willows (Salix) and birches (Betula glandulosa) dominate the vegetation (Welsh and Rigby 1971). Interspersed among the willows and extending into the wet meadows are the plants: Andromeda polifola, Cardamine purpurea, louseworts (Pedicularis lanata, P. sudetica, P. labradorica), and Rubus chamaemorus. Labrador tea (Ledum decumbens) and crowberry dandanh (Empetrum nigrum) fill in between the willows and the birches.

The Canadian Wildlife Service classification (Hawkings 1999) describes the following three classes found in lakes that have drained.

Dead Willow, Wet Sedge (7) Dead willow shrubs with an understory of wet sedge, usually water sedge. This community forms in lakes that have drained and are in the process of reflooding as permafrost invades and impedes drainage.

Figure 10.4 Click on photo to enlarge and see photos of other vegetation types.

Moist Graminoid Calamagrostis (8) Meadows dominated by Calamagrostis in drained lake basins. Usually has some scattered medium to tall willow and alder k'oh shrubs.

Shrub Thicket (12) Dense stands of medium to tall shrubs, usually willow or alder. The densest stands are found in recently drained lake basins, along rivers, and on levees. Understory species are variable, including water sedge in recently drained lake basins.

Major streams entering Old Crow Flats from the mountain slopes have produced belts of gravel, spread by the meandering streams (Welsh and Rigby 1971). These are willow-covered and barren, meander-scarred plains in the peripheral areas, but tree-covered with white spruce and cottonwood trees in the central and southern part of the area.

Though the effect of streams normally doesn't reach much beyond the active running water, in the Old Crow Basin where many streams are braided, the effect is more broadly felt. Several different willows dominate the stream banks, and where meanders develop, there may be considerable development of willow-dominated vegetation. The larger stream-side plants may be up to 5 m tall, especially the feltleaf or Alaska willow, Salix alaxensis, with densely hairy branchlets, and the cottonwood, Populus balsamifera. Neither of these reach far from the stream. The feltleaf or Alaska willow is the tall, abundant willow species along all of the drainages, from high elevations down to low ones. Even along the Old Crow River, where other species become important, this willow continues as one of the dominant species.

Other willows which grow along the drainages gradually disappear as the moisture decreases away from the stream and the drier heath or tussock tundra replaces them. The green or sitka alder k'oh, Alnus crispa, grows tall enough to be part of the thicket along streams, and in the Old Crow Flats, extensive alder growth occurs where river bars are not annually scoured. River bars which receive annual scouring do not support vascular plants and are barren throughout the growing season. In the openings among the willows which line the stream banks and bars are many colourful plants: sweet vetch, milk vetch, buttercups, and chickweeds.

Hawkings (1999) classifies these wet areas as Wet Barrens (15): Unvegetated or sparsely vegetated saturated soils, usually in very recently drained or drying lakes, or along rivers. Between the riverbank (riparian) vegetation and the natural pale or grey-green of the tussock tundra is a distinctive dark, dull, green stripe of heathland tundra (Welsh and Rigby 1971). This community is dominated by willow, Dryas, crowberry dandanh, Labrador tea lidii masgit, bearberry shoh jak, birch, and bog cranberry or lingonberry natl'at (Vaccinium vitis-idaea), and bog blueberry jak zraii (V. uliginosum). Secondary flowering species include the yellow Arnica and Oxytropes, white Castilleja, bluebell (Mertensia) and blue lupine, pale cream lousewort (Pedicularis capitata), and pink louseworts and bear root trih (Hedysarum).

Tussock or hummocky terrain occupies about 30 % of northern ecoregions based on recent satellite imagery. Tussocks are cottongrass-dominated structures while hummocks are earth-dominated. The structure and ecology of tussocks is further discussed in Geomorphology and Ecology. South of the treeline the mature forest consists of open stands of black spruce, with ground lichens on the hummocks and mosses in the inter-hummock troughs. Near tundra areas there are small patches of open white or black spruce. In the tundra, the characteristic vegetation on imperfectly or poorly drained locations are tussocky cottongrass (Eriophorum vaginatum) and mosses. On better drained areas dwarf ericaceous (heather family) plants, willows, and birch (Betula glandulosa) are common (Zoltai and Tarnocai 1974).

In the Old Crow Basin area, hummocky or tussock tundra covers more land surface than any other vegetation type (Welsh and Rigby 1971). Hummocks develop as one moves away from either lakes or streams. The composition of the vegetation of this hummocky tundra is varied, depending on soil moisture, depth of thaw, and slope. Near the streams and lakes there is a transition from the tallish, lush vegetation characterizing these sites to the hummocky tundra type. Slopes along streams are characteristically transitional to tussock tundra also, either abruptly or subtly. Generally, feltleaf willow (Salix alaxensis) gives way abruptly along the wet sites, but the other species of willow, which may grow to a height of a metre along the streams, continue to dominate the rounded hummocks of the tundra, but seldom reach a height much above the hummocks. Other species that are found in this hummocky tundra are; bog cranberry (Vaccinium vitis-idaea), bog blueberry (V. uliginosum), birch (Betula glandulosa), mountain avens (Dryas integrifolia), alpine bearberry (Arctostaphylos alpina), lousewort (P. lanata), Maybell's oxytrope (Oxytropis maydelliana), bitter-cress (Cardimine digitata), bistort (Polgonum bistorta), and sedges (Carex species).

In higher, better-drained ridges and slopes, the willow hummocks give way to a cottongrass tussock tundra dominated by tussocky cottongrass (Eriophorum vaginatum) and sedges (Carex) species (Welsh and Rigby 1971). Shrubs are not lacking in the tussock tundra, but their role is subordinate to the cottongrass. The same shrubs occur in the tussock tundra as in the wetter hummocky tundra. Depressions or hollows and moist flats in the tussock tundra appear green and lush in comparison to the well-developed tussock tundra, due to the presence of tall cottongrass (Eriophorum angustifolium) and, in some places, common horsetail (Equisetum arvense).

Hawkings (1999) describes the above vegetation using the following classifications:

Moist Graminoid Cottongrass Tussock (9) is extensive meadows of tussocky cottongrass (Eriophorum vaginatum) tussocks on uplands underlain by mineral soils.

Moist Graminoid, Shrub (10) moist graminoid communities (8 and 9 above) with higher coverage of shrubs.

Tundra polygons form in areas of low relief and poor drainage due to the annual freeze-thaw cycle. The outlining trench is often water-filled, or at least more moist than the surrounding area, and features tall cottongrass (Eriophorum angustifolium) and sedge (Carex species), and often Sphagnum moss. Other plants found in the wetter polygons are marsh cinquefoil or marsh fivefinger (Potentilla palustris), louseworts (Pedicularis species), and buckbean (Menyanthes trifoliata). The trenches in drier polygons often support a community of plants similar to that along a stream channel, with for example, willow, birch, crowberry, Labrador tea, and bog blueberry or cranberry (Welsh and Rigby 1971).

Lichen Heath (14) Lichen mats underlain by relatively well-drained soils. Usually found in the form of high-centered or flat-centered (low-centered) polygons.

Click photo to enlarge

On ridge tops plants are widely spaced. Density and composition varies with the most dense vegetation in hollows and on north-facing slopes (Welsh and Rigby 1971). Common species are one-flowered ciquefoil (Potentilla uniflora), draba (Draba palanderiana and Draba lactea), vetch (Astragalus australis), blackish oxytrope (Oxytropis nigrescens), saxifrage (Saxifraga reflexa), prickly saxifrage (S. tricuspidata), primrose (Douglasia arctica, D. ochotensis), rock-cress (Parrya nudicaulis), Siberian phlox (Phlox sibirica), and forget-me-not ? (Eretrichium nanum). The main sedges and grasses are Carex, Kobresia, fescue (Festuca), and bluegrass (Poa). Springs and seeps are densely covered with willow, birch, crowberry, mountain avens, and sedges. Herbs in the area of springs include the same species as in wet meadows: louseworts, bittercress, anemone, Senecio, sedges, Kobresia, shooting star (Dodecatheon), figwort (Lagotis), rockcress (Parrya), Jacob's ladder (Polemonium), and forget-me-not (Myosotis). Talus and rockstripe areas of the alpine tundra are either barren or vegetated in clumps, stripes or patches, the major species being mountain avens, crowberry, birch, and sedge. Vegetation on slopes is commonly oriented in stripes running up and down the slope.

In the alpine tundra, each exposed geological layer or stratum tends to support a distinctive flora, with limestones supporting the greatest number of species (Welsh and Rigby 1971). The geological control of vegetation in the Old Crow Basin is perhaps best demonstrated on outcrops of rock formations on ridge or mountain tops where conditions for growth are poor. While some plants seem to do well on any substrate, others "prefer" certain geological types. Among the many plants living on limestone formations in alpine sites are: Alyssum americanum, sandwort (Arenaria rossii var. elegans), rock cress (Braya purpurascens), sedge (Carex petricosa), alpine fescue (Festuca brachyphylla), fescue (F. ovina), nodding lychnis (Silene uralensis), Lychnis fucata, and tufted saxifrage (Saxifraga caespitosa). Intermediate in terms of numbers of plants supported are the shales and schists. Plants found on these formations include: sandwort (Arenaria macrocarpa, A. rubella), primrose (Douglasia arctica), cushion saxifrage (Saxifraga escholtzii), and groundsel/ragwort (Senecio fuscatus). The siliceous conglomerates, granite and sandstone are definitely poorer in species, and the list of apparently restricted species contains alpine azalea (Loiseleuria procumbens), cinquefoil (Potentilla elegans), and saussurea (Saussurea angustifolia), and, primarily on shale, the saxifrages Saxifraga exilis and thyme-leaved saxifrage (S. serpyllifolia) (Welsh and Rigby 1971).

Only one of Hawkings (1999) classification types refers to alpine tundra. Dry Barrens (16) are unvegetated or partially vegetated dry soils, most commonly found in exposed alpine areas or gravel bars along rivers. Small areas are found in drained lake basins. Some areas of cottongrass tussock tundra may be found in this class if there is a high percentage of frost boils (small, round, low elevation structures caused by frost action).

MacHutchon (2000) describes three habitat types in the Thomas Creek area that are based on a herb tundra herb or mountain ridge type of vegetation. Horsetail–Mountain-avens Tundra Herb occurs on moisture-holding sites on upper mountain slopes. These slopes likely retain snow or moisture late in the season. Medium willow and ericaceous shrubs, mountain-avens, and horsetail dominate it. Sedge–Mountain-avens Tundra Herb occurs on exposed sites and areas that experience rapid drainage. Severe environmental conditions prevent tree and shrub growth so these sites are predominantly ground shrubs and short perennial plants. There is a large amount of exposed rocky soil. Mountain-avens, sedges and lichens are common in this habitat.

Sparsely Vegetated Mountain Ridge occurs on mountain ridges and peaks and are mostly non-vegetated (less than 15% vegetation), lichen, and bare rock. There are scattered patches of ground shrubs (e.g., willow, mountain cranberry, alpine azalea (Loiseleuria procumbens), and blueberry).

Welsh and Rigby (1971) recognized three main types of boreal forest in the northern Yukon; white spruce stands on south-facing slopes, open parkland tundra (i.e., taiga), and riparian (river) woodland. The spruce woodland on the southern bases of hills and on south-facing slopes in valleys is made up of isolated stands in the northern parts of the area and continous stands southward in the Old Crow Flats. The dominant species obviously is the white spruce. Spruce is also the dominant species in the open parkland, which differs mainly in the frequency of the species encountered. Even in the riparian woodland, the spruce is the dominant conifer, but other tree species are important, especially white birch (Betula papyrifera), and cottonwood (Populus balsamifera). The understory in the riparian woodland is composed of glandular or resin birch (Betula glandulosa), willows, Rosa, Labrador tea (Ledum sp.), crowberry, and Lapland rosebay (Rhododendron). Streamside herbaceous plants include; wheatgrass (Agropyron macrourum), hairy rock-cress (Arabis hirsuta), sandwort (Arenaria physodes), alpine milk-vetch (Astralagus alpinus), Barbarea orthoceras, paintbrush (Castelleja pallida), Eskimo potato (Hedysarum alpinum, H. boreale), long-stalked stichwort (Stellaria longipes), and Dandelion ch'at'an tsoo (Taraxacum ceratophorum) (Welsh and Rigby 1971).

Open Shrub (11) variable communities are dominated by medium to tall shrubs. Understory communities may be grasses and sedges (graminoid), low ericaceous shrubs, or mosses (bryophytes). Often includes scattered spruce.

Open Spruce Forest (13) Open canopy spruce forests with understory of ericaceous shrubs, cottongrass tussocks, or lichen.

Click on photo to enlarge.

In the Thomas Creek area, MacHutchon (2000) describes six habitat types based on an open white spruce vegetation. Three examples are given here: Open White Spruce–Kinnikinnick Forest occurs on steep, well drained slopes in the Thomas Creek valley. This habitat is characterised by the presence of bearberry or kinnikinnick (Arctostaphylos uva-ursi), Lapland rosebay, and blueberry. Open White Spruce–Horsetail Forest occurs on gradual slopes receiving abundant upslope moisture. Abundant water flow and/or a warmer aspect create a sufficiently deep active layer for tree growth. This habitat is similar to the Open White Spruce–Horsetail Forest that occurs on alluvial terraces on the valley bottom. Open White Spruce–Blueberry Forest occurs on the side slope of mountains or hills. It occurs on well but not excessively drained sites with a deep active layer. Open White Spruce Spruce–Blueberry Forest is characterised by the presence of less than 15% tree cover and scattered medium or low shrub cover. Mountain heather (Cassiope tetragona) and lichens are usually present. This habitat is similar to the Open White Spruce–Mountain –avens Forest that occurs on alluvial terraces on the valley bottom.

PLANT COMMUNITIES AND THEIR CLASSIFICATION

Treeline

Plant Communities