Tactic P-2: Understanding the Tundra Environment
An understanding of the tundra environment is critical when choosing tactics and strategies for treating a spill. Following is an overview of four generalized tundra types and their characteristics. Although this discussion focuses on Alaska’s North Slope (arctic tundra), the planning, treatment, and monitoring, tactics in this manual also apply to tundra environments elsewhere in Alaska, including alpine tundra.
What is tundra?
Tundra is a Russian word translated as “treeless plain” (http://www.merriam-webster.com) or “marshy plain” (Billings 1974). Tundra in this manual is used to describe ecosystems where the indigenous plant cover consists of low herbaceous, dwarf shrub, or lichen vegetation in places which have summers too cold to allow tree growth. Tundra includes the circumpolar treeless region north (and south) of the latitudinal treeline and the less extensive mountain landscapes above altitudinal treeline (Murray 1978).
Alaska’s North Slope stretches from the crest of the Brooks Range north to the Arctic Ocean
(Fig. 4). The Arctic Coastal Plain area is flat and wet with abundant oriented thaw lakes. In contrast, the Arctic Foothills Area is a broad expanse of valleys and hills. The climate is characterized by extreme winter cold, strong winds, and brief summers (about 90 days [June–August]) when the air temperature is generally cool and there is relatively little precipitation. The soil at depth remains perennially frozen (permafrost) but an “active layer” of surface soil, varying in depth from a few inches to a few feet, thaws each summer and refreezes each winter (Fig. 5). The rooting depth of plants and most of the activity of soil microbes are limited by the depth of the active layer (i.e., thaw depth). Although annual precipitation is low, surface water is abundant, because permafrost limits water infiltration and movement. Tundra vegetation consists of low-growing plants including mosses, lichens, grasses, sedges, and dwarf shrubs. Compared to most other environments, relatively few plant species have adapted to the extreme conditions of the tundra (Fig. 6). Soils develop slowly in the Arctic, because the cold climate and short growing season limit the decay of dead plant matter.
What are the types of tundra?
This manual classifies tundra into four types: aquatic, wet, moist, and dry (Figs. 7–10). These generalized types are based on a hierarchical tundra vegetation classification scheme developed by Walker (1983, 1985). They occur in three major geographic provinces on the North Slope of Alaska: 1) the coastal plain, 2) the foothills, and 3) the mountains of the Brooks Range, as well as on the Seward Peninsula.
Wet tundra is the most common type on the coastal plain, due to the low topographic relief and the presence of a shallow, saturated active layer. Patterned ground features (i.e., polygons bounded by ice wedges, Fig. 6) are abundant. In the foothills province, moist tundra predominates on slopes, wet tundra in low areas, and dry tundra on exposed hilltops and ridges. Patterned ground is less common here. In the Brooks Range and above treeline in other mountain ranges in Alaska, dry tundra predominates. High shrub thickets develop on floodplains, in sheltered areas or where snow accumulates and protects plants from harsh winter winds. In the braided channels of active floodplains, the soil surface is frequently barren. Figure 11 illustrates topographic features and subsurface conditions associated with a few of the common plant community types on the North Slope.
- Occurrence: Frequently forms marshes along the margins of ponds, lakes and streams, and may form a mosaic with wet tundra.
- Common Plants: Arctic pendant grass (Arctophila fulva), water sedge (Carex aquatilis), and mare’s tail (Hippuris spp.).
- Soils: Thick layer of aquatic sediments and peat.
- Active Layer: Deep at maximum thaw (late summer). A thaw basin of unfrozen soil may be present in the vicinity of ponds, lakes, and streams.
- Occurrence: Where shallow (< 1 ft) surface water persists through all or most of the growing season, in troughs, low centers of polygons, and in wet areas within drained lake basins. Wet tundra is the most common tundra type on the coastal plain. May form a mosaic with moist tundra where the soil is saturated but without standing water.
- Common Plants: Water sedge (Carex aquatilis), tall cottongrass (Eriophorum angustifolium), Fisher’s tundra grass (Dupontia fisheri), and arctic pendant grass (Arctophila fulva).
- Soils: A mat of roots and organic matter approximately 1 ft thick, underlain by mineral soils. The organic soil layer and rooting zone are thicker in wet tundra than in dry or moist tundra. Ponds and standing water are common within wet tundra areas, and soil pore spaces are saturated with water during the growing season.
- Active Layer: Moderate to deep at maximum thaw. The high thermal conductivity of water may melt the top of permafrost in the summer despite the insulating effects of the highly organic root mat, especially if the surface has been physically disturbed. This active layer is often about 1 foot (12 inches) in depth (Fig. 12), but may extend to about 3 feet below the tundra surface in wet tundra.
- Occurrence: Usually where the soil is saturated in a portion of the active layer throughout the growing season, but standing water is absent or present for only a part of the growing season. Areas of moist tundra on the North Slope include the slopes of hills, on high-centered polygons, and the rims of low-centered polygons.
- Common Plants: Sedges (Carex aquatilis and C. bigelowii), cottongrasses (Eriophorum angustifolium and E. scheuzeri), and dwarf shrubs including willows (Salix spp.), birch (Betula spp.) and mountain-avens (Dryas spp.). Tussock tundra is a common type of moist tundra on the North Slope, especially in the foothills. It is dominated by tussock cottongrass (Eriophorum vaginatum), dwarf shrubs, mosses, and lichens.
- Soils: A dense, compressed mat of roots and organic matter overlies mineral soils.
- Active Layer: Relatively thin due to the dense insulating organic mat and moderate soil moisture content.
- Occurrence: Where good drainage creates relatively dry soil conditions throughout the growing season. On the slopes of mountain ranges, on ridges and hilltops in foothills, stabilized sand dunes, pingos, and other well-drained sites on the coastal plain.
- Common Plants: Dwarf shrubs including birch, willow, mountain-avens, blueberry and cranberry (Vaccinium spp.), Labrador tea (Ledum palustre ssp. decumbens), crowberry (Empetrum nigrum), arctic bell-heather (Cassiope tetragona), and bearberry (Arctostaphylos spp.), along with lichens, mosses, and grasses.
- Soils: Thin root mat and low organic matter content compared to soils of moist and wet tundra. Ample drainage reduces the ability of the thin root mat to hold moisture.
- Active Layer: The active layer in dry tundra is usually comparable to wet and moist tundra, but can be as deep as 3 feet.
Sensitivity to Disturbance
Tundra environments can be especially sensitive to disturbance for several reasons:
- Short growing season
- Extreme winter wind and cold temperatures
Tundra vegetation and soil insulate the permafrost layer from the sun and warm surface air during the growing season. Actively growing plants cool the soil by drawing water from the soil (evapotranspiration). Surface disturbances can interfere with these processes, causing ice in the soil to melt and resulting in subsidence (thermokarst). Drainage patterns are affected by subsidence, leading to further changes in topography and hydrology. For example, thermokarst in dry or moist tundra can lead to formation of wet or aquatic tundra, but thermokarst in wet tundra also can lead to drier conditions.
Difficulty Treating Spills
Spills on tundra can be difficult to treat for several reasons:
- Short summer season, when most treatments are easier to implement.
- Low temperatures limit the rate of microbial breakdown of hydrocarbons (biodegradation).
- Remote locations present practical challenges for cleanup efforts.
- Patterned ground features or tussocks make treatment more complicated.
- Soils and vegetation may be physically damaged, which can impede achievement of the treatment goals.