Common (Translated Scientific) Name: North American Boreal Shrubland & Grassland Macrogroup
Colloquial Name: North American Boreal Shrubland & Grassland
Hierarchy Level: Macrogroup
Type Concept: Vegetation of this boreal type mostly occurs on sites where ecological drivers such as extreme cold, snow, wind, or insolation result in vegetation atypical of the prevailing boreal forest. The vegetation can also develop post-fire and persist due to difficult conditions for tree regeneration. Shrubs predominate in most communities and include Juniperus communis and Juniperus horizontalis on warm-aspect grassy slopes, alvar or dunes, Betula nana and/or Betula glandulosa at high elevations and in cold valleys, Alnus viridis ssp. sinuata, Salix glauca, or Salix pulchra on avalanche tracks or local areas of high snowpack, and Kalmia angustifolia or Empetrum spp. heath. Grasses and herbs of dry sites include Arctostaphylos uva-ursi, Artemisia frigida, Bromus inermis var. pumpellianus, Calamagrostis purpurascens, Carex obtusata, Carex richardsonii, Chamerion latifolium, Elymus trachycaulus ssp. trachycaulus, Festuca altaica, Festuca saximontana, Koeleria macrantha, Leymus mollis, and Saxifraga tricuspidata, among others. Herbs and grasses of moist sites include Calamagrostis canadensis, Carex bigelowii, Chamerion angustifolium, Equisetum arvense, Geranium erianthum, Heracleum maximum, Polemonium acutiflorum, Rubus arcticus, and Sanguisorba canadensis. These moist-site herbs can also occur in meadows in areas of high snow cover.
Diagnostic Characteristics: Upland sites in the boreal and subarctic regions of North America where ecological drivers produce shrub or herb vegetation within a forest climate. These drivers include extreme cold, deep snowpack, dry, well-drained soils, wind, high insolation, or post-fire conditions that limit tree regeneration.
Rationale for Nominal Species or Physiognomic Features:
Classification Comments: Betula glandulosa is considered part of Betula nana in this macrogroup. The limit of this type northward into the Arctic needs further review. In Alaska, both Betula glandulosa and Betula nana are valid species (USDA NRCS 2011).
Similar NVC Types:
Physiognomy and Structure: Shrub and herb communities of this macrogroup vary considerably in vegetation cover from open to closed, but are usually of moderate to high cover. Shrub communities may be comprised of broad-leaved deciduous, broad-leaved evergreen or needle-leaved shrubs, and can include or be dominated by dwarf-shrubs. Herb communities can be graminoid- or forb-dominated, and usually include both. Mosses and lichens can occur and are prevalent in heath, or on drier sites where the leaf litter is light.
Floristics: The floristics vary by environmental site conditions: Dry sites, including steep warm aspects, dunes, and alvars, often have a shrub cover comprised of drought-tolerant species such as Juniperus communis, Juniperus horizontalis, or Arctostaphylos uva-ursi. With slight increases in soil moisture, Shepherdia canadensis, Dasiphora fruticosa ssp. floribunda (= Pentaphylloides floribunda), or Rosa acicularis become more prevalent. Grasses and herbs can dominate some of these sites and also co-occur with shrubs, including Artemisia campestris ssp. borealis, Artemisia frigida, Calamagrostis purpurascens, Carex obtusata, Carex richardsonii, Carex supina, Elymus trachycaulus ssp. trachycaulus, Geum triflorum var. triflorum, Hesperostipa comata, Koeleria macrantha, Festuca altaica, Festuca saximontana, Leymus innovatus, Saxifraga tricuspidata, and Senecio lugens.
Mesic herbaceous meadows include Artemisia norvegica, Bromus inermis var. pumpellianus, Carex bigelowii, Calamagrostis canadensis, Chamerion angustifolium, Leymus mollis, or Festuca rubra.
Higher elevations and valleys with cold-air drainage/ponding in the western mountains often have Betula nana- or Betula glandulosa-dominated scrub. Associated shrubs include Vaccinium uliginosum, Ledum palustre ssp. decumbens, Salix pulchra or other Salix spp., and the dwarf-shrubs Arctostaphylos alpina var. alpina (= Arctous alpina), Empetrum nigrum, and Vaccinium vitis-idaea. Herbaceous species are sparse and may include Artemisia arctica, Festuca altaica, and Anthoxanthum monticola ssp. alpinum. Feathermosses (Hylocomium splendens and Pleurozium schreberi) and lichens are common.
In areas of the western Cordillera with a moister climate and high snowpack, shrub stands can be dominated by Alnus viridis ssp. sinuata or willows, including Salix barclayi, Salix alaxensis, Salix commutata, Salix glauca, Salix richardsonii, or Salix pulchra or a combination of any of these species. The understory species include Achillea millefolium var. borealis, Angelica lucida, Calamagrostis canadensis, Chamerion angustifolium, Equisetum arvense, Geranium erianthum, Heracleum maximum, Rubus arcticus, and Sanguisorba canadensis. Forb- or fern -dominated patches also occur; species include Angelica lucida, Athyrium filix-femina, Dryopteris expansa, Equisetum arvense, Heracleum maximum, and Veratrum viride.
In Newfoundland, heath characterized by Kalmia angustifolia is prevalent in areas of increased fire frequency. Associated species include Ledum groenlandicum (= Rhododendron groenlandicum) and Vaccinium angustifolium. Closer to the coast, where wind is also a factor, the heath is dominated by Empetrum nigrum, sometimes with Vaccinium uliginosum. On the most exposed headlands, a heath dominated by Empetrum eamesii and Racomitrium lanuginosum occurs. All these heath types also have patches of lichens with Cladonia boryi, Cladonia arbuscula (= Cladina arbuscula), Cladonia arbuscula ssp. mitis (= Cladina mitis), and Cladonia rangiferina (= Cladina rangiferina) being most characteristic.
Inland dunes of the boreal and subarctic regions vary in their overall floristics, depending upon history and regional flora, but plant cover is mostly sparse and discontinuous. The active dunes throughout are characterized by the wide-ranging grasses Bromus inermis var. pumpellianus, Festuca rubra, and Leymus mollis. Other herbs specific to Alaska include Chamerion latifolium, Carex obtusata, Carex lachenalii, Festuca brachyphylla, and Astragalus alpinus. Other herbs of the Athabasca Lake dunes include Calamagrostis stricta, Deschampsia cespitosa, Hudsonia tomentosa, Stellaria longipes, Artemisia campestris var. borealis, Achillea millefolium var. megacephala, and Tanacetum bipinnatum ssp. huronense. Additional herbs characteristic of Carcross, Yukon dunes are Calamagrostis purpurascens, Carex sabulosa, Elymus calderi, Festuca saximontana, Achillea millefolium var. borealis, Artemisia frigida, Solidago simplex, and Stellaria longipes. Shrubs can occur in all the dune areas, including Salix glauca, Salix alaxensis, and Salix niphoclada in Alaska; Salix brachycarpa var. psammophila, Salix silicicola, and Betula nana in Lake Athabasca dunes, and Juniperus communis and Juniperus horizontalis in Yukon. On all dune systems, dry-site mosses, e.g., Polytrichum piliferum, or lichens of the genera Cladina, Cladonia, and Stereocaulon also occur.
In the boreal and boreal transition regions of Alaska and western Canada, common shrubs include Juniperus communis, Juniperus horizontalis, Arctostaphylos uva-ursi, Shepherdia canadensis, Dasiphora fruticosa ssp. floribunda, Vaccinium vitis-idaea, or Rosa acicularis. More mesic dips or swales might have Symphoricarpos occidentalis, Amelanchier alnifolia, Prunus spp., or Elaeagnus commutata. Common herbaceous species may include Carex spp., Koeleria macrantha, Hesperostipa comata, Hesperostipa spartea, Festuca altaica, Festuca rubra, Calamagrostis purpurascens, Leymus innovatus, and Hesperostipa curtiseta.
Dynamics: Fire is a common disturbance agent in the western boreal of North America, and although communities of this macrogroup may burn, there are generally other factors strongly influencing vegetation development, including extreme cold, deep snowpack, wind, and high insolation. Warm-aspect scrub and grassland is likely promoted by fire frequency, but these communities are most prevalent in areas with a very dry boreal climate. Fire likely plays a factor in eliminating trees from cold-air valleys, but it is the frequency of frost that restricts tree establishment and favors the development of scrub birch (Betula nana) communities. Fire likely occurs over the alvars, influencing the vegetation; however, the vegetation is also responding to the shallow soils and limestone bedrock.
Fire is normally rare in the eastern boreal; however, Newfoundland heaths became much more prevalent with the arrival of European settlement and an associated increase in fire frequency (as well as the seasonal timing of fires) and forest harvesting. Severe wind exposure is also a factor in promoting some of the heath. A similar situation occurs in Quebec, where transformation of once productive, relatively diverse forest stands into heaths dominated by Kalmia angustifolia or Ledum groenlandicum following harvesting or wildfire is considered a significant forestry issue, as described by Thiffault and Jobidon (2006). They state that the rapid and lasting domination of disturbed sites by these ericaceous shrubs, and the associated stunted growth of conifers, pose a threat to the maintenance of forest productivity and biodiversity.
Snow-avalanching is a factor in promoting Alnus viridis stands in the western Cordillera.
Wind creates and maintains dune communities through the transport and deposition of sand. Several of the large dune systems originated under conditions after the Pleistocene glaciation. The dunes have persisted since then, although much reduced in size as the sand deposits were stabilized by forest and tundra vegetation.