Betula nana



INTRODUCTORY


  © Jürg Stöcklin
AUTHORSHIP AND CITATION:
Tollefson, Jennifer E. 2007. Betula nana. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].

FEIS ABBREVIATION:
BETNAN

SYNONYMS:
For Betula nana ssp. exilis:
Betula exilis Sukatsch. [31,34,44]
Betula nana var. sibirica Ledeb. [34,44]
Betula tundrarum Perfiljev [44]

NRCS PLANT CODE [85]:
BENA

COMMON NAMES:
dwarf birch
Arctic dwarf birch
dwarf alpine birch
swamp birch

TAXONOMY:
The scientific name of dwarf birch is Betula nana L. (Betulaceae) [31,34,44,50,92]. There are 2 subspecies:

Betula nana subsp. exilis (Sukatsch.) Hult. [31,34,44,92]
Betula nana subsp. nana [31,34]

Dwarf birch hybridizes with bog birch (Betula glandulosa) where their ranges overlap [31,44,92]. Horne birch (Betula × hornei Butler) is the hybrid between dwarf birch and paper birch (Betula papyrifera) [92].

LIFE FORM:
Shrub

FEDERAL LEGAL STATUS:
No special status

OTHER STATUS:
Information on state-level protected status of plants in the United States is available at Plants Database.

DISTRIBUTION AND OCCURRENCE

SPECIES: Betula nana
GENERAL DISTRIBUTION:
Dwarf birch is a widespread arctic species with a circumpolar distribution. In North America, Betula nana subsp. exilis is native throughout Alaska and across northern Canada to Baffin Island, Labrador, and Greenland. In Alaska, it occurs along the coast and in interior mountains from the northern part of southeastern Alaska to the western end of the Alaska Peninsula and the Bering Sea, and north to the Arctic coast [44,78,92]. Betula nana subsp. nana is native only to Greenland and Nunavut in North America [21,31]. Flora of North America provides a distributional map of dwarf birch and its infrataxa.

ECOSYSTEMS [33]:
None

STATES/PROVINCES: (key to state/province abbreviations)
UNITED STATES
AK

CANADA
AB BC MB NF NT NS NU PQ SK YK

BLM PHYSIOGRAPHIC REGIONS [5]:
None

KUCHLER [53] PLANT ASSOCIATIONS:
K094 Conifer bog

SAF COVER TYPES [29]:
12 Black spruce
13 Black spruce-tamarack
18 Paper birch
201 White spruce
202 White spruce-paper birch
203 Balsam poplar
204 Black spruce
217 Aspen
222 Black cottonwood-willow
251 White spruce-aspen
252 Paper birch
253 Black spruce-white spruce
254 Black spruce-paper birch

SRM (RANGELAND) COVER TYPES [76]:
901 Alder
904 Black spruce-lichen
907 Dryas
911 Lichen tundra
912 Low scrub shrub birch-ericaceous
913 Low scrub swamp
916 Sedge-shrub tundra
917 Tall shrub swamp
918 Tussock tundra
919 Wet meadow tundra
920 White spruce-paper birch
921 Willow

HABITAT TYPES AND PLANT COMMUNITIES:
Dwarf birch is characteristic of low, open, mixed-shrub and tussock tundra communities. It is also found in black spruce (Picea mariana) and white spruce (P. glauca) communities including black spruce-birch (Betula spp.), the most widespread ecosystem type in Alaska [1,36,84,88,91].

Dwarf birch is listed as a dominant species in the following vegetation classifications:

United States
Alaska: Canada
Yukon:

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Betula nana
GENERAL BOTANICAL CHARACTERISTICS:
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g., [44,92]).

Dwarf birch is a deciduous, low and spreading shrub. Plants are strongly branching and range from 0.5 to 3 feet (.15-1 m) tall. Twigs are resinous and slightly hairy. Leaves are thick and leathery and range from 0.2 to 0.5 inch (5-12 mm) long and 0.2 to 0.6 inch (5-16 mm) wide. The inflorescences are catkins. Male catkins are 0.4 to 1 inch (10-25 mm) long, and female catkins are 0.2 to 0.4 inch (6-10 mm) long. Fruits are narrow-winged, single-seeded samaras [21,31,44,92]. Dwarf birch has an extensive underground system. Rhizomes and roots account for 80% of total plant biomass [15,16]. Roots are ectomycorrhizal, an adaptation to arctic and alpine soils that are generally low in inorganic nitrogen and phosphorus [20,83].

RAUNKIAER [74] LIFE FORM:
Phanerophyte
Chamaephyte

REGENERATION PROCESSES:
Dwarf birch reproduces by seed and vegetatively by branch layering and sprouting. Vegetative reproduction is more common than reproduction by seed, resulting in widespread clones [6,9,10,54,98]. In Eagle Creek, Alaska, dwarf birch allocated 99.6% of its annual production to vegetative growth and 0.4% to flowers and fruits over a 3-year period. Sexual reproduction in dwarf birch was lowest out of 8 species studied [18].

Pollination: Dwarf birch is wind pollinated.

Breeding system: Dwarf birch is monoecious [49]. Birches in general are strongly self incompatible [23].

Seed production: Dwarf birch is a prolific seed producer [9,98]. Plants at high altitudes and in cold climates produce few seeds (Elkington 1968, cited in [23]).

Seed dispersal: Dwarf birch seeds are dispersed in their samaras. Wind, water, and sometimes gravity disperse the samaras. Samaras may blow across crusted snow [24,49,63].

Seed banking: In a review of the literature, Karrfalt [49] states that birch seeds may be abundant in the soil, but the seeds are generally short lived. Dwarf birch was not found in the seed bank at an Alaskan arctic study site [28]. At another site near Eagle Summit, Alaska, however, dwarf birch seed was absent from the aboveground vegetation but present between 0 and 24 inches (0-60 cm) in the soil [61]. Dwarf birch also germinated from the seed bank at a northern Finland research site [86].

Germination: Seed germination in dwarf birch varies from 21% to 95% [18,21]. Optimum germination temperature for many arctic species is 59 to 86 °F (15-30 °C) [6]. Stratification improves germination of birch seeds [49]. In a greenhouse experiment, stratification for 5 to 15 days broke seed dormancy in dwarf birch. Stratification for 15 days was required for maximum germination in 14 days at 54 °F (12 °C). A longer period of stratification was required for maximum germination at lower temperatures. Germination occurred at a high temperature (75°F (24 °C)) with no special treatment [48].

Seedling establishment/growth: Although dwarf birch produces abundant seed and seed viability may be as high as 95%, successful establishment from seed is rare [18,28,98]. Dwarf birch seedlings are slow growing [10,21,97]. Dwarf birch had the greatest number of active apical meristems producing leaves, stems, or inflorescences of 8 species studied at Eagle Creek, Alaska. It also had the lowest annual aboveground production [18]. In the spring, nitrogen and phosphorus stored in dwarf birch rhizomes and roots are transported into new leaf and stem tissue aboveground. After early July, the leaves are net exporters of nutrients to the rest of the plant, and by late August belowground tissues have regained their initial spring nitrogen and phosphorus concentrations [15].

Growth rates in dwarf birch increase under increased nutrient availability. In experimentally treated plots in Alaskan moist tussock tundra, dwarf birch became dominant following fertilization with nitrogen and phosphorus. Dominance was attributable to increased growth of individuals already present and not the recruitment of new individuals. Under unfertilized conditions, most axillary buds on dwarf birch plants grow into short shoots. The branching pattern in dwarf birch can change, however, when nutrients are not limiting. Under fertilized conditions, axillary buds that would have produced short shoots are stimulated to produce longer, structural branches. Long shoots produce 2 to 3 times more leaf area than short shoots, increasing the plant's total photosynthetic capacity and allowing a dense dwarf birch canopy to form [10,11]. Dwarf birch also produces more long shoots in response to experimental warming [17,36,42].

Vegetative regeneration: Dwarf birch reproduces vegetatively by layering and by sprouting from the root crown and/or rhizomes after fire and other top-killing disturbances [6,9,27,98].

SITE CHARACTERISTICS:
Dwarf birch is found from 70 feet (20 m) elevation in the Canadian Arctic to at least 4,300 feet (1,300 m) in Alaska [21]. It occupies a wide variety of sites, ranging from rocky arctic and alpine tundra to deep, organic soils [44,45,92]. Dwarf birch occurs on moist, acidic, and nutrient-poor sites including muskegs, bogs, and the understory of black and white spruce taiga communities [44,52,92,95]. It is also found on well-drained, upland sites including moraines, steep banks, and dry, rocky slopes [13,43]. It grows extensively on sites underlain with permafrost [64,70], including palsas and frost polygons in Alaskan arctic tundra [27].

In bogs near Fairbanks, Alaska, dwarf birch abundance decreases as soil moisture increases. Dwarf birch is more "vigorous" in communities that support taller tussocks [12,46]. In a review of the literature, de Groot and others [21] state that bog birch does not appear tolerate continuous flooding but does appear to tolerate periodic drought.

Dwarf birch occurs in circumpolar regions with long, cold winters and short, cool summers. In a review of the literature, de Groot and others [21] state that the optimum temperature for photosynthesis in dwarf birch is 50 to 55 °F (10-13 °C). Dwarf birch appears to tolerate colder temperatures than bog birch. Annual precipitation across the range of dwarf birch varies from 12 to 16 inches (300-400 mm) in circumpolar regions to 47 to 78 inches (1,200-2,000 mm) in the British Isles.

SUCCESSIONAL STATUS:
Dwarf birch is noted in several successional stages. Dwarf birch colonizes disturbed sites via seeds dispersed from off site [28]. On a glacier foreland in Sweden, however, dwarf birch was not found in any area younger than 80 years. Many young plants were found on a 130-year-old site [98]. On a glacier foreland in south-central Norway, dwarf birch replaces pioneer species after >220 years [38].

SEASONAL DEVELOPMENT:
Bud break in dwarf birch is a function of air temperature. Growth chamber studies show that after a period of winter chilling, dwarf birch buds break in response to warming air temperatures in the spring [71]. In a review of the literature, de Groot and others [21] state that leaf growth in dwarf birch begins soon after snow melt. Fine root growth begins about 1 week after bud break. As the early leaves expand, single female inflorescences form at the ends of short shoots. Additional leaves develop throughout the growing season as shoots elongate. Male inflorescences are formed on long shoots as shoot elongation slows. Female catkins expand in early June, and seeds ripen in August [21]. Samaras disperse from late summer until the following spring [49].

The phenology of dwarf birch across its range is summarized below.

Seasonal development of dwarf birch [23]
bud break mid-May to mid-June
new shoot growth mid-June to early August
female catkins in flower early June
seeds ripe August
leave senescence August-September
leaf abscission complete by late September

In tussock tundra in central Alaska, dwarf birch leaves expand in June, mature in mid-July, and begin to senesce in early August [65]. Flowering and fruiting dates are given below.

Timing of flowering and fruiting in dwarf birch in central Alaska [65]
Onset of flower bud swell Onset of flowering

Onset of fruiting

28 May-15 July 17 June-31 July 9 July-17 July

FIRE ECOLOGY

SPECIES: Betula nana
FIRE ECOLOGY OR ADAPTATIONS:
Fire adaptations: Dwarf birch regenerates after low- and moderate-severity fire by sprouting from the root crown and rhizomes [72]. The samaras are dispersed by wind and can invade burned areas from off site [4]. Successful establishment from seed, however, is rare in dwarf birch [18,28,98].

Fire regimes: Dwarf birch is adapted to a wide range of fire regimes, from subarctic and alpine areas that seldom burn to boreal environments that burn frequently [22,24]. Black spruce-birch (Betula spp.) is the most widespread forest type in interior Alaska and also the type with the highest frequency of fire [91]. Native Americans were an important cause of fires in the black spruce-birch ecosystem [59]. Fire frequency increased with the increase in mining activity in the 1800s [89]. Today, most fires are lightning caused [39,58]. Between 1940 and 1969, lightning was responsible for 78% of the area burned in interior Alaska [89].

Fires occur in interior Alaska between 1 April and 30 September. Most fires occur in May, June, and July, corresponding with the highest annual temperatures, longest day length, lowest humidity and precipitation, and high winds [32,89]. Fires can occur, however, whenever fuels are not covered with snow and are exposed to sufficiently warm temperatures and drying winds [89].

Fire years are sporadic in occurrence but tend to occur at least once every decade [40]. “Exceptional fire years” are characteristic of the black spruce-birch ecosystem. In Alaska, 6 years (1941, 1950, 1957, 1969, and 1977) accounted for 63% of the total area burned between 1940 and 1978 [93]. The average acreage burned each year in interior Alaska is approximately 1 million acres [59]. Fires tend to be large and may spread over thousands to hundreds of thousands of acres or more [40,57,87].

Estimated fire-return intervals in the black spruce-birch ecosystem vary from 50 to 200 years [40,93]. Fires occur every 50 to 70 years in black spruce-white spruce/bog birch/reindeer lichen communities in interior Alaska [32]. Heinselman [40] estimates a fire-return interval of 130 years for open black spruce/reindeer lichen forest and 100 years for closed-canopy black spruce forest. Mean fire-return intervals in lowland black spruce forests on the Kenai Peninsula, Alaska, range from 89 to 195 years [2,60].

Black spruce-birch communities experience high-severity, stand-replacing fires. These communities are highly flammable due to the abundance of ericaceous shrubs, the prevalence of dead, low-hanging branches on the black spruce trees which are often covered with highly flammable epiphytic lichens, and the thick moss and lichen mats that cover the forest floor and become highly flammable after periods of low rainfall [57,58,90]. There is often nearly continuous fuel from the forest floor to the tree crowns [93]. Most fires in black spruce-birch communities are either crown fires or ground fires severe enough to damage or kill aboveground vegetation, including overstory trees. Fires may be severe enough to completely expose the mineral soil layer [26,40,87,93].

The following table provides fire return intervals for plant communities and ecosystems where dwarf birch is important. For further information, see the FEIS review of the dominant species listed below.

Fire-return intervals for plant communities with dwarf birch
Community or Ecosystem Dominant Species Fire Return Interval Range (years)
birch Betula spp. 80-230 [82]
tamarack Larix laricina 35-200 [68]
black spruce Picea mariana 35-200
conifer bog* Picea mariana-Larix laricina 35-200
jack pine Pinus banksiana <35 to 200 [19,26]
aspen-birch Populus tremuloides-Betula papyrifera 35-200 [26,94]
*fire return interval varies widely; trends in variation are noted in the species review

POSTFIRE REGENERATION STRATEGY [81]:
Small shrub, adventitious buds and/or a sprouting root crown
Rhizomatous shrub, rhizome in soil
Initial off-site colonizer (off site, initial community)

FIRE EFFECTS

SPECIES: Betula nana
IMMEDIATE FIRE EFFECT ON PLANT:
Dwarf birch is easily top-killed by low- and moderate-severity fires [72].

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
No additional information is available on this topic.

PLANT RESPONSE TO FIRE:
Dwarf birch regenerates after top-kill by fire by sprouting  by from the root crown and rhizomes [72]. In a review of the literature, de Groot and others [21] state that dwarf birch regenerates quickly after low- and moderate-severity fire. In Alaska, dwarf shrub-heath and low shrub communities recover rapidly after fire due to the rapid sprouting response of dwarf birch and other shrubs [6].

The few studies that address the effects of fire on dwarf birch have shown that dwarf birch may either increase or decrease after fire. In the Kenai Mountains, Alaska, prescribed burning was conducted at 17 sites between 1979 and 1984. In postfire surveys conducted in 1998 and 1999, dwarf birch cover had increased by an average of 2% across the burns. In 10 burns there was no change; in 1 burn there was a 4% decrease in dwarf birch cover; and in 6 burns there was an increase in dwarf birch cover that ranged from 1% to 10% [9]. Dwarf birch also increased after a July 1977 tundra fire on the Seward Peninsula, Alaska [73].

Percent frequency (and cover) of dwarf birch at 3 postfire successional stages [73]
  postfire year 1 postfire year 3 postfire year 24
Site 1 40 (1) 60 (1) 100 (7)
Site 2 trace (0) trace (0) 3 (10)
Site 3 trace (0) trace (0) 6 (20)
Site 4 40 (trace 60 (trace) 80 (11)
Site 5 40 (1 50 (1) 60 (6)

In other studies in similar habitats, dwarf birch decreased after fire. In tussock tundra northwest of Fairbanks, Alaska, production of leaves and buds in dwarf birch was significantly less in burned tundra than in unburned tundra both 1 year after fire (P<0.05) and 13 years after fire (P<0.01) [30].

Dwarf birch production (g/m²) in burned and unburned tundra 1 and 13 years after fire [30]
  Burned Unburned
1970 (postfire year 1) 1.0 4.8
1982 (postfire year 13) 10.8 21.2

Dwarf birch frequency was greater in unburned than in burned tundra sites 1 year after a July 1977 wildfire on the Seward Peninsula. Frequency of dwarf birch measured in May and June 1978 is given below [99].

Dwarf birch frequency (%) in burned and unburned stands 1 year after fire [99]
  Burned Unburned
late May 3 87
mid-June 8 80

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
No additional information is available on this topic.

FIRE MANAGEMENT CONSIDERATIONS:
No information on fire management of dwarf birch is available. Based on information available for bog birch, however, it is likely that prescribed burning at regular intervals can reduce dwarf birch cover [8,23,25]. In very wet places dwarf birch stands may be natural fire breaks [37].

MANAGEMENT CONSIDERATIONS

SPECIES: Betula nana
IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Numerous wildlife species eat dwarf birch. Moose and caribou browse dwarf birch in Alaska [9,55,77,84]. In south-central Alaska, dwarf birch comprises 13% of vegetation and accounts for 9% of the moose diet [79]. Dwarf birch buds, leaves, and sprouts are preferred food items for caribou in Alaska in the spring and early summer. Leaves are available until mid-September but are less palatable than willow (Salix spp.) leaves by then [77].

Ptarmigan eat the buds, catkins and twigs of dwarf birch in Alaska. Dwarf birch and bog birch buds and catkins comprised 11% of the food in rock ptarmigan crops in Alaska in spring, 12% in summer, 45% in fall, and 79% in winter. For willow ptarmigan the 2 birches comprised 0% of food in crops in spring, 3% in summer, 4% in fall, and 12% in winter [96]. Spruce grouse eat dwarf birch seeds in central Alaska [96].

Dwarf birch foliage provides forage for a number of herbivorous insects. In Alaska, the total number of herbivorous insects decreases with increases in latitude and altitude and distance from the white spruce forest zone. More detailed information on insects found on dwarf birch foliage is available in Koponen [51].

Palatability/nutritional value: Carbohydrate concentrations are high in dwarf birch stems and roots in the spring and decline with shoot and root growth. Carbohydrates increase again in the fall and are stored in roots and stems during the winter [14]. Protein content in dwarf birch foliage on the Kenai Peninsula, Alaska, ranges from 7.1% to 16.8%. Nutrient content of dwarf birch stems and leaves collected over 2 summers on the Kenai Peninsula is provided in the table below [67].

Nutrient content (ppm) of dwarf birch stems and leaves [67]
Ca K Mg Na Cu Fe Mn Zn
631-3,920 5503-5,550 646-1,730 55-74 11-15 61-86 5-109 45-64

Dwarf birch produces carbon- and nitrogen-based antiherbivore compounds that deter browsing [21].

Cover value: No information is available on this topic.

VALUE FOR REHABILITATION OF DISTURBED SITES:
Based on information available for bog birch [7], it is likely that dwarf birch has some erosion control potential.

Information on dwarf birch cultivars is available in Santamour and McArdle [75].

OTHER USES:
No information is available on this topic.

OTHER MANAGEMENT CONSIDERATIONS:
Dwarf birch is vulnerable to a variety of human impacts. In a mixed shrub-sedge tussock bog in Wrangell-St Elias National Park, Alaska, ATV use results in extensive breakage of dwarf birch stems [1]. In the Tuktoyaktuk Peninsula region, Northwest Territories, dwarf birch cover was reduced by 75% to 90% in vehicle tracks and along seismic lines associated with oil and gas exploration [41]. The presence of crude oil in the soil results in a reduction of mycorrhizal root biomass, lower respiration rates, and premature leaf senescence in dwarf birch [56]. Dwarf birch can recover from oil-induced death of the apical bud, however, by sprouting from rhizomes or layering [45].

Dwarf birch is predicted to increase with a warming climate due to increased availability of nitrogen and phosphorus [10]. Bud break is predicted to occur earlier, allowing dwarf birch to become more abundant in arctic communities over time [71]. Increased growth of dwarf birch results in greater carbon storage in woody biomass, which may offset some of the carbon released by increased decomposition in boreal soils [10].

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