Specimens of Betula glandulosa have been reported from the St. Lawrence Valley, but I have not seen them.
Wherever Betula glandulosa comes in contact with B . pumila , it forms a bewildering swarm of plants, known as B . × sargentii Dugle, having intermediate states of most vegetative characters.
Plants intermediate between Betula glandulosa and B . nana subsp. exilis make up a continuum of forms linking the typical forms of Betula nana and B . glandulosa in parts of Alaska where the ranges of these species overlap. Wherever they occur in isolation, the species remain reasonably distinct and easy to identify. In southern Greenland, Betula glandulosa hybridizes with B . nana subsp. nana and with B . pubescens .
Betula × eastwoodiae Sargent (= B . glandulosa × occidentalis ) occurs in montane meadows and marshes in Alberta, British Columbia, Northwest Territories, Saskatchewan, Yukon, Alaska, Colorado, and Wyoming, where the range of the parents overlap.
Betula × dugleana Lepage (= Betula glandulosa Michaux × B . neoalaskana Sargent) is common throughout Alaska and the Yukon, where the parent species frequently come into contact (E. Hultén 1941--1950, vol. 4; E. Lepage 1976).
Betula × dutillyi Lepage [= Betula glandulosa Michaux × B . minor (Tuckerman) Fernald] is a putative hybrid that occupies the same general range as Betula minor . Like that species, however, it has not been studied experimentally. Careful examination of the entire complex to which this taxon belongs will be necessary before any of its parts can be truly understood. Betula × dutillyi exhibits many of the same characteristics as B . minor , but it is slightly smaller in habit, and its leaves are smaller with somewhat blunter tips and more cuneate bases (E. Lepage 1976).
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g., [31,48,60,75,76,77,159,162,168]).
Resin birch is a deciduous, long-lived shrub. Plants are low and spreading to erect with 1 to several main stems. Resin birch ranges from 8 inches (20 cm) tall on upland sites and in arctic environments to 10 feet (3 m) in drainages and in more southern areas [31,44,60,76,77,77,102,137,159,168]. The bark is thin, smooth, and does not peel readily [51,75,159,168]. Leaves are thick and leathery and range from 0.2 to 1.2 inches (0.5-3 cm) long and 0.2 to 0.8 inch (0.5-2 cm) wide [75,76,77,159]. The inflorescences are catkins. Male catkins are 0.4 to 1 inch (10-25 mm) long, and female catkins are 0.3 to 0.8 inch (7-20 mm) long [31,159,168]. Fruits are narrow-winged, single-seeded samaras 1 to 1.5 mm long and wide [31,41,104]. Rhizomes are 0.8 to 2.4 inches (2-6 cm) thick and are found in the top 2.4 inches (6 cm) of soil [44]. Resin birch has an extensive root system [24,42,104]. Roots are ectomycorrhizal, an adaptation to arctic and alpine soils that are generally low in inorganic nitrogen and phosphorus [37,145].
Fire adaptations: Resin birch can survive low- to moderate-severity fires. On many sites, resin birch has deep roots and rhizomes that are protected from all but high-severity fires [12,42]. Resin birch regenerates after fire by sprouting from the root crown and from dormant buds on the rhizomes [43,44,82,112,165]. In arctic and boreal ecosystems, resin birch increases sprout production, sprout height, and aboveground biomass production during the first 1 to 2 years after a fire. Resin birch responds to top-kill by sprouting from dormant buds on the root crown and rhizomes after top-kill release. Burned plants may produce large leaves that senesce later in the fall than leaves on undisturbed plants, thereby maximizing photosynthate production [43]. Resin birch samaras are dispersed by wind and can invade burned areas from off site [11]. Although bog birch can establish from seed after fire, seedlings are susceptible to both drought and shade [43,44].
FIRE REGIMES: Resin birch is adapted to a wide range of FIRE REGIMES, from subarctic and alpine areas that seldom burn to boreal environments that burn frequently [42,44]. Wetland areas where resin birch grows burn infrequently due to the high moisture content of the vegetation and soil. These sites sometimes act as firebreaks. Fires do occur, however, during dry summers or in the spring and fall when the vegetation is dry [35,43,44,86,104,143].
In interior Alaska, resin birch is found on poorly drained and permafrost underlain sites occupied primarily by black spruce stands, muskegs, and bogs. These types are widespread in Alaska and burn frequently [154,158]. Black spruce-birch (Betula spp.) is the most widespread forest type in interior Alaska and also the type with the highest frequency of fire [158]. Native Americans were an important cause of fires in the black spruce-birch ecosystem [96]. Fire frequency increased with the increase in mining activity in the 1800s [154]. Today, most fires are lightning caused [70,95]. Between 1940 and 1969, lightning was responsible for 78% of the area burned in interior Alaska [154].
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 [55,154]. Fires can occur, however, whenever fuels are not covered with snow and are exposed to sufficiently warm temperatures and drying winds [154].
Fire years are sporadic in occurrence but tend to occur at least once every decade [71]. ÂExceptional 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 [160]. The average acreage burned each year in interior Alaska is approximately 1 million acres [96]. Fires tend to be large and may spread over thousands to hundreds of thousands of acres or more [71,94,150].
Estimated fire-return intervals in the black spruce-birch ecosystem vary from 50 to 200 years [71,160]. Fires occur every 50 to 70 years in black spruce-white spruce/bog birch/reindeer lichen communities in interior Alaska [55]. Heinselman [71] 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 [4,97].
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 [94,95,155]. There is often nearly continuous fuel from the forest floor to the tree crowns [160]. 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 [50,71,150,160].
The following table provides fire return intervals for plant communities and ecosystems where resin birch is important. Find fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Fire-return intervals for plant communities with resin birch Community or Ecosystem Dominant Species Fire Return Interval Range (years) birch Betula spp. 80-230 [142] tamarack Larix laricina 35-200 [113] Great Lakes spruce-fir Picea-Abies spp. 35 to >200 northeastern spruce-fir Picea-Abies spp. 35-200 [50] Engelmann spruce-subalpine fir Picea engelmannii-Abies lasiocarpa 35 to >200 [7] black spruce Picea mariana 35-200 conifer bog* Picea mariana-Larix laricina 35-200 [50] jack pine Pinus banksiana <35 to 200 [34,50] Rocky Mountain lodgepole pine* Pinus contorta var. latifolia 25-340 [16,17,144] aspen-birch Populus tremuloides-Betula papyrifera 35-200 [50,161] quaking aspen (west of the Great Plains) Populus tremuloides 7-120 [7,62,103] Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [7,8,9] *fire return interval varies widely; trends in variation are noted in the species reviewPrescribed burning can reduce resin birch cover. Naturally occurring fires controlled the spread of resin birch on Canadian Rocky Mountain rangelands prior to active fire exclusion. Today, prescribed fires are used to reduce the spread of resin birch and other shrubs and to restore and maintain native grasslands [45]. The effects of prescribed burning on resin birch vary depending on burning conditions, fire season, severity, and postfire growing conditions. Burning resin birch stands in spring, when carbohydrate reserves are lowest, apparently promotes postfire sprouting and growth. Increased fire severity and fall burning both increase mortality in resin birch [43].
Prescribed burning at 3- to 6-year intervals in the Rocky Mountain foothills of Alberta has decreased shrub cover and increased forage production [45]. Resin birch cover decreased by 35% following a moderate-severity, prescribed spring fire in wood bison habitat in Fort Providence, Northwest Territories. After 3 months, resin birch cover increased by 26% [59]. Due to resin birch's "vigorous" sprouting response, burning at regular intervals is necessary to minimize its regrowth [29].
Fuel potential of resin birch is low because leaf moisture content is high [143]. Moisture content of resin birch measured near Inuvik, Northwest Territories, is given in the table below [164].
Moisture content (%) of resin birch in dry and wet tundra [164] 18 July 1 August 15 August Dry tundra 44 47 47 Wet tundra 56 50 51Resin birch occupies a wide variety of sites, ranging from rocky subarctic and alpine tundra to deep, organic, boreal soils [44]. It is typically a wetland species occurring most commonly on moist, acidic, and nutrient-poor organic sites including fens, swamps, bogs, muskegs, wet meadows, lake and stream margins, and seepage areas [22,31,48,60,75,76,77,159,168]. Bog birch is also found on upland sites including eskers, till ridges, rock outcrops covered with shallow soil, cliffs, sandy hillsides, and rocky ridges [5,31,51,82,137]. It dominates open valley bottoms in the Canadian Rocky Mountains [43] and is the most common shrub at treeline in interior Alaska, forming a nearly continuous zone between the treeline and alpine tundra in many areas [156].
Elevation: Resin birch occurs between 1,300 and 11,000 feet (400-3,400 m) across its range [66,75,79,80,123,125,168]. Elevational ranges are summarized below.
Elevational ranges for resin birch by state or province
State Elevation (feet) California 6,500-7,500 [75,125] Sierra Nevada 6,500-8,500 [79] Colorado 5,700-11,400 [46,66,80] Montana 4,900-8,000 [46] Utah 6,000-11,000 [46,168] Wyoming 6,400-10,500 [46] Nova Scotia 1,300 [123]Temperature: Resin birch is tolerant of cold temperatures. It is common in black spruce forests in the Yukon where the mean annual temperature is 27 °F (-3 °C) [6]. Frost tolerance in resin birch is high, and resin birch grows abundantly over large areas of permafrost [87]. Resin birch tolerates severe winter temperatures by withdrawing water from the protoplast and freezing it in the cell walls [25].
Moisture: Although it is primarily a wetland plant, resin birch does not appear to tolerate continuous flooding. In bogs near Fairbanks, Alaska, resin birch abundance decreases as soil moisture increases. Resin birch is also more "vigorous" in communities that support taller tussocks [32]. In the Cariboo Forest Region of British Columbia, resin birch is common in wetlands that have no standing water late in the season [139]. In Montana, however, the water table is often within the rooting zone of resin birch throughout the summer, and resin birch grows in soils that remain flooded until midsummer or are saturated year-round [64]. In a willow (Salix spp.)-resin birch community near Churchill, Manitoba, the depth of the water table averaged 3 inches (6.5 cm) below the surface, and soil moisture in the organic layer was 63% [24]. Resin birch is an indicator of "substantial groundwater" in the North Thompson River valley, British Columbia [98].
Annual precipitation ranges from 4 to 9 inches (109-230 mm) on 2 northern Canadian study sites where resin birch is abundant [6,24]. While some authors describe resin birch as drought intolerant [141], in a review of the literature de Groot and others [41] state that resin birch appears tolerant of periodic drought.
Soils: Resin birch grows in a variety of soils, ranging from sandy and gravelly loam on river terraces to poorly drained, organic soils in bogs, muskegs, and other wetland habitats [43,64,114,121,141]. It is tolerant of moderate salinity [24] and pH ranging from 3.1 to 6.5 [105,141].
Resin birch is only lightly to moderately browsed by most classes of livestock [19,107]. It accounted for 2.7% of summer cattle forage, for example, on the Red Rock Lakes National Wildlife Refuge in Montana [47]. Browse production may be moderate to high in some resin birch communities. However, cattle tend to avoid the boggy soils associated with this species unless the soil becomes dry enough to walk on, usually in late summer [40,64,86,104]. Cattle eat resin birch in riparian wet meadows in the southern Blue Mountains, Oregon [120].
Numerous wildlife species eat resin birch, including moose, mule deer, white-tailed deer, Rocky Mountain elk, mountain goats, caribou, grizzly bears, American black bears, small mammals, birds, and insects [14,68,74,81,91,109,126,146,159]. Resin birch is a "preferred" browse species for game animals in Teton County, Wyoming [18]. It is dominant in tamarack swamps in southwestern Manitoba. These swamps provide habitat for moose, jumping mice, northern river otters, shrews, Canada jays, black-capped chickadees, white-throated sparrows, and Connecticut warblers [22].
Moose: Resin birch accounted for 11.8% of summer and 0.7% of winter moose forage on the Red Rock Lakes National Wildlife Refuge [47]. It is preferred browse in Banff and Jasper National Parks, Alberta [53], but is not preferred by moose in Alaska [30].
Caribou: Buds, leaves, and sprouts of resin birch are preferred foods for caribou in Alaska in the spring and early summer. The rumens of 6 caribou examined in mid-June contained almost exclusively bog birch. Caribou eat the leaves extensively into June and July, but by mid-September the leaves are less palatable than willow (Salix spp.) leaves [135,159]. Caribou also eat resin birch in summer and winter in northern Canada [20,36,72,128]. Heavy browsing by the Rivière George caribou herd in northern Quebec depleted winter carbohydrate reserves in resin birch, leading to decreased resin birch growth in spring [36].
Birds: Several species of ptarmigan and grouse eat resin birch in Alaska, Canada, and the contiguous northern United States [107,159]. Sharp-tailed grouse and greater prairie-chickens eat resin birch buds in Wisconsin in the winter [127], and spruce grouse eat resin birch seeds in central Alaska [163]. Resin birch and resin 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 [163].
Small mammals: American beavers eat resin birch [109]. Resin birch is a preferred winter food of snowshoe hares in the southwestern Yukon [39,88,122,136]. Eastern heather voles eat resin birch bark in the winter in Canada [57]. White spruce/resin birch communities in the Kluane Region, Yukon, provide habitat for a number of small mammals including deer mice, northern red-backed voles, meadow voles, and heather voles [89].
Fish: Resin birch provides overhanging shade and cover for fish along low-gradient streams in western Montana [28].
Insects: Insect herbivores can cause "moderate" damage to resin birch. During the 1976 to 1980 growing seasons, resin birch plants in northern Quebec lost 20% to 50% of leaf biomass to insects [118]. In Alaska, the total number of herbivorous insects decreased with increases in latitude and altitude and distance from the white spruce forest zone. More detailed information on insects found on resin birch foliage is available [85].
Resin birch importance rankings for 9 ungulate species in British Columbia are provided below.
Importance of resin birch in the diets of ungulates in British Columbia [27] Sitka black-tailed deer low mule deer low white-tailed deer low mountain goat low bighorn sheep low Roosevelt elk low Rocky Mountain elk moderate moose high caribou moderatePalatability/nutritional value: The palatability of resin birch in several states is as follows
Palatability of resin birch for livestock and wildlife [28,38,46,67,125]: California Colorado Montana Wisconsin Wyoming cattle poor fair poor ---* fair domestic sheep fair-poor fair fair --- fair horses poor poor poor --- fair white-tailed deer --- --- poor --- --- mule deer --- --- poor --- --- moose --- --- -- --- high elk --- --- poor --- --- pronghorn --- --- poor --- --- rabbits --- --- --- high --- * No data available.The energy and protein values of resin birch are low [28]. Sugar content in resin birch leaves declines in late summer. Nitrogen concentration in leaves peaks early in spring then declines throughout the growing season [118]. Nutritive values measured in resin birch plants near Inuvik, Northwest Territories, are given in the table below [129,130].
Nutritive values in resin birch twigs and leaves [129,130] Plant part Month Cu Mo Fe Mn Zn K Mg Ca P crude fat crude fiber crude protein ppm % twigs July 2.8 0.23 50 157 87 0.70 0.20 0.34 0.08 8.9 26.1 3.5 twigs Aug. 4.6 0.21 161 67 178 0.21 0.10 0.41 0.09 10.7 27.4 5.4 twigs Nov. 3.7 0.31 332 121 206 0.09 0.11 0.62 0.06 4.9 33.7 4.2 twigs Feb. 4.6 0.21 205 78 160 0.18 0.10 0.47 0.06 9.3 30.6 4.9 twigs May 4.7 0.23 102 92 152 0.23 0.10 0.47 0.09 9.8 28.5 6.0 leaves May 4.4 0.21 83 151 108 0.66 0.34 ---* --- --- --- --- leaves July --- --- --- --- --- --- --- 0.44 0.13 7.3 12.7 10.4 leaves Aug. --- --- --- --- --- --- --- 0.63 0.15 7.9 15.8 12.1* No data available.
Resin birch produces carbon and nitrogen-based antiherbivore compounds that deter browsing [41]. Sugar and nitrogen content is highest in the leaves in early spring. Resin birch allocates the greatest portion of its photosynthate to the production of antiherbivore phenolics at that time; otherwise, leaves would be susceptible to browsing insects [118].
Cover value: The table below summarizes thermal or feeding cover values of resin birch.
Cover values of resin birch for wildlife in 3 western states [28,46] Colorado Montana Wyoming elk good poor poor mule deer poor poor poor white-tailed deer ---* poor poor upland game birds good fair fair waterfowl good poor small nongame birds good fair good small mammals good fair good * No data available.Resin birch provides cover for willow, rock, and white-tailed ptarmigan in southwestern Yukon [115]. Grizzly bears in the central Canadian Arctic constructed their dens under resin birch cover more than any other plant species. Resin birch was present at 84% of 52 den sites, and it was the highest in percent cover around den entrances. Resin birch roots formed ceilings of several dens studied [102].
In the boreal forests of interior Alaska and Canada, resin birch is found in many
black spruce (Picea mariana) and white spruce (P. glauca)
communities and is especially common at the northern and altitudinal limit of
trees [3,6,117,152,154,159]. In these
northern environments, permafrost prevents the percolation of water, resulting
in the development of muskegs, bogs, and ponds that often impede the growth of
trees but support resin birch and other low-growing shrubs [55,159].
Resin birch is characteristic of many mixed shrub and tussock tundra
communities in Alaska and northern Canada [1,2,10,149]. In southwestern Canada and the contiguous United States,
resin birch often occurs on wetland sites including bogs, fens and carrs, within
lodgepole pine (Pinus contorta), Engelmann spruce (P. engelmannii),
or subalpine fir (Abies lasiocarpa) forest types and is often associated
with alders (Alnus spp.) and willows (Salix spp.) [15,22,84,86,110].
Resin birch is listed as a dominant species in the
following vegetation classifications:
black spruce/resin birch/feather moss (Hylocomium spp.) vegetation type
black spruce/resin birch-marsh Labrador tea/sphagnum (Ledum palustre/Sphagnum spp.) vegetation type
black spruce-white spruce/resin birch vegetation type [152]
black spruce-white spruce/resin birch/feather moss community [157]
black spruce-white spruce/resin birch/reindeer lichen (Cladonia spp.) community [55]
black spruce-white spruce/thinleaf alder (Alnus incana ssp. tenuifolia)-resin birch/Schreber's big
red stem moss (Pleurozium schreberi) vegetation type
resin birch vegetation type
resin birch-blueberry/rough fescue (Vaccinium spp./Festuca altaica)/feather moss-lichen vegetation type
resin birch-blueberry/Bigelow's sedge (Carex bigelowii) vegetation type
resin birch-bog blueberry-black crowberry (Vaccinium uliginosum-Empetrum nigrum)-
marsh Labrador tea/lichen vegetation type
resin birch-bog blueberry/sedge (Carex spp.)/sphagnum vegetation type
resin birch-bog rosemary (Andromeda polifolia)/sphagnum vegetation type
resin birch-diamondleaf willow (Salix planifolia)-bog blueberry vegetation type
resin birch-mountain cranberry-cloudberry (Vaccinium vitis-idaea-Rubus chamaemorus)/sphagnum vegetation type
resin birch-marsh Labrador tea-blueberry vegetation type
resin birch-marsh Labrador tea/mountain cranberry/wideleaf polargrass (Arctagrostis latifolia) vegetation type
resin birch/Schreber's big red stem moss-mountain-fern moss (H. splendens) vegetation type
resin birch-sweet gale (Myrica gale)-bog rosemary/sphagnum vegetation type
resin birch-sweet gale/sedge/sphagnum vegetation type
resin birch-willow (Salix spp.)-marsh Labrador tea/Bigelow's sedge/feather moss-lichen vegetation type
resin birch-willow-thinleaf alder vegetation type
mountain alder (Alnus viridis subsp. crispa)-resin birch-marsh Labrador tea/sphagnum vegetation type
paper birch/resin birch/feather moss vegetation type
shrubby cinquefoil (Dasiphora fruticosa ssp. floribunda)-
sweet gale-resin birch-black crowberry/sphagnum vegetation type
shrubby cinquefoil-sweet gale-resin birch-marsh Labrador tea/feather moss vegetation type
white spruce/resin birch/feather moss-reindeer lichen vegetation type
white spruce/resin birch/reindeer lichen vegetation type
white spruce/resin birch/sphagnum vegetation type
white spruce/resin birch/mountain-fern moss vegetation type [152]
diamondleaf willow-resin birch shrub association [13]
willow-resin birch type [100]
Wolf's willow (S. wolfii)-resin birch-shrubby cinquefoil shrub association [13]
resin birch/woollyfruit sedge (C. lasiocarpa) community type [33]
resin birch dominance type [64]
resin birch/beaked sedge (C. rostrata) community type [28,116]
resin birch/tufted hairgrass (Deschampsia caespitosa) community type [116]
resin birch/woollyfruit sedge community type [33]
grayleaf willow (S. glauca)-resin birch-shrubby cinquefoil shrub type [63]
resin birch-kinnikinnick (Arctostaphylos uva-ursi) shrub carr association [139]
white spruce/willow (Salix spp.)-resin birch zone [111]
resin birch/star reindeer lichen (Cladonia alpestris)-moss community (Kershaw 1984, cited in [65])
birch (Betula spp.) vegetation association [83]
balsam poplar (Populus balsamifera)/resin birch/arctic lupine (Lupinus arcticus) vegetation type
black spruce/resin birch-marsh Labrador tea vegetation type [138]
resin birch/rough fescue community [49]
resin birch-bog blueberry-reindeer lichen vegetation type
cloudberry-resin birch-bog blueberry vegetation type
lichen-resin birch-marsh Labrador tea vegetation type [138]
white spruce/resin birch/water sedge (Carex aquatilis) community [49]
Resin birch decreases with grazing. Resin birch cover was significantly (P=0.01) greater on ungrazed
sites (88%) than on grazed sites (47%) within the summer range of the
Rivière George caribou herd in
northern Quebec and Labrador, Canada. Browsing and
trampling by caribou have opened the closed canopy of resin birch and reduced leaf
biomass by 60% [99]. Resin birch plants heavily browsed by snowshoe hares near Kluane, Yukon,
exhibited rapid growth of new twigs when hare numbers declined [136].
Expanding resin birch populations
on Canadian Rocky Mountain rangelands reduce forage for elk, bison, and
other grazing animals. Removal of bog
birch increases the production of forage grasses [43].
In resin birch, leaf growth begins soon after snow melt, and growth continues throughout the growing season as shoots elongate [41]. Male catkins develop in late summer or fall and expand with or before leaf development the following spring. Female catkins appear with the leaves in the spring [78,104]. Flowering dates vary and are summarized below.
Fruits mature between July and October and can persist through the winter [104,106,159,166]. Samara dispersal occurs in the fall, just prior to snow fall, and in the following spring soon after snow melt [78,166]. Leaves begin to senesce in late summer, and leaf abscission is complete by late September [118].
Phenological stages for resin birch in a valley-bottom floodplain in west-central Alberta are summarized below.
Seasonal development of resin birch in west-central Alberta [44] 5 May most plants initiating leaf-break 10 June male catkins dropped; female catkins small and turning darker green 29 June female catkins at mature size 11 Aug. female catkins brown; terminal buds large 1 Sept. half of leaves on most plants yellowFire has a substantial influence on resin birch growth and population dynamics [43,44]. Resin birch survives most low- and moderate- severity fires by sprouting from the root crown and/or rhizomes after top-kill by fire [43,44,82,112,165]. It flowers "profusely" from young sprouts [11] and produces large leaves after burning. Resin birch leaves were up to 3 times larger 1 year after fire than leaves on unburned plants near Inuvik, Northwest Territories, a response that may be linked to the increase in available nutrients following the fire [164,165]. A large proportion of phosphorus released into the soil after fire is absorbed by the roots of resin birch and then incorporated into new stem and leaf tissue. Changes in resin birch root biomass, root phosphorus concentration, and root phosphorus mass with burning of a mature 140-year-old black spruce/star reindeer lichen woodland at Schefferville, Quebec, were as follows [11]:
Resin birch increases after low- to moderate-severity fires [164]. Repeated fires near treeline and on some wet sites in Alaska and northern Canada result in thickets of resin birch, mountain alder, and willows (Salix spp.) [82,154]. On tundra sites near Inuvik, Northwest Territories, total vascular plant cover on a burned area was more than twice that on an adjacent unburned area. The increase was due in large part to resin birch, which increased 8.8% after the burn [92].
Because of its ability to sprout from the root crown and rhizomes, resin birch is among the first plants to regenerate after fire in many communities [26,43,44,69,105,149,150]. Resin birch also persists into middle and late successional stages [23,105,133,149,150]. It was present in all postfire successional stages observed in a black spruce/reindeer lichen woodland in northern Quebec, but was most abundant in the intermediate stages between approximately 20 and 50 years after fire [56]. Frequency of resin birch at each successional stage is summarized below.
Frequency (%) of resin birch at 4 postfire stages [56] postfire year 5 postfire year 20 postfire year 50 postfire year 90 24 63 69 1In subarctic black spruce forests of western Labrador, resin birch was most abundant 18 to 40 years after fire. Mean canopy volume of resin birch between 2 and 140 years after fire is summarized below [132].
Mean canopy volume (m³) of resin birch across 5 postfire successional stages [132] postfire year 2 postfire year 18 postfire year 40 postfire year 80 postfire year 140 0.01 2.23 1.04 1.01 0.00Low-severity fire and spring burning promote sprouting in resin birch. In a study conducted during the 1992 growing season in the Rocky Mountains of Alberta, resin birch plants were burned in low-, medium-, and high-severity treatments. Plants burned earlier in the growing season and in low-severity treatments produced more and taller sprouts by the end of the first year after burning than plants burned late in the growing season or in severe fire treatments. Resin birch in the high-severity treatments sprouted latest. Following high-severity fire, new sprouts originated from the bottoms of rhizomes, indicating mortality of buds closer to the soil surface. No sprouting occurred on plants burned after late June, which may be related to seasonal variation in plant hormones that release buds from dormancy and promote stem extension in resin birch. Fall burning resulted in greatest plant mortality than spring and summer burning. Some plants burned in the fall sprouted the following year [43,44].
Resin birch was more abundant in "lightly" burned areas than in "heavily" burned areas following a June 1971 wildfire in black spruce forest near Fairbanks, Alaska [151]. Density of resin birch for 4 years following the fire is provided below.
Resin birch density (stems/ha) after wildfire in heavily and lightly burned areas [151] postfire year 1 postfire year 2 postfire year 3 postfire year 4 heavy 125 1,625 750 1,625 light 1,125 4,500 1,750 3,375The response of resin birch to fire in a valley-bottom floodplain in the Rocky Mountains of Alberta varied with fire severity. Resin birch stem density increased for 2 years after a spring prescribed, low-severity fire in 1984 due to abundant sprouting. Following high-severity burns in 1987 and 1993, however, both stem density and canopy cover sharply declined. Results of this study are given in the figure below [29].
Although survival of resin birch plants decreases when fire severity is high, seedlings establish more easily on the bare mineral soil that is exposed after a high-severity fire [23,42].
On some sites, including in Wisconsin fens, resin birch increases in the absence of fire [38]. In the Rocky Mountains of Alberta, resin birch forms extensive, closed-canopy stands where fire has been excluded [29].
Resin birch reproduces by seed and vegetatively by branch layering and sprouting [26,44]. Reproduction by seed is more common in southern populations, and vegetative reproduction is more common in northern populations [73,166,167].
Pollination: Resin birch is wind pollinated. In a resin birch population on Baffin Island, Northwest Territories, female catkins were smaller and contained 50% fewer flowers than were contained in female catkins from a more southern site in subarctic Quebec. There was an estimated 10-fold difference in pollen dispersed between the 2 sites. At the northern extent of its distribution, resin birch is clonal, and the distance between genetically distinct individuals is great. In these areas, female catkins are more likely to receive incompatible pollen, preventing fertilization from occurring [167].
Breeding system: Resin birch is monoecious [64,104]. Plants are not self fertile [167].
Seed production: Resin birch produces numerous catkins, each of which yields 30 to 50 samaras [166]. Seed production is generally high in more southern parts of its range [30,42,73]. In more northern areas, production of viable seed is limited by the shorter growing season, lower temperatures, and distance between genetically distinct individuals [166].
Seed dispersal: Resin birch seeds are dispersed in their samaras. Wind, water, and sometimes gravity disperse the samaras. Samaras may blow across crusted snow [11,44,104].
Seed banking: Resin birch produces numerous, tiny seeds and has a transient seed bank. In a review of the literature, Karrfalt [41] states that birch seeds may be abundant in the soil but the seeds are generally short lived. Rowe 1983 [124] states that viable resin birch seeds are "rare" in the soil seed bank. Resin birch seeds were present, however, in the first 1.2 inches (3 cm) of soil collected from alpine sites on the Gaspé Peninsula, Quebec [106]. Results of this study are provided in the table below.
Resin birch seed production and density on sites in Quebec [106]
Site Total seeds/m² Viable seeds/m² % cover in aboveground vegetation 1 3 0 0 2 275 0 18 3 1,263 13 10 4 1,003 6 10 5 6 0 0Germination: Prechilling improves germination of resin birch seeds. Optimum germination temperature for many arctic species is 59 to 86 °F (15-30 °C) [26]. The germination rate of resin birch seeds collected from alpine sites in the White Mountains, New Hampshire, was 25% for refrigerated seeds and 4% for unrefrigerated seeds. Days required for germination ranged from 14 to 28 for refrigerated seeds and from 27 to 299 for unrefrigerated seeds [108].
Seed viability varies with latitude. At the northern range limit of resin birch on Baffin Island, <0.5% of seeds were viable. Very few samaras contained seeds with fully developed embryos. At a southern site in subarctic Quebec, 70% of seeds were viable [166,167]. Seeds that overwinter on plants remain viable until they disperse the following spring [166].
Seed germination and samara weight may be correlated. In a germination study in Kuujjuaq, Quebec, no seeds from samaras weighing <0.09 mg germinated, few samaras weighing <0.12 mg had seed that germinated, and all samaras weighing >0.34 mg had seed that germinated [166]. Germination of wind-dispersed seeds may be highest on exposed mineral soils [104].
Seedling establishment/growth: Seedling recruitment rates in resin birch populations are usually very low. Site disturbance by fire increases the likelihood of seedling establishment [44]. Although recruitment from seed is almost nonexistent in northern resin birch populations, plants of all age classes were evident in a southern Quebec population [73,166]. Seedling growth is very slow, and seedling mortality is often high [41,44].
Vegetative regeneration: Resin birch reproduces vegetatively by branch layering and sprouting from dormant buds on the root crown and rhizomes [26,44]. Resin birch is clonal in the northern parts of its range [166].
Resin birch is shade intolerant [42,44,87]. It is characteristic of canopy openings in black spruce woodlands in boreal Canada [3]. It establishes from seed or, more commonly, by sprouting after fire and other disturbances [23,43,148,149] and in many communities persists through subsequent successional stages. In many black spruce communities in central Alaska and northern Canada, resin birch appears soon after low- to moderate-severity fires and is dominant in the vegetation 6 to 25 years after fire. Trees begin to dominate after 25 to 30 years, but the low shrub layer of resin birch and associated species continues to expand and increase in cover [149,150]. In black spruce woodlands in the Northwest Territories, resin birch is most common 15 to 20 years after fire but is also present in stands as old as 300 years [23].
The table below summarizes an analysis of 5 stands representing a vegetation chronosequence on gravel outwash of the Muldrow Glacier in Denali National Park, Alaska. Resin birch was not present in the earliest successional stage but was abundant in intermediate stages and persistent in the oldest stands [153].
Frequency (%) and cover (%) of resin birch at 5 successional stages [153] Successional stage Frequency Cover Pioneer stage (25-30 years) 0 0 Meadow stage (100 years) 80 <5 Early shrub stage (150-200 years) 100 50-75 Late shrub stage (200-300 years) 100 50-75 Climax tundra (5,000-9,000 years) 100 25-50The scientific name of resin birch is Betula glandulosa
Michx. (Betulaceae) [31,54,60,61,75,76,77,137,159,168].
Resin birch hybridizes with arctic resin birch (Betula nana subsp. exilis and
Betula nana subsp. nana) where
their ranges overlap [54,77,159]. Resin birch
also hybridizes with paper birch (Betula papyrifera) in interior Alaska [159].
Numerous other hybrids have been described including:
Betula ÃÂ sargentii Dugle (B. nana ÃÂ B. pumila)
Betula ÃÂ eastwoodiae Sargent (B. nana ÃÂ B. occidentalis) [31,51,54]
Betula ÃÂ dugleana Lepage (B. nana ÃÂ B. neoalaskana)
Betula ÃÂ dutillyi Lepage (B. nana ÃÂ B. minor, a
putative hybrid) [54]
The erosion control potential for resin birch is high. In Montana, the dense underground network formed by resin birch and rhizomatous sedges help stabilize streambanks [28]. Because resin birch grows slowly, its short-term (1-3 years) revegetation potential is low. Resin birch is, however, suitable for long-term (>3 years) revegetation of exposed mineral soil [28,101].
Black spruce seedling survival after fire in the boreal forest may be facilitated by shading from resin birch and other shrubs that reproduce vegetatively and grow quickly [134].
Betula glandulosa, the American dwarf birch, also known as resin birch or shrub birch, is a species of birch native to North America.
American dwarf birch is a multi-stemmed shrub typically growing to 1–3 m (3.3–9.8 ft) tall, often forming dense thickets. The trunks are slender, rarely over 5–10 cm (2–4 in) diameter, with smooth, dark brown bark. The leaves are nearly circular to oval, 0.5–3 cm (1⁄4–1+1⁄8 in) long and 1–2.5 cm (3⁄8–1 in) broad, with a toothed margin. The fruiting catkins are erect, 1–2.5 cm (3⁄8–1 in) long and 5–12 mm (3⁄16–15⁄32 in) broad.
It is closely related to the dwarf birch (Betula nana), and is sometimes treated as a subspecies of it, as B. nana subsp. glandulosa. It is distinguished from typical B. nana by the presence of glandular warts on the shoots and longer leaf petioles. Hybrids with several other birches occur.[2]
This plant occurs in arctic and cool temperate areas from Alaska east to Newfoundland and southern Greenland, and south at high altitudes to northern California, Colorado, and the Black Hills of South Dakota in the west,[3] and locally south to northern New York in the east. In the Arctic, it occurs down to sea level, while in the south of the range, it grows as high as 3,400 metres (11,200 ft) altitude.
It is typically a wetland species, growing in bogs, muskegs, or other moist and nutrient-deficient soils, but can be found in a wide variety of sites, ranging from rocky subarctic locales to boreal forests with deep, organic-rich soils.[2] It is the most common shrub at treeline in interior Alaska, forming a nearly continuous zone between the treeline and alpine tundra.[2]
Many species of wildlife rely on this shrub as a food source, particularly ungulates such as moose, deer, caribou and elk.[2] Its preference as a food source varies depending on the specific population; moose in Alberta, for example, are known to graze on it heavily, whereas moose in Alaska are observed to eat it at low levels.[2] Bears, small mammals, birds, and insects also rely on the shrub as a food source.[2]
American dwarf birch provides cover for ptarmigans, and grizzly bears use it consistently to construct their dens.[2]
This species is valued for its erosion control potential, and has been used to help stabilize stream-banks.[2]
Betula glandulosa, the American dwarf birch, also known as resin birch or shrub birch, is a species of birch native to North America.
