Forest Products Commercial Species Alaska Paper Birch (Betula papyrifera, var. humilis) Written By: Eugene E. Wheeler, Retired USDA Forest Service Program Manager for Cooperative Forestry Programs, (Alaska) Region 10. Currently a consultant for Idaho Panhandle Forestry. Introduction This booklet contains a brief discussion of the botanical and wood structural characteristics, forest associations, major pests and problems, and some of the commercial uses of paper birch. The booklet is nontechnical and provides an introduction to paper birch, and the Alaska forest environment in which it grows. There are two varieties of paper birch which have potential commercial significance in interior and central Alaska, the Alaska paper birch and the Kenai paper birch. In addition, one variety, western paper birch, exists in southeast Alaska. However, because of similarities in their wood and botanical characteristics, the three varieties will be treated as one (Alaska paper birch-Betula papyrifera, var. humilis) in this booklet. Alaska paper birch (Betula papyrifera, var. humilis) is one of only four hardwood species growing in Alaska which have a potential commercial value. The birches of Alaska's interior boreal (taiga) forests which stretch from the Kenai Peninsula across the Alaska Range to the south slopes of the Brooks Range, including the drainages of the Yukon, Kuskokwim and Copper River systems, are the hardest of the Alaskan hardwoods. The most extensive stands are to be found in the upper Cook Inlet forests. Paper birch is normally found in association with other tree species such as white spruce on the low rolling slopes and benchlands below 1,500 feet (457.4 meters) elevation. It will also occur as pure stands following fires, regenerating from either seed or sprouting. Paper birch is one of the few hardwood species found in Alaska. In comparison with hardwoods such as oak, its nearly colorless wood is moderately light in weight and is straight grained with a fine, even texture which makes it easy to split or work. These characteristics make it well suited for speciality products such as spools, toothpicks, tongue depressors and chopsticks in which hardness or decorative appearance are not of prime consideration. It is now recognized as potentially valuable for other uses such as cabinetry, furniture, and flooring where hard finishes can protect the surfaces. The Climate Interior Alaska is a moderately dry area with extreme temperatures. Total precipitation varies from 6 inches to 25 inches (15.2 to 63.5 centimeters). Summer temperatures range from 35°-100°F (2°-38°C), while winter temperatures can drop below -70°F (-57°C). The growing season is short (90 to 125 frost-free days). However, long periods of daylight (20-24 hours) provide the solar energy required for tree growth. Growth is rapid on the favored, well-drained soils and 20 inch leaders are not uncommon. Birch does not normally occupy those soils underlain with permafrost and is often used as an indicator of frost-free soils. Permafrost, forming under moss and timber mats that inhibit solar energy from reaching the soils, sets a physical barrier to root penetration, limiting access to soil nutrients and impeding growth. The Forest The boreal forests (taiga) of Alaska occur from the Kenai Peninsula to the southern slopes of the Brooks Range and from the Canadian border to the Bering Sea. White spruce (Picea glauca), paper birch (Betula papyrifera) and aspen (Populus tremuloides) are the main species on the warmer, well-drained sites. Mixtures of balsam poplar (Populus balsamifera), black cottonwood (Populus trichocarpa), white spruce, black spruce (Picea mariana) and eastern larch (Larin laricina) develop on the bottomlands and flood plains of the many rivers. Various willows and small alders also grow throughout the forest. Approximately 105.8 million acres (42.8 million hectares) of Alaska are classified as boreal forest. Of this, 22.5 million acres (9.1 million hectares) are presently considered commercial forest (capable of producing 20 cubic feet or more of wood per acre, per year). The total net sawtimber volume of all species in Interior Alaska is estimated at 31 billion board feet (5.4 billion cubic feet), with birch accounting for 8% of this volume. Bottomland Spruce-Hardwood Association The bottomland spruce-hardwood forest types, consisting of variable width strips along the major rivers, account for approximately 17; (18 million acres or 7.3 million hectares) of the boreal forest. These are the most productive sites of that vast forest area. The high productivity of these sites is due, in part, to the frequent flooding of the rivers, which adds nutrients to the soil and removes the accumulated litter layer. Very little permafrost exists in the river bottom soils which are well drained and support vigorous vegetation. However, productivity of the bottomland spruce-hardwood forest sites does vary considerably throughout the boreal forest area. The most productive commercial forest sites are located near Fairbanks on the Tanana River. The least productive sites are those adjacent to the Porcupine River northeast of Fort Yukon. Upland Spruce-Hardwood Forest Types The upland spruce-hardwood forest types account for approximately 16% of the boreal forest (64.4 million acres or 26 million hectares). These types occupy sites to 1,000 feet (304.9 meters) in elevation along the lower Yukon River and up to 3,500 feet (1,067.2 meters) near the Alaska-Yukon Territory border. The average productivity of upland forests is less than that of the bottomland forest sites. Factors which contribute to this lower productivity include lower soil nutrient levels, cooler temperatures, and lower precipitation. The Tree Paper birch (Betula papyrifera), including varieties and hybrids is the predominate species on over five million acres. Birch is most commonly associated with spruce and aspen, however, it may seed or sprout as a pure stand following fire. In those situations, it is usually a pioneer species and will be replaced with spruce when the birch trees become decadent. Birch is a small to medium sized tree that matures in 80 to 100 years. It will attain heights of 60 to 80 feet and diameters of 12 to 18 inches. Trees reaching 24 inches in diameter are not uncommon on the better sites, although average stand diameters are more likely to be in the 8 to 10 inch range. Paper birch, commonly known as white birch, gets its name from its unique bark which separates into paper thin layers. The bark has a creamy, white, chalk-like color, often tinged with pink, yellow or grey. Wood Properties The sapwood of paper birch is whitish to pale yellow while the heartwood is light brown to reddish-brown. The wood is without any characteristic odor or taste, diffuse/porous (growth rings not very distinct), and has a fine, even texture that takes stains and finishes well. The wood of paper birch is moderately light in weight, moderately strong in bending and compression strength, stiff, moderately hard, high in shock resistance, and has a very large shrinkage. It is ranked as moderately easy to kiln dry, highly stable, difficult to glue, but has a high nail-holding capacity. Seasoned paper birch lumber gives good service in ordinary construction, although it is considered very low in decay resistance. The wood is diffuse/porous in structure, has a fine, even texture and takes finishes and stains well. The wood works easily and the straight uniform grain is most satisfactory for uses such as veneers and plywoods. Seasoning Paper birch is generally easy to dry and can be air dried to some extent in the interior region where it grows. Recommended kiln drying schedules begin with dry-bulb temperatures of 130° to 140°F and wet-bulb depressions of 5° to 7°F. Kiln drying times vary from 3 to 10 days. Other Properties The following wood properties are averages derived from previous reports concerning paper birch. Figures have been derived from tests on Alaska birch from the Anchorage area. They are published herein as guidelines. Variables are caused by growing and handling conditions. I. Weights A. Solid Wood - Average weight in pounds per cubic foot (kilograms per cubic meter). Wood lbs./cu.ft. kg./cu. m. Green (58% moisture content) 48 768.9 Seasoned (12% moisture content) 38 608.7 B. Logs (Approximated) Average weight per log in pounds per thousand board feet (MBF). (kilograms) Sealing Diameter Green (lbs.) Green (kg.) Dry (lbs.) Dry (kg.) 12" (30.5 cm.) 3,100 5,942 10,200 4,627 18" (45.7 cm.) 9,200 41,731 7,200 3,266 C. Lumber - Average weight in pounds (kilograms) per MBF. Wood lbs. kgs. Green 4200 1,905 Air Dry 3,250 1,474 II. Specific Gravity - Based on vlume when green and weight when ovendry. Wood Specific Gravity Green (50% MC*) 0.49 Seasoned (12% MC) 0.55 III. Shrinkage - Percent from green to seasoned, based on original green dimensions. (Tangential)* 9.9 Thickness (Radial)** 6.5 Length 0.1 Volume 16.7 *Moisture Content - MC **Flat-grain board. Reverse for quarter-sawn or edge grain board. IV. Basic Strength Values and Mechanical Properties of Alaska Birch. (Strength properties of Alaska birch increase as the wood is dried out.) A. Hardness - Load required to embed a 0.444 inch (1.1 cm.) ball to 1/2 its diameter. Wood End Grain Side Grain Green 550 lbs. (249.5 kg.) 560 lbs. (254.0 kg.) Seasoned 860 lbs. (390.1 kg.) 840 lbs. (831.0 kg.) B. Static Bending Wood Fiber Stress at Elastic Limit psi* (ksc**) Green 3,800 psi (267.2 ksc) Seasoned 7,800 psi (548.4 ksc) Modulus of Rupture Wood Fiber Stress at Elastic Limit psi* (ksc**) Green 7,100 psi (499.2 ksc) Seasoned 13,800 psi (970.3 ksc) Modulus of Elasticity Wood psi (ksc) Green 1,350 psi (94.9 ksc) Seasoned 1,900 psi (133.6 ksc) Work to Elastic Limit Wood Work to Elastic Limit(in.-lb. per cu. in.) Work to Elastic Limit(cm.-kg. per cu. in.) Green .60 .007 Seasoned 1.87 .0202 Work to Maximum Load Wood Work to Maximum Load(in.-lb. per cu. in.) Work to Maximum Load(cm.-kg. per cu. in.) Green 11.6 .126 Seasoned 13.3 .150 C. Impact Bending - Height of drop causing complete failure - 50 lb. (22.7 kg.) hammer. Wood Height Inches (Centimeters) Green 37 in. (94.0 cm.) Seasoned 40 in. (101.6 cm.) D. Compression Parallel to the Grain Fiber Wood Stress at Elastic Limit Maximum Crushing Strength Green 1,970 psi (138.5 ksc) 3,030 psi (213.0 ksc) Seasoned 4,900 psi (344.5 ksc) 7,510 psi (528.0 ksc) E. Shearing Strength Wood Parallel to Grain Green 920 psi (64.7 ksc) Seasoned 1,420 psi (99.8 ksc) F. Tension Wood Perpendicular to the Grain Green 200 psi (14.1 ksc) Seasoned 670 psi (47.1 ksc) G. Compression (Perpendicular to grain, fiber stress at elastic limit) Wood psi (ksc) Green 430 psi (30.2 ksc) Seasoned 830 psi (58.4 ksc) * Pounds per square inch - psi ** Kilograms per square centimeter - ksc Uses of the Tree The international trade market is just now becoming aware of the potential of Alaska hardwoods. The first sale of hardwoods from interior Alaska to a Pacific Rim country occurred in early 1985. Through recent trade missions, the international market is finding out that Alaska hardwoods such as Alaskan birch are excellent stock for specialty products such as chopsticks and turned woods. Alaska birch would make excellent pulp regardless of the pulping process. Alaska birch also has a growing domestic market. It is now used to some extent for kitchen cabinets, dried and finished lumber, and flooring materials for buildings of light and medium construction. The future of Alaska birch is somewhat dependent upon the stocks of mature birch in Canada and the lower 48 which are more accessible to the world market at present.