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, and some of the commercial uses of three of the four hardwoods of Alaska which have potential commercial value. These are balsam poplar, black cottonwood, and quaking aspen. The booklet is nontechnical and provides an introduction to the three species, and the Alaska forest environment in which they grow.
Balsam poplar (Populus balsamifera), black cottonwood (Populus trichocarpa), and quaking aspen (Populus tremuloides) are similar in wood characteristics and habitat. However, their habitats may differ. Because of the similarities, information presented in this booklet applies to all three species. Differences such as habitat will be discussed when relevant. Information presented in this booklet is drawn from previous U.S.F.S. publications on all three species.
The three species are fast growing hardwoods 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. Black cottonwood is the largest hardwood in Alaska, attaining heights of 80 to 100 feet (24.4 to 30.5 meters) and diameters to 36 inches (91.4 centimeters). However, very little black cottonwood is found in interior Alaska, where its habitat is limited to coastal riverbottom lands. The majority of black cottonwood in Alaska is found in the valley bottoms of southeast Alaska. On the other hand, balsam poplar is the most widely spread hardwood species in interior Alaska, attaining elevations higher than that of white spruce and extending further north into the Brooks Range than any other species. Aspen is nearly as widespread as balsam poplar.
In comparison with most other hardwoods, the clean straight trunks of balsam poplar and aspen make high-quality logs which are light in weight and long in fiber. These characteristics make them well suited for products such as pulp and veneers. However, because of the distance to manufacturing facilities, very little use has been made of either species in Alaska. Some interest in all three species appears to be developing from the Pacific Rim Countries for speciality products.
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 well-drained soils on which the three species grow and 20 inch leaders are not uncommon.
The climate of the interior is favorable to the establishment of all three species. They are all found on sites with wide variation in temperatures and precipitation. Aspen does better in dryer sites while the two poplars do better on the wetter sites. Black cottonwood does not grow well away from the humid influence of coastal rain and fog.
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 (Larix laricina) develop on the bottomlands and flood plains of the many rivers. Various willows and small elders 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 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 the two poplars accounting for 9% and quaking aspen 2% 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 riverbottom 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 area are those adjacent to the Porcupine River northeast of Fort Yukon.
Balsam poplar is an important tree species of this association, reaching its greatest size and abundance on the flood plains of the rivers. It invades sandbars and grows rapidly to heights of 80 to 100 feet (24 to 30 m.) and diameters of 24 inches (60 cm.). It is eventually replaced by white spruce on most sites. Black cottonwood may grow in place of balsam poplar in this association in the Susitna Valley. Hybrids of the two may also occur there.
Upland Spruce-Hardwood Forest Types. The upland spruce-hardwood forest types account for approximately 61% 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 bottomiand forest sites. Factors which contribute to this lower productivity include lower soil nutrient levels, cooler temperatures, and lower precipitation.
Aspen stands develop on south facing slopes of upland areas following fires and a willow stage. The aspen stands mature in 60 to 80 years and are eventually replaced by white spruce except on those sites which are excessively dry. There, aspen may persist. Aspen and balsam poplar may invade most other sites following fire or other catastrophes. Both species will occur as pure or mixed stands with the poplar favored on the wetter sites and aspen favored on the dryer sites. White spruce will eventually replace both species on most sites.
Black Cottonwood and Balsam Poplar. It is difficult to distinguish between black cottonwood and balsam poplar. The two species have similar appearances and habitats. However, the two have different ranges, overlapping only in the Cook Inlet area where hybrids may occur. Both species are prolific seeders, with high germination potential. However, the germinative capacity of both is short lived and the seeds require moist sites with bare mineral soil to become established. Black cottonwood and balsam poplar will also regenerate from stump sprouts and from cuttings. The two species are very intolerant of shade and will not reproduce under the canopy of an existing stand.
The two poplars are medium lived trees, maturing in 75 to 150 years. Growth is rapid for the first 40 to 50 years and may continue well past the age of maturity. However, heart rot occurs in most stands nearing maturity, especially those of black cottonwood.
Aspen. Aspen is a short lived tree, maturing in 60 to 80 years. Growth is rapid with trees attaining maximum diameters of 16 to 18 inches (40.6 to 45.7 centimeters) and maximum heights of 70 to 80 feet (21.3 to 24.4 meters). The average stand diameters are likely to be nearer to 6 to 8 inches (15.2 to 20.3 centimeters). Like the two poplars, aspen is a prolific seeder with a high viability. Aspen also needs bare mineral soil to become established and will regenerate from stump sprouts and cuttings. This species is very intolerant of shading and will eventually be replaced by the taller, more tolerant species such as white spruce.
Wood Properties. Although the wood of all three species has a characteristic odor when green, it has no odor or taste when seasoned. The woods are diffuse-porous (growth rings not very distinct), with fine, even texture.
The wood of all three species is light in weight when seasoned, moderately weak in bending and compression strength, low in shock resistance, and soft but tough when properly seasoned. The poplars have a low shrinkage rate when seasoned, while aspen has a moderately high shrinkage rate. All three species are easy to glue, take finishes and stains well but have a low nail-holding capacity and are low in decay resistance. The wood works easily and the straight, uniform grain is most satisfactory for uses such as veneers and plywoods.
Seasoning. All three species are moderately easy to season. However, bacterial wetwood in aspen can be a problem in seasoning because of its tendency to warp and twist. The woods have moderately high to very high moisture content when green. Recommended kiln drying schedules begin with dry-bulb temperatures of 170°-190°F (77°-88°C) and wet-bulb depressions of 4°-22°F (2°-12°C). Kiln drying times vary from 7 to 11 days. The lumber or veneer should be air dried to near the fibre saturation point prior to kiln drying. The wood is then easier to dry with little or no degrade.
Close stickering is necessary for air-drying, as is good ventilation and yard sanitation to prevent degradation from warp or damage from fungi.
Other Properties. Little, if any, published work has been done in Alaska to determine wood qualities of the three species presented in this booklet. A limited amount of data exists on balsam poplar for Alaska. With the exception of the limited data for balsam-poplar, the following wood properties are averages derived from previous U.S.F.S. reports on the three species from other states and Canada. They are published herein as guidelines only.
A. Solid Wood - Average weight in pounds per cubic foot (kilograms per cubic meter).
1. Balsam Poplar
(608.7 kg./cu. m.)
(384.4 kg./cu. m.)
(688.8 kg./cu. m.)
(432.5 kg./cu. m.)
II. Specific Gravity - Based
on volume when green and weight when oven-dry.
III. Shrinkage - Percent from
green to seasoned, based on original green dimensions.
* Moisture Content -
** Flat-grain board. Reverse for quarter-sawn or edge grain
IV. Basic Strength Values
and Mechanical Properties of Black Cottonwood, Balsam Poplar,
and Aspen. (Strength properties of white spruce increase
as the wood is dried out.)
* Pounds per square inch - psi
** Kilograms per square centimeter - ksc
Uses of the Trees. 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 aspen are excellent stock for specialty products such as chopsticks and turned woods. Aspen and the poplars would also make excellent veneers, core stock, plywood, and pulp.
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