This article, which appeared in the Aug/Sept. 2008 issue of Sawmill & Woodlot Magazine, is part of S&W Publishing LLC’s “Tree Species” series and is not intended for reprint or republication. It is posted here with permission from Sawmill & Woodlot Magazine. To access copies of Sawmill & Woodlot’s “Tree Species” articles, visit http://sawmillmag.com/treespecies.php
THERE ARE THREE commercially important birch species: yellow birch (Betula alleghaniensis, which means from the Alleghenies), paper (or white) birch (B. papyrifera, meaning paper-bearing), and sweet birch (B. lenta, meaning flexible, referring to the twigs). Properties (especially strength) vary considerably among these three species, with white birch being the lightest weight and the weakest. In fact, weight of lumber is often the key to the separation of paper birch from other species. A sawmill in a birch region may also encounter small quantities of river birch (B. nigra) and gray birch (B. populifolia); the lumber from these two would be sold as birch lumber. For many centuries, birch has provided North Americans with important forest products. The bark of paper birch was used for canoes. Paper birch wood is also easily worked with hand tools, and has been used for bowls and other woodenware. Yellow birch has been widely used for centuries for furniture and cabinets. Seeds are consumed by songbirds including the common redpoll, pine siskin, and chickadees. Ruffed grouse feed on seeds, catkins, and buds. The yellow-bellied sapsucker uses yellow birch as a summer food source. Yellow birch, and therefore lumber at sawmills, is found from eastern Minnesota south to northeastern Iowa, northern Illinois, northern Indiana; eastward into Ohio, Pennsylvania, and New Jersey; and south through the Appalachians to northern Alabama and Georgia; and northward into Quebec, Ontario, New Brunswick, Maine, upper Michigan, and New York, with about 50% of the growing volume in Quebec. In fact, yellow birch is the official tree of Quebec. Paper birch is the most widespread of the three main species. It is found scattered throughout Canada, in Alaska, and south to North Carolina. It is the official tree of Saskatchewan and New Hampshire.
PROCESSING: Suggestions and Characteristics
DENSITY. Yellow and sweet birch average about 43 lbs. per cubic foot at 7% MC. This is roughly the same as oak. Paper birch averages 38 lbs. per cubic foot— 15% lighter than the other two birches. A board foot of dried and planed yellow birch will weigh just over 2-1/2 lbs.; paper birch, about 2-1/4 lbs.
SAWING. The birches are easy to saw using standard sawing techniques. As the core may have decay or discoloration and a lot of knots, the clearest lumber with good widths will be obtained by sawing one or two high quality pieces from a face and then turning the log to a new face (rather than sawing too deeply on one face). It is advisable to identify the species from the bark and leaves and then keep the various species separate during sawing and through marketing. This is really important for paper birch, which differs so much from the other birch species. Generally, the best markets will be for 4/4 and 5/4 thicknesses. However, with white birch, carving blocks that are 12/4 or so can be a potential outlet.
DRYING. All the birches dry very easily. Slow drying can result in some brown stain and sticker stain development, especially with paper birch. Likewise, initial temperatures over 120 F will cause some darkening. There is only a little risk of surface checking. Warping in lower grades is a risk. Shrinkage in drying is 5%. Final moisture contents for birch should be between 6% and 7%. Slight MC variations outside this range are not suggested due to birch’s high shrinkage.
GLUING & MACHINING. Yellow birch and sweet birch are moderately difficult to glue and require flat, true surfaces that have been recently prepared. Pressures must not be too low. Paper birch glues a bit more easily. Yellow and sweet birch machine quite well with sharp tools. Paper birch machines even more easily, and has been used for turned products as well. The wood veneers well, providing smooth surfaces.
STABILITY. Birch is subject to modest size changes when the MC changes— about 1% size change for each 3% MC change running across the grain parallel to the rings (tangentially), and about 1% size change for each 4% MC change across the rings (radially).
STRENGTH. The birches are quite strong. For yellow birch, bending strength (MOR) averages 16,600 psi. Hardness averages 1,260 lbs. Stiffness (MOE) averages 2 million psi. Sweet birch is 16,900 psi, 1,470 lbs., and 2.2 million psi. Yellow birch and sweet birch are stronger and stiffer than hard maple. Paper birch is 12,300 psi, 910 lbs., and 1.6 million psi, considerably lower than the other two, but higher than soft maple. Birches, especially yellow and sweet, are known for their tendency to split when nailed. Using blunt-pointed nails or predrilling the holes for screws or nails will offset any risk.
COLOR & GRAIN. Birch wood is fine and uniform in texture, with the annual rings making only light colorations. Yellow birch has some reddish coloration at times in its heartwood. Sweet birch is darker in color than yellow birch. Paper birch is a little lighter in color than either.
PESTS & DISEASES The birches host an impressive number of insect pests, and among these the bronze birch borer stands out as the most common and widespread cause of tree death. With the exception of river birch, all of our featured species are quite vulnerable to attacks by this insect. The borer preys on stressed hosts, which are often in abundance owing to outbreaks of defoliators, old age, drought, or, especially in urban settings, poor site quality (e.g., compacted soil). The borer larvae wreak havoc by feeding on the tree’s food-conducting inner bark. This behavior effectively girdles the tree. The borer’s presence can be readily detected because larvae pupate within their feeding galleries, and the slender, bronze adults create a characteristic D-shaped hole when they exit the tree. The complete list of insect defoliators that can stress (but rarely kill) birches is extensive— some of its prominent members include forest tent caterpillar, birch leafminer, gypsy moth, and Japanese beetle. Another noteworthy pest is the Columbian timber beetle, which tunnels into the sapwood, causing considerable losses in birch wood quality in the eastern U.S. This beetle is a threat to vigorous as well as stressed trees. As is the case with insect pests, birches host a staggering array of fungal pathogens, and only the most common diseases can be highlighted here. Nectria canker (a target canker) frequently causes extensive damage to birch stems, often predisposing them to wind breakage. Additionally, the stems of older birch trees are typically colonized by hardwood trunk rot (caused by Phellinus ignarius), which produces woody hoofshaped conks, and sterile-conk trunk rot (caused by Inonotus obliquus), which produces amorphous black conks most frequently in branch stubs, wounds, and in the center of Nectria cankers. In general, the birches are also susceptible to shoestring root rot (caused by Armillaria mellea). Notably, among our featured species, river birch appears to be least vulnerable to root rot and to birch pathogens in general.
MARKETS. The best market will be for lumber, usually No.1 Common grade, which will eventually be used for cabinets or furniture. This lumber should be flatsawn rather than quartersawn to develop the most desirable grain patterns. Lower grade lumber should probably be dried and then remanufactured into smaller, nearly clear pieces, which can be sold to hobbyists and cabinet manufacturers. A common length piece for cabinets is 23 in., but check with your customer to find out the best sizes. There is a small market—which takes some effort to locate—for carving blocks of paper birch. This particular market likes 8/4 and 12/4 short length pieces (under 30 in.). Avoid overdrying these products. It may be necessary to carry a small inventory of carving blocks so that orders can be filled promptly. Paper birch is also used for tongue depressors, ice cream sticks, and toothpicks. Sweet and yellow birch are heavy, hard, and strong and are used for cabinets, paneling, and veneer.
Gene Wenger is a Professor Emeritus at the University of Wisconsin-Madison and president of The Wood Doctor’s Rx. He can be reached at [email protected].
Eric Kruger is a professor in the Department of Forest Ecology & Management at the University of Wisconsin-Madison. He can be reached at [email protected].