Hardwood, Softwood, Fuelwood

A strange thing happens at my house every October: the woodstove seems to get bigger. It's been an inconspicuous little black thing in the corner of the living room all summer, but come snowfall, it's a brute right out in the middle. And it looks hungry.

If there's any science whatsoever in that observation, it's psychology. Important objects may be perceived as larger than they really are, and that stove certainly grows in importance as temperatures decline.

Other sciences apply more exactly to heating with wood. Between recent bouts of hauling stove-length chunks of birch and spruce into winter shelter, I reviewed some of what scientist Neil Davis had to impart about wood as fuel in his book Energy/Alaska.

I was willing to categorize birch as a hardwood from the first moment my splitting maul bounced uselessly off a birch log, but the book told me that isn't the usual way to tell hardwoods from softwoods. From a forester's point of view, it's all in the seeds. Softwoods are gymnosperms, a nice technical term meaning they have naked ovules--the female protoseeds--before pollination. That's a more primitive approach to reproduction than the one used by hardwoods, which are angiosperms. Angiosperms, the book told me, have closed ovaries.

Fortunately, the practical Alaskan wood-harvester doesn't have to delve so intimately into the sex lives of trees to tell a softwood from a hardwood. Our conifers--spruce, cedar, hemlock, and the other trees with needles and cones--are softwoods. The leafy deciduous trees--poplar, willow, birch, alder--are hardwoods.

Other than forewarning of the bouncing-maul problem, knowing whether a given log comes from a softwood or a hardwood has a few other implications for people planning to use that log for fuel. What we really care about is the wood's energy content.

A live, growing tree may be nearly half water; cut and thoroughly dried, the remaining tree tissue is approximately two-thirds holocellulose and one-quarter lignin. Holocellulose consists of long-chain carbohydrate molecules and lignin is the tough stuff that makes wood structurally strong, but their real interest for fuelwood seekers lies in their elemental composition. Lignin is about 60 percent carbon, 7 percent hydrogen, and 33 percent oxygen. Holocellulose contains approximately 45 percent carbon, 6 percent hydrogen, and 49 percent oxygen. For fuel purposes, what's wanted are energy-rich hydrocarbons--the less oxygen present in the fuel substance, the better.

So that difference in oxygen content means lignin has more energy per unit weight than holocellulose does--about 11,000 British thermal units per pound compared to 7500 for holocellulose. (Methane gas, which contains carbon and hydrogen but no oxygen, has an energy content of 23,700 Btus per pound--indeed, oxygen content does make a big difference.) That should mean that better fuelwood would come from tree species with higher ratios of lignin to holocellulose. In general, softwoods have proportionately more lignin. Thus, per pound, conifers are the best sources of fuelwood. Davis reports that a pound of dry white spruce (27 percent lignin) contains 8890 Btu, while a pound of equally dry paper birch (19 percent lignin) has 8330 Btu.

But spare that spruce tree for a minute. Consider again the clue of the bounced maul: density makes a difference in energy content measured by volume. A birch log of the same dimensions as a spruce log will weigh more. One cubic foot of dry birch weighs in at 37.5 pounds; a cubic foot of spruce attains only a hair under 27 pounds.

So, if one considers wood by the standard volumetric measurement of the cord--128 cubic feet, which comes down to somewhere between 80 and 90 cubic feet of actual wood--birch is better for heating. It's worth sharpening the old splitting maul for, anyhow.