There are tens of thousands of species of trees around the world, and the wood from all of them is combustible. When wood burns, it gives off heat energy that can be used for many things. A tiny wood fire can heat a cup of tea to warm a thirsty hiker. Where wood products are abundant, wood-fired electrical generators produce electricity for entire communities. However, the wood from each species of tree burns differently; one of the most notable differences is whether the tree that produced the wood was a hardwood or softwood.
Hard Versus Soft
There’s no one criteria that categorizes wood from specific species of trees as hard or soft woods. The hardness of wood is a continuum from one of the hardest, Lignum Vitae, to the softest, balsa wood; in between are all other woods. (See References 1 and 2) Overall, the wood from deciduous trees is harder than wood from coniferous trees, but there is some overlap. The wood from some fast-growing deciduous species, such as aspen and cottonwood, is softer than slower growing conifers like jack pine or tamarack. In general, the faster a tree grows, the softer the wood it produces will be.
A better definition of hard versus soft woods can be expressed by its density, which is weight per unit volume. For wood, density is often expressed as pounds per cubic foot. As weight increases, the hardness of the wood increases. The weight of an average piece of lignum vitae is 84 pounds per cubic foot; most species of oak, a common hardwood used for firewood, weighs in at around 47 pounds per cubic foot; the species of pine often used for basic construction is 25 pounds per cubic foot; and balsa is only 8 pounds per cubic foot. (See Reference 1) All of these weights are based on a 12% moisture content. Although there’s no strict dividing line, wood heavier than 35 pounds per cubic foot is hardwood; lighter than that, it’s softwood.
Consider firewood to be an energy storage material; solar energy from the sun is collected by the tree and stored in the wood. When the wood is burned, the heat and light produced is the sun’s energy being released. Once you grasp that, it’s easy to understand that the longer it takes a tree to grow, the more energy from the sun is being absorbed and stored in the wood, and when the wood is burned, the more energy will be released. (See Reference 3) Slower growing trees with harder wood store more energy.
If the amount of heat released per pound of wood is the only measure of efficiency used, there is little difference between hardwood and softwood. A pound of hard oak will produce a nearly identical number of BTUs of heat as a pound of soft pine. However, firewood is usually measured and sold by volume, such as cords, face cords, cubic feet or ricks. (See Reference 4) If you measure the efficiency of firewood based on volume, hardwood will deliver greater efficiency based on the amount of heat per volume.
- Jupiterimages/Comstock/Getty Images