Our deciduous trees include aspen, birch, willow and alder. But before considering whether you should be stacking more aspen in the woodpile just because it is a hardwood, consider the chemistry and composition of the tree. Almost all species of wood have the same energy content when all of the moisture is dried out of them. Softwoods contain slightly more energy per kilogram because they contain more pitch, which is composed mainly of carbon and hydrogen, just like oil. They are, when measured by weight.
Hardwoods contain more energy per cubic metre than softwoods because they are usually denser.
Wood is measured by volume, and a cord of hardwood has almost twice as much energy as a cord of softwood. That s because it can weigh about twice as much, depending upon the particular species of wood. What it really comes down to is the density of the wood. Dense woods last longer in a fire, and produce more coals. Low-density woods tend to ignite more quickly.