Low-density woods tend to ignite more quickly. They are preferable for kindling,and they will also heat up the stove more quickly. Now back to the question of whether you should be loading up the woodpile with aspen, our main deciduous tree. According to the Solid Fuels Encyclopedia by Jay Shelton, aspen delivers 16.3 million Btus per cord, which is less than black or white spruce, both of which deliver 17.1 million Btus. If you come across a dead paper birch in the woods, it can be worth dragging home as this species has an energy content of 23.6 million Btu per cord.
Some softwoods are also relatively dense.
Douglas fir, for example, has energy content per cord of 20.6 million Btu, the same as bigleaf maple. While you cannot do anything about the basic chemistry of the wood, there is one major variable that you can control - the amount of moisture in it. When you cut down a live tree, it contains about 50 percent water. You definitely want to reduce that amount as much as possible before throwing the wood onto the fire. When you burn wet wood, the heat initially just dries off the moisture in the wood.