Not All Firewood Is Equal
If you're making decisions about woodlots, keep in mind that some types of wood burn better than others.
The following are heat values (in million BTUs) per cord for various species of trees. The higher the value, the "better" the wood.
Ash, Green 22.8
Elm, American 19.8 Difficult to split
Elm, Red 20.6 Difficult to split
Elm, Siberian 20.9 Difficult to split
Locust, Black 28.3 Difficult to split
Maple, Sugar 24.0
Maple, Silver 18.9
Oak, Red 24.0
Oak, Bur 24.9
Oak, Post 25.6
Osage Orange (Hedge) 32.6 Sparks; do not use in open fireplace
Sycamore 19.5 Difficult to split
Walnut, Black 21.8
For more information read the Kansas Forest Service publication Managing Your Woodland for Firewood.
Also check out Making Good Choices About Firewood.