Trees considered deciduous or hardwoods tend to burn hotter and longer than trees considered to be evergreen or softwood (there are some exceptions). Wood also tends to burn hotter when dried.
Wood heat value is measured in BTUs or British Thermal Units. The higher the value, the more heat you get per unit of wood. Here is a list of tree species ranked by their heating abilities:
Five Best Burning Species
·  Hickory - 31 to 32 mm btu/cord
·  Oak - 30 to 31 mm btu/cord
·  Black Locust - 28 mm btu/cord
·  Beech - 27 mm btu/cord
·  Elms/Maples - 21 to 26 mm btu/cord
Five Worst Burning Species
·  White Pine - 15 mm btu/cord
·  Cottonwood/Willow - 16 mm btu/cord
·  Hemlock - 17 mm btu/cord
·  Sassafras - 18 mm btu/cord
·  Yellow Poplar - 18 mm btu/cord