Why does some wood burn hotter than other wood?

[CheeseDonkey]

Say I have two pieces of wood from two different deciduous trees. Said logs have been drying for the same amount of time and they are both relatively the same size.

Why would log A burn hotter than log B? What is the mechanism that causes that?

[Leaffan]

Deciduous hardwoods burn hotter than coniferous softwoods. I’m willing to bet the hardness also varies between species of deciduous trees.

And this from Wiki:

There is a correlation between density and calories/volume. This makes the denser hardwoods such as oak, cherry, and apple more suited for camp fires, cooking fires, and smoking meat as they tend to burn hotter and longer than softwoods such as pine or cedar.