Without air there is no oxidization, but there is pyrolosis. these are both processes that occur in combustion of organic matter. pyrolosis is charring, the matte undergoes a chemical change where it releases a number of volotile gasses such as hydrogen and methane and CO, and liquids such as tar and water. What's left behind is a semi-crystaline structure that is about 80% carbon, and very brittle -useless for reinforcing. This will occur in wood at about 580 degrees Fahrenheit, a little lower for grasses and such.
Oxidization normally occurs after pyrolosis. Essentially, the heat breaks down the solids into volatile compounds which will freely bond with oxygen. Without Oxygen, the carbon will not burn into CO2.