Here's why:

* Oxygen is a reactant: Combustion is a chemical reaction where a fuel (wood in this case) reacts with oxygen to produce heat, light, and ash. Oxygen is essential for the reaction to occur.

* Collision theory: Chemical reactions occur when molecules collide with sufficient energy. Reduced oxygen means fewer oxygen molecules are available to collide with the fuel molecules, leading to fewer successful collisions and a slower reaction rate.

* Limited supply: Oxygen acts as a limiting reagent, meaning it's the component present in the least amount and restricts the amount of product formed. Less oxygen means less combustion can occur.

In practical terms, a log will burn more slowly and with less intensity if the oxygen supply is reduced.

Think of it like this: imagine you're trying to light a campfire with limited air. It will take longer to get going and the flames will be smaller.