What provides the main heat source for a decent size fire in the growth stage?

In the growth stage of a fire, the primary heat source is indeed the fuel that is present in the environment. During this phase, the fire is transitioning from the ignition point and increasing in intensity as it finds and consumes available combustible materials. The fuel can be in various forms, such as solids (like wood), liquids (like gasoline), or gases (like propane), all of which release heat as they combust.

While oxygen is critical for the combustion process and heat is a result of combustion, they do not independently serve as the main source of heat during the growth stage. The combustion process itself is the mechanism through which fuel reacts with oxygen to produce heat, but it is the fuel that ultimately acts as the fuel source that drives the growth of the fire. Therefore, recognizing the role of the fuel in providing energy is key to understanding the dynamics of fire development during this stage.