The ground is cold, when native people are in cold situation they lay fiber and other items down before they put down their sleeping cloths. The heat from a body will draw cold up. So the wood puts a barrier between the engine and the cold that rises. Metal is a cold sponge, it takes it in like a cold blooded rep takes heat. The wood pushes the cold away. And if their is snow on the wood it acts like insulation when the engine is warm and sitting on the wood. As the tractor cools it draws the cold up from the ground.