Probably without knowing it, the ground or soil becomes humus due to the decomposing leaves that integrate into the soil underneath. Also animal matter may decompose and mix in with the soil. This top layer is known as humus. When the sun shines down, it causes the process of the animal decay and dead leaves to decompose at a faster rate than if the humus is in a shadier area.
Therefore, take these two places: a cool, temperate forest and a tropical rainforest soil. The heat produced in the tropical rainforest soil increases the decomposition to decay and evaporate and therefore there is less humus in this situation. In the cool temperate forest, the humus is cool and stays around a lot longer so the amount of humus would be more.