In Journey to the Center of the Earth, what is the difference between the mushroom forests and the forests found on the surface?

Readers are not told too much about the mushroom forest, so we are forced to apply some of our own scientific knowledge to the unique forest. What we are told about the forest does ring similar to a forest on Earth's surface.

but here we had white mushrooms thirty or forty feet high, with caps of the same width. There were thousands of them. No light could pierce their dense cover, and complete darkness reigned beneath those domes, crowded together like the round roofs of an African city.

The mushroom forest isn't tall by forest standards, and its densely packed organization is similar to rain forests and jungles that are found on Earth's surface as well. The key difference is that the mushroom forest is underground. This means that no light feeds energy into the system. The mushrooms have to be obtaining energy in a different way than surface forests. Surface forests are made of plants. Plants are autotrophic organisms capable of photosynthesis. This means that they use water, light, and carbon dioxide to produce their own food (glucose). Mushrooms are not plants. They are a fungus; therefore, the mushroom forest is heterotrophic. It is not capable of producing its own food. The forest essentially has to "eat" some kind of organic material in order to keep itself alive.

As Axel Lidenbrock, his uncle Otto, and their guide Hans proceed on their epic journey, they encounter all kinds of strange, incredible sights. In chapter 30, they come across a world whose natural features strongly resemble those found on the earth's surface, except that they are much much bigger. Gazing around in wonderment as he stands in this gigantic crater, Axel is awestruck by the sheer vastness of everything; it is as if he is on another planet, like Neptune or Uranus.
Soon Axel's attention is drawn to an enormous forest. It is only a forest of mushrooms, says Otto, casually. But these are no ordinary mushrooms. They are giant mushrooms, thirty to forty feet high, crowned with a cap of equal diameter. There are thousands of them that are so densely packed together that they blot out the light when you stand beneath their giant canopies.