Answer: The forest would eventually die.
Step-by-step explanation:
Fungi (along with bacteria) are responsible for "recycling" all the nutrients in an ecosystem.
For example when an animal or a plant dies, fungi consume all the organic matter and they break it down into Carbon, Nitrogen, Oxygen, Phosphorus and other essential elements, and then release it into que soil or the atmosphere.
If they are removed from an ecosystem, plants wouldn't be able to get these elements, because they would all be "trapped" in complex molecules. As a result, the plants and every organism they act as food to, would die.