The relationship between the tree roots and the fungus is called mutualism.

Here's why:

* Mutualism: A symbiotic relationship where both organisms benefit.

* The tree roots gain access to water and nutrients that the fungus can extract from the soil more effectively.

* The fungus receives sugars (carbohydrates) produced by the tree through photosynthesis.

This specific type of mutualism is called mycorrhizae, which literally means "fungus root".