Moss and lichen do not affect tree growth or health and no control is necessary. The link provided will give you more information. 

Tree roots need to have air exchange to maintain health. Different tree species will exhibit different root growth. Roots may also be more visible on the ground if the soil is rocky or more compacted. Feel free to send a photo of the tree or trees in question so we can help advise you further. 

You can provide a mulch ring around the base of the tree to help protect the roots from mowing, string trimming or foot traffic compaction, but you don't want to necessarily bury them. Mulch around the base of a tree should not be packed up against the trunk. We refer to this as a "mulch volcano," and even though they are commonly seen, they are not good for the tree's health. It can cause roots to grow up higher onto the trunk of the tree, searching for air, then wrap around the trunk, causing girdling roots. As the roots continue to grow, they choke the tree and restrict water and nutrient exchange in the tree, ultimately causing death. We have a helpful blog post that explains this in more detail: Free the Flare. 

We also have a page about properly mulching trees that would be helpful to refer to. If adding mulch to the base of a tree, you don't want it to be thicker than about 3-4 inches. This will help suppress weed growth but also allow for air exchange. It can also help maintain soil moisture. 

Let us know if you have further questions. 

Emily