We'd like to plant at/around/where 3 Birch trees stand some more sun (of one side) and shade (for the other) plants in the area.

I was wondering whether we can add some dirt (e.g., in a mogul/mound bed), but will the Birch suffer of die if we cover their roots at the surface of the ground? If there are limits to the depth of soil we can add, what are they?

If, on the otherhand we'll need to keep those roots free and clear and thus the ground flat 'as is,' won't the roots of the birch and the roots of the plantings conflict with each other? Also if flat, how does one plant the new plants without disturbing the birches' existing roots?

Thank you in advance!