We have a 40' easement across a neighbors property. The have planted large trees on the land. Can we ask that they be removed?

Easements allow a non-owners a right to property for a specified purpose, ( utilities, access, view).. It sounds like your use of the easement is being eroded by a gradual use by the burdened party,
(they are putting things into the easement that interfere with your use). You should have a talk with your neighbor in a non-confrontational manner and explain your concerns.
If that fails, consult a real estate lawyer in your area and bring copies of all easements and plat maps to the lawyer.