Legal Question in Real Estate Law in California

Technically your are not allowed to place any permanent structures on the easement. In practice the utility may not need to use the easement for decades. They will give you a warning if they do need to use it and what they want removed. They can not use it in such as matter to cause damage to trees outside of the easement, but if part of the tree is on the easement they can take reasonable efforts to achieve usage of the land.

I wouldn't be able to give you definitive answer without reviewing the document creating the easement, whether that be an easement deed, or a reservation.