Typically, when the easement does not specify who will maintain the easement, the owner of the easement (that would be you) has the duty to maintain the easement and the owner of the land has no duty to maintain the easement for your use. You are responsible for the cost to clear your view. But, you should try to coordinate your efforts to trim the trees with the landowner. Also, be sure that whoever you hire to do the trimming is knowledgeable. You cannot kill or seriously damage the trees. While you have the right to a view, you do not have the right to permanently destroy the property owners vegetation.