Thanks for your question.
Generally, if the branches of a neighbour’s tree start to grow over to neighbouring property, the neighbour can cut them back to the boundary point between the properties, as long as the tree is not under a tree preservation order.
However, the land is not your presumably as you have a easement to display a sign on a 3rd party's land. You would need to get the land owner to cut the tree back (unless there is a provision in the deed of grant allowing you to do this - that there should be nothing on the land to obscure the sign).
I hope this helps.
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It depends on the wording of the easement. You may just have a simple right to display a sign. It is not possible to comment further without seeing the wording of the easement. The fact it is registered means it is legal but that in itself will not imply other rights.
If you have to enter the land to remove the branches then you will be trespassing if you do so without the land owner's express permission.