Hello, I am a solicitor with 20 years experience. I will try to answer this for you.
This sounds more like a positive covenant rather than a restrictive one. What si the real purpose of teh restriction to ensure that the benefiting land can enjoy looking at trees, have shade, privacy?
This is also pretty much empty of meaning as suggested. What is prevent you or a future owner getting rid of the trees and replacing them with dwarf varieties and putting them in the far corner of he garden. As it is an unenforceable burden you might think that there is no harm in agreeing to it.
Reasonableness isn't really the test with covenants.