I'd think that the aerodrome owner hires a surveyor to check the airport and it's protected surfaces (and I don't know, but regular checks like that is probably a condition of their licence). The surveyor puts in their report, noting that a row of trees has grown into those surfaces. A NOTAM is issued, a tree surgeon is hired to lop the top off said trees and the NOTAM is susequently removed. It's a cycle of life thing.