It depends on the “lay of the land” to be developed, but it’s common for clearing of the boundaries, tree removal and preservation, and new fencing to be Conditions attached to Development Consents; entirely at the expense of the Developer. So maybe your Developer was just getting in early, and additionally sharing the costs of the above with the neighbours.
You said that the O/C and individual Lot Owners have been notified of the DA, so why not put in a Submission requesting some suitable mature trees on Developer’s side of the fence as replacements?