That triangle is govt land, so no one is supposed to cut them except govt.
A friend living nearby suspects it was the "Yan Yan" family.
If the cutters aren't govt workers you can call the police; though I don't know how seriously they will take it.
They're not enthusiastic about getting involved.
Lands Dept is supposed to look after it, but they act much too slowly to be effective.
You could send a complaint to 1823:
http://www.1823.gov.hk/eng/Include photos, especially if you have some of those doing the cutting.