They bend and flex more than any of the U-lock or mini-U-locks suggested upthread.

But since you're not the OP anyway, If you're looking for suggestions on "Effective/light locks to secure to trees," why don't you start your own thread? It's certainly a peculiar lock requirement which other tree-lockers-- and I'm sure they're out there-- would be happy to see come out of the lock underground into the light. As an urban cyclist with an abundance of purpose built lock points and lock points otherwise suitable to a variety of common locks, I admit I'm guilty of being unaware of the particular needs of cyclists in less developed communities and those who lock to trees and lamp posts, so such a thread would perhaps be enlightening to me as well, although I heavily suspect that there are few suitable options which I'd consider light. Like I said, I'm just delighted to have more appropriate lock points available for my needs, but don't want to crowd-out the voices of the less fortunate, so I look forward to your thread.