I am not aware of legislation to quote but it is so obviously not your fault or responsibility that I would just stick with saying that unless someone else here comes up with something better. So long as you keep diligently up to date with rent and anything else you are responsible for, I would just stick with not paying for a tree.
If someone, not you, stole the letterbox and you promptly reported it, you would not have to buy a new letterbox. On the other hand you could reasonably ask the owner (or their agent) to replace the letterbox so you could receive letters.
Yes, you are expected to keep the garden in reasonable condition, but no, you should not be expected to replace a stolen tree.
If the agent gets heavy about this, you say that two highly esteemed 😉 ‘strata gurus’ on Flat-Chat have advised that you don’t need to replace a stolen tree.
Relax and enjoy Christmas.