It takes 4 months till it have to be replanted.
You should never "till." Instead, use until.
Then,Afternext 14 yea
rs, the tree is prepared to be cut down.
Your thoughts are very organized. However, I would suggest varying you sentence structures a little bit. So, for example, you could combine "There the tree is cut for thin "slats" and it is treated. It stays inactive for 60 days." like:
"There, the tree is cut
forinto thin "slats." The slats are treated and remain inactive for the next 60 days."
What is this essay for? I see that it is for IELTS, but do you have a prompt?