OK so replanting trees is a feature that would be harder to implement than I first anticipated. Just because it touches too many systems for me to feel comfortable just before I realise the next patch (read: I'm scared to break other stuff). But I have implemented natural regrowth. So if you go crazy on cutting down an entire forest, it will grow back eventually, unless you built onto it or it is now a roofed area (e.g., if you paved paradise and put down a mega walled fortress).