I’m interested in this but not knowledgeable. What is a “tipping point” in this context? The article doesn’t say. How does the overall size of a forest affect

I’m interested in this but not knowledgeable. What is a “tipping point” in this context? The article doesn’t say. How does the overall size of a forest affect its ability to re-expand? Wouldn’t this happen at the edges once those are no longer maintained, regardless of the overall size? Does “unrecoverable” mean “via natural processes”? Wouldn’t it be possible for human intervention to reforest? Thx.