''I'm not following your logic. There has to have been a point in 1000 AD where there was no forest, or else Chrono and team would never have planted the seed. If you go back in time and change something, the changes are evident only after you make the change. Just as the Chrono series explains, every time something in a timeline is changed in the past, it creates a "new dimension" proceeding from that point forward. The characters return to the new dimension where there is a forest, and the old dimension without a forest is relegated to the Darkness Beyond Time. I believe.''
There, there it is the answer. It goes like this:
1st. Time Line (Without forest):
1,000
1) Crono arrives here with the seed. No forest.
2) He travels to the year 600.
First time line from 600 onward is sent to the DBT, 2nd time line is created.
2nd Time Line (With Forest):
600
1) Crono arrives from 1,000.
2) Crono plants the seed.
3) Crono returns to 1,000.
600-1,000
1) Forest grows.
1,000
1) By TTI, Crono arrives with the seed. He sees the forest.
2) Crono decides there is no need to plant it anymore.
3) By TB, Crono disappears the moment the other one, who did travel, went to 600.
4) By TTI, the Crono who did planted the seed returns to find the forest.
There, it works that way.