Question on Thread

okay, revisiting this series after having read it many, many years ago - finding a bunch of things I missed the first time around.

i do, however, have a question….

since thread voraciously consumes all organic material and can denude entire forests in a short time why is/was there any organic material left on Pern when the colonists landed?

Before dragons thread fell unchecked - shouldn’t there at least have been massive swaths of denuded barren land??