At one time in the past, it was richer. But the forest ecosystem was too stable, causing it to continue to grow until it had consumed all of the resources, lea

At one time in the past, it was richer. But the forest ecosystem was too stable, causing it to continue to grow until it had consumed all of the resources, leaving the soil poor. But the forest remains, and remains almost maximally stable, so any time that anything dies, freeing up resources, they're almost immediately taken back up by the system and converted into more forest.