As you likely know Inonotus is a tree pathogen that attacks live tree tissue. Chaga can be repeatedly harvested until the tree dies, and after harvesting typically fully regrows within 3 to 10 years.
It has been shown that Chaga infection rates in birch stands vary greatly, but typically range from 1-20% of the trees showing signs of infection. The only study that I am aware of that has evaluated the sustainability of wild-harvested Chaga was completed in 2004 by David Pilz. Pilz concludes that Chaga's resources are so immense that it is very unlikely to be over-harvested to the point of species endangerment. His results showed that even using pessimistic values, the biological resource is exceedingly abundant and at no risk of over-harvesting, even with dramatically increased harvest levels. The birch forests of the world are vast and mostly untouched.
Over-harvesting is most likely to only affect pockets of birch located near rural areas or roads, whereas areas with poor access and rugged terrain will not have the same issues. It is simply not economical to go deeper and deeper into forests to collect Chaga, it will become too expensive, thus preserving a great amount of the Inonotus species.
