Chaga (Inonotus obliquus) is distributed across the circumboral region of the northern hemisphere. It is found in a number of countries, such as; United States, Russia, China, Japan, Korea, and Canada. In Europe, it is often found in Central and Northern Europe, but less often in Western and Southern Europe (Lee et al. 2008).
Chaga is mainly parasitic on birch (Betulina spp.) trees (Sinclair et al, 2005) (Brydon-Williams, 2019). It is occasionally encountered in alders (Alnus spp.), beech (Fagus spp.), oaks (Quercus spp.), and poplars (Populus spp.) (Ryvarden and Gilbertson, 1993). To the sapwood it causes limited damage which is usually around the area of the initial bark damage as the sterile conk emerges 3-5 years after infection (Lee et al, 2008). It is these black conks that are harvested and are of commercial interest. These conks are formed primarily only on birch trees and rarely on alders (Lee et. al, 2008. Brydon-Williams, 2019). In other tree species, chaga conks have not been reported, except when other fungal species like birch polypore are misidentified.
While the host tree still lives, only conks will be produced while the mycelium spreads inside the heartwood. This means that it is to be expected that the quality of wood from infected trees will be inferior to non-infected trees. However should the price for chaga conks remain steady, the loss in value of the wood will be offset by the conk production.
It is believed that chaga only produces spores, and is actively infectious, after the host tree dies (Shigo, 1965. Brydon-Williams, 2019). These basidiospores are transmitted to new trees via wind or insect transmission. Infection occurs through existing wounds in the bark, such as caused by frost, wind, and human activity (Brydon-Williams, 2019).
While Chaga is considered a parasite, there is little evidence that it causes significant tree mortality. It has been postulated that chaga could drive an increase in tree mortality (Drenkhan, 2020) although there have been several studies which have found no clear correlation between birch tree mortality and chaga presence (Niemelä et al., 1995. Brydon-Williams, 2019). Furthermore, birch trees usually live for 90-100 years (Beck, 2016), while it has been reported that chaga will cause decay within a live tree for between 10-80+ years (Shigo, 1965. Lee, 2008. Brydon-Williams, 2019). Given that the first sign of chaga infection is the formation of conks 5-10 years after initial infection, this implies chaga does not cause a significant decrease in the trees maximum lifespan.
In regard to Chaga inhibiting tree growth, Brydon-Williams (2019) found that chaga presence did not effect the yearly basal area increment of birch trees. Given that chaga has a propensity to infect more mature damaged trees it is most likely safe to assume that chaga poses minimal risk to saplings or young stands of trees (Balandaykin et al, 2015).
Overall, a conservative estimate for the incidence rate of chaga in birch would be 1-5% (Kuz’michev et al. 2001, Brydon-Williams 2019). While higher incidence rates have been reported, they are not common, and estimates are complicated by chaga often not included in Russian and European forest inventory data (Pilz 2004). Several studies have noted that chaga is more likely to be found in mature stands, and that the distribution positively correlates with anthropogenic disturbance (Balandaykin et al, 2015, Brydon-Williams 2019). Perhaps because of this positive correlation with human disturbance, it is was found that in the United States that harvesting areas for birch wood had no significant negative impact on Chaga presence (Brydon-Williams, 2019).