There’s an internet myth that Chaga is only found on 1 to every 10,000-20,000 birch tree (<0.01%). The source of this information seems to stem from an issue of Mycophile magazine from 2006 which stated: “according to one Chaga product site, only one birch tree in 15,000 bears Chaga”. Safe to say, this figure is purely anecdotal.
One report out of Russia stated that 20% of all birch trees are infected with Chaga. The Finnish Forest Research Institute has said that anywhere from 6-30% of birch forests in Sweden and Finland are infected with Chaga. Given this data, I would assume Canada to contain similar levels. This is a multiple magnitude difference than the estimates above and is based on actual data from reputable sources.