From a blog I follow:
Tree hive beekeeping is a 1000 year old art of keeping bees in slots cut into living pine and oak trees. Over 70,000 tree hives were recorded in a Polish 1827 census. The Polish tree hive tradition was lost in the 1930’s, but in 2002 Dr Hartmut Jungius of the World Wide Fund for Nature (WWF) discovered tree beekeeping still being practised in Bashkortostan, while setting up a 22,000 hectare woodland nature reserve. There, over 700 hives can be found, of which 30% are naturally populated by swarms and managed by the Bashkir.
Read more here.