Not a geologist: A bit of reading did not find me a pure academic term. Felsenmeer and Blockhalde are in use in the German language. In French there is Chaos de pierres, de rochers, de rocs. English term seems block field (from this engineering paper).
I see there is ecological dispute on the subject in South Korea. If you want to produce a map of those areas from openstreetmap data, maybe the Key:geological - OpenStreetMap Wiki can offer you a tool? E.g. rock_glacier is also a morphological term. Can mix with wood/forest or just laid criss-cross.
I would not use natural=boulder_field when the area is covered with trees.