 For Garthwaite, forestry was always part of the spectrum of nature and during his time in Burma he co-edited Smythies's Birds of Burma, which is still a standard work of ornithology, as well as authoring other works.He worked in Burma until 1947 although his career was interrupted by war service in the Indian Army Royal Engineers, in which he attained the rank of lieutenant-colonel.When he returned to Britain, Garthwaite joined the Forestry Commission at a time when its projects were expanding rapidly.