Rackham was a prolific historical ecologist whose prime interest was the function, history, and management of British woodlands. He kept a series of notebooks, which he began during his youth and continued until his death, in which he recorded observations on plants seen in his home surroundings and on his travels, in addition to information about the weather and his college duties. Arising from his research on Hayley Wood in Cambridgeshire, he developed the concept of ancient woodland, rich in plant diversity and managed through traditional practices. His 1980 book "Ancient Woodland, its History, Vegetation and Uses in England" led to the recognition of such areas by the Forestry Commission and in planning legislation. It also helped to alter forestry industry views about woodland conservation. The Woodland Trust became a larger woodland owner to ensure conservation. He argued for the preservation of traditional management techniques like coppicing, to let light in to increase in the diversity of the herb layer.