He taught courses on dendrology, agrostology, and plant taxonomy. He is also President of the International Dendrology Society. Marshall has been called the "Father of American Dendrology". In 1965 Berry joined the International Dendrology Society (IDS). Dendrology dated the timbers to between 1750 B.C.E and 1285 B.C.E. He also made contributions as a botanist, especially in the field of dendrology.