Friedrich Casimir Gustav Heyer (11 March 1826 – 10 July 1883) was a German forestry professor and son of who was also a famous forester. He was a professor of forestry at the University of Giessen. Gustav was the first son of Karl Heyer and was born in Giessen where his father taught forestry. After school he went to study science under the guidance of his father. He graduated in 1847 with a D. Phil and worked briefly at Darmstadt. In 1849 he joined Giessen University to teach forest science. He became a full professor in 1857. He stayed on despite offers from the Polytechnic at Zurich. His principal contributions were in forest value estimation and statistics. He was also an expert on forest soil science.