Johannes E.S. Henkel, universally known as John S. Henkel, was the eldest son of the forester Caesar C.H. Henkel* and his wife Auguste Radue. He was educated at Dale College, King William's Town, where he passed the Civil Service Entrance Examination. In July 1888 he entered the Department of Forestry as an assistant in the office of the conservator of forests at King William's Town. He was promoted to district forest officer at King William's Town in June 1893. Around this time he compiled survey maps of Port Alfred (1894) and of the Bathurst forest area (1895). In February 1896 he married Juanita Gutsche, with whom he had one child. In August 1898 he was transferred to Stutterheim as district forest officer. During the Anglo-Boer War (1899-1902) he served in the Stutterheim District Mounted Troops with the rank of Captain and was awarded the Queen's medal. After the war, in August 1902, he was selected to study forestry at the Royal Indian Engineering College at Cooper's Hill, near London, where he obtained the Diploma in Forestry.
Upon returning to the Cape Colony Henkel was appointed acting assistant conservator of forests in the Eastern Conservancy, with headquarters at King William's Town, in October 1905. His appointment was confirmed the next year. In July 1907 he was transferred as assistant conservator of forests to the Western Conservancy, with headquarters in Cape Town. There he also served as lecturer in forestry at the South African School of Forestry (under the auspices of the South African College) until February 1908, and as chairman of the school's Board of Management to January 1909. The next month he was sent to the Midlands Conservancy, with headquarters at Knysna. There he succeeded C.B. McNaughton* as assistant conservator and in April 1912 was promoted to conservator of forests. That same year he described "The indigenous high forest situated in the divisions of George, Knysna and Humansdorp, Cape Province" in the Report of the South African Association for the Advancement of Science (1912, pp. 68-76).
In January 1915 Henkel was transferred to Pietermaritzburg as conservator of forests for Natal. In this capacity he recommended and initiated various afforestation schemes in the Zululand coastal belt, especially with Eucalyptus and other exotic species. This work was later described in "Afforestation in Zululand" (Rhodesia Agricultural Journal, 1920, Vol 17, pp. 50-52). A paper by him on "Forest progress in the Drakensberg" was published in the Report of the South African Association for the Advancement of Science (1916, pp. 179-186). In March 1920 he left Natal to take up an appointment as chief of the newly established Forest Service of Southern Rhodesia (now Zimbabwe), where he remained until his retirement in 1931. During these years he developed a forestry service of great economic importance. He travelled widely to select suitable sites for afforestation and in the process became well acquainted with the country's vegetation.
Henkel became a foundation member of the South African Association for the Advancement of Science in 1902 and by 1918 was a member of its council. In 1927 he served as secretary of Section C (which included forestry and botany) at the association's annual congress held in Salisbury (now Harare), and the next year was president of the same section at the annual congress held in Kimberley. His presidential address dealt with "The relation of vegetation to water supply in Southern Rhodesia" (South African Journal of Science, 1928, Vol. 25, pp. 35-51). In reaction to his address the association adopted a resolution recommending that he be permitted by the authorities to complete his vegetation survey, which he did. It was the first detailed vegetation survey of any African country. The results were published in his paper "Vegetation types in Southern Rhodesia", in the Proceedings of the Rhodesia Scientific Association (1931, Vol. 33). The accompanying map, on a scale of 20 miles to the inch (1: 1 244 160), included the territory's districts, rivers, roads, railways, towns, etc.
In 1907 Henkel became a member of the South African Philosophical Society. He remained a member when this society became the Royal Society of South Africa in 1908, and was elected a Fellow of the latter in 1930. In 1916 he became a foundation member of the South African Biological Society and served on its council. He joined the Rhodesia Scientific Association in 1920. The University of South Africa awarded him an honorary Doctor of Science (DSc) degree in 1932. In 1917, and again in 1936, he served on the Board of Trustees of the Natal Museum. He was an extremely hard worker who felt the need to personally confirm every observation and record it in his notebook. His remarkable observing powers, memory for detail, wide field experience, stamina, and willingness to share his knowledge made him an excellent field guide and companion.
After his retirement Henkel settled in Pietermaritzburg and wrote A field book of the woody plants of Natal and Zululand (Durban, 1934, 252p), a leaf key to over 700 species of trees and shrubs. As a personal friend of J.W. Bews*, professor of botany at the Natal University College, he assisted the latter in his investigations of the ecology of the Natal vegetation. With A.W. Bayer he made an ecological study of the wattle bagworm. He compiled a map of the Hluhluwe Game Reserve in 1936 and that same year, with S. Ballenden and A.W. Bayer as co-authors, published "An account of the plant ecology of the Dukuduku forest reserve and adjoining areas of the Zululand coast belt" in the Annals of the Natal Museum (Vol. 8, pp. 95-125). From about 1944 until his death he worked on a leaf key to the grasses of southern Africa. The plants he collected are in the Bews Herbarium of the University of Natal.