Voorhoeve has studied the plants of Liberia for over 50 years, joining the staff of the then College of Forestry of the University of Liberia in 1960 to teach botany.
The same year he also served as the tree expert for a German Forestry Mission to Liberia. Voorhoeve had the good luck to meet a local Gio man, David Kwewon, who had an unparalleled knowledge of the forest, its composition and uses. Kwewon joined the German Liberian inventory crew as tree spotter; later he joined the staff of the College of Forestry as Keeper of the Arboretum. Voorhoeve and Kwewon combined forces to collect the information that ultimately resulted in the 1965 publication "Liberian high forest trees".
After a rich and varied career Voorhoeve decided, in 2005, to re-edit his 1965 book, trying to make it more user friendly and extending the number of species. The result of this endeavour is the "Tree Atlas of Liberia and surrounding regions", a combined effort with Dr. Carel Jongkind and Fauna & Flora International.