This is a very old paper now, but might be of interest:
(1971). Forestry and the war in South Vietnam. Journal of Forestry, 69(11), 777-783.
https://academic.oup.com/jof/article-abstract/69/11/777/4659836Westing, A. H.
This is a RG link to a paper of the same area and time period:
War and defoliation in the South Vietnam (1961-1971). The sources of history
Tran, T., Amat, J. & Pirot, F. (2007). Guerre et défoliation dans le Sud Viêt-Nam, 1961-1971: Aux sources de l'histoire. Histoire & mesure, vol. xxii,(1), 71-107.