I do not know much about the great green wall. Most of my information came from agricultural videos on Farmer Managed Natural Regeneration in the Sahel and parts of Ethiopia. Probably a part of it. FMNR has had a lot of success. You may Google the following: Yacouba
Sawadogo, FMNR, Zai holes/pits/technology, stone berms for more information.
My understanding of the Zai holes is that they are like tiny swales you
amend and plant in. Since they manage water runoff and allow for water infiltration, they may allow your trees to be more resilient to little rainfall.
See this story in Kenya: