They fix nitrogen and improve soil quality.
And, to make things fun, the species they’re using practices “reverse leaf phenology.” The trees go dormant in the rainy season and then grow their leaves again in the dry season. This means you can plant crops under the trees, in that nitrogen-rich soil, and the trees don’t compete for light because they don’t have any leaves on.
And then in the dry season, you harvest the leaves and feed them to your cows.
Crops grown under acacia trees have better yield than those grown without them. Considerably better.
So, this isn’t just about stopping the advancement of the Sahara - it’s also about improving food security for the entire sub-Saharan belt and possibly reclaiming some of the desert as productive land.
Of course, before the “green revolution,” the farmers knew to plant acacia trees - it’s a traditional practice that they were convinced to abandon in favor of “more reliable” artificial fertilizers (that caused soil degradation, soil erosion, etc).
This is why you listen to the people who, you know, have lived with and on land for centuries.
