Acacias are said to be nitrogen fixers, since they are legumes, and I see them mentioned in this forum every now and then. I guess people are planting them as part of tree guilds. Living in Chile (mediterranean to semiarid), there is a bountyfull of trees that only grow here, lots of them draught hardy and even edible, and the local Acacia Caven is one of them. A spiny shrub, slow growing, hated by all.
Now I have come over some research abouth the A. Cavens role in the local forests. The author found a close relationship to other young trees growing under the canopy of the Acacias, thus providing shade, humidity and maybe nitrogen. The paper then proposes that the Acacias are nurse trees for the other trees, which will eventually outgrow them.
Of course I wrote an email to the author of the paper, who was so friendly to reply. According to her observation, the younger trees growing under the canopy of Acacia had their stems literally only centimeters away from the Acacia stem. I understand this is much much closer than the permaculturists would recommend, e.g. if you are planting a forest garden, or any sort of tree guilds. Furthermore, in her opinion, the Acacias provide mainly humidity, but there is no significant effect of nitrogen fixing
So I am wondering: is it advantageous to plant Acacias next to other trees in general, the saplings being 5cms away from each other?
Open link to the paper: https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecs2.1667