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I’m a bit of a skeptic of this method but it seems to work fairly well in warm, wet climates.
In climates with any level of aridity, overly dense tree plantings can create dry, stressed trees that are vulnerable to pests and fire.
In addition, the costs and level of labor involved mean it’s not the best choice for larger landscapes. For tiny urban plots or similar situations in appropriate climates it’s a cool idea. But I think it’s a bit over-hyped at the moment.
I wondered before if the solution to retain humidity in dry climates comes from plant density, but what you described is what happened to me.
Instead, I find the best solution was to combine plants that grow at different heights, as the larger ones give shade and the lower ones with shallow roots keep the land humidity from evaporating.
(That's with some tricks mixed in for water conservation on dirt mixture, using a layer of porous rocks, using ditches of thatch, ceramic pearls, etc..)