You say 100 trees per acre, but Afforestt says 100 trees per 30 sq meters (0.0074 acre)[0][1]! (This is equivalent to 121,000 per acre.) A mature forest may only need 100 trees per acre, but it may take a much larger density to get it started. You may need the soil shaded to keep the soil and leaf fall moist. You may need the trees to shade eachother. You may need the trees to share resources through their roots. You may need the trees to protect each other from the wind.

[0] https://fellowsblog.ted.com/how-to-grow-a-forest-really-real...

[1] https://www.dropbox.com/s/pgofw7noxmpfwxg/Miyawaki%20Methodo...