So I managed to make 2 trees stop growing permanently. I did this by planting saplings (circled in blue) next to them when they had grown to their medium size. Or so I thought, because when I tried to recreate this, the trees always grew to their full size OR remained saplings. I even tried recreating the surroundings, placing the full grown trees in the same patterns around them, but to no avail.
Does anyone know how this works? I'd so like to recreate this across my island, I think it looks nice and adds layers to the landscape.