I do not have the expertise to validate this claim. But I don't think that they're suggesting that in 10 years there will still be 4 trees per square meter, or even one per square meter.
Where did you see this? I see
> three to five saplings per square meter
> 300 trees of 42 species in a 93-square-meter plot
I wouldn't nitpick but it's an order of magnitude.
Still, there are 4000+ square meters to an acre. That remains two orders of magnitude larger than a mature forest should have.
In fact, 100 trees per acre is the minimum to be classified as a forest in a particular program, and that's in Southern California, which is especially sparse land for trees. http://mountainsfoundation.org/uploads/media_items/landowner...