There is no direct adjacency needed to spread seeds from one plot of land to another, that's one of your made up hurdles. Small seeds naturally get blown far in the wind. Larger seeds still get carried large distances by animals, which was why I specifically copy pasted the bit about birds carrying acorns ...

To claim that it would be difficult and expensive to let a wild forest grow in those meadows is absurd, that's what this was about for me. It's a convenient excuse to do nothing, but it's not a very good excuse.