The original trees were all placed fully grown, before the tree-growing mechanic was added, and are therefore exempt from normal growing rules (They're exempt anyway since they're ALREADY fully grown).
I'd say your best bet is to place trees around 4-6 tiles apart.
You see a masterful engraving by Blaze. On it is Blaze, bears, boars, and foxes. Blaze is striking down the bears. The boars are screaming. The foxes are in a fetal position. The image relates to the return of Blaze in the late winter of 2009.