[libertine]

Well with some quick google search, in the US there's an average of 4816 trees per acre of primarily forest land, so 1 trillion trees you'd just need 207.641.196 acres.

The total area of forest land in US is 501 million acres, with 2.5 trillion trees of all types.

So you'd need to just find 207 million acres of viable land, 1 trillion saplings and enough people to plant them.

In my point of view, that's a bit challenging - but I'm no expert in such matters.

[RTGthelame]

I think it is important to note that the 4816 figure - presumably from the Forest Services report on New England forests - includes on average 4600 immature seedlings and saplings which will not reach maturity due to space requirements, and 200 trees with trunks larger than 5". Even if we consider an aggressive distribution of 750 trees/acre you're planting a landmass equivalent to 10x California. Challenging indeed!

[joubert]

You don’t have to plant all the trees in the US. There’s one atmosphere.

[libertine]

Of course, I used the US as a reference.