Let us put the idea into context:
"Between 2000 and 2012, 2.3 million square kilometres (890,000 sq mi) of forests around the world were cut down."
Makes roughly 191.000 km2 per year.
A hex is roughly 34.641 km2
So it seems definitely within humanities scope of abilities.
Yup nice. Especially also want the ability to PLANT forests, if of course the planet supports it.
Trees tend to need some time to grow. In the 300 turns range (50 years)
But maybe a realy competitive local plant, we could assume the competitiveness is part really quick growth?
First of all I don't mind if it takes just 3 turns for it to grow. Game first, modelling reality (or sci fi fantasy in this case) second.
Secondly: Ethiopia has planted 353 million trees in 12 hours, so a turn being 2 months is plenty of time.
Third, on a planet with trees (which is what we need as the planet needs to be friendly towards them) you'd be able to get plenty of saplings and young trees instead of seeds.
I don't know what you envision, but a lot of trees get planted as young trees, not seeds. So as soon as they're planted, there's no real need for them to grow anymore (in terms of infantry getting cover). That can also be done in the game. At least enough for the hex to become lightly forested (do it again for heavy forest).
The game doesn't make a difference beyond 'light forest', '(heavy) forest' and 'no forest', so it's not as if smaller details would matter (as in, 5 million trees or 20 million or 50 million or 100 million or 250 million or 500 million: it's all either light forest or heavy forest).
Of course gameplay comes first. But I got the feeling the timescales and realism are somewhat important for Vic. There is a sort of "minimum realism" he wants to keep.
- They claim to have planted 350 million saplings. The claims are unreliable at best and may be inflated as much as kill-claims during war.
- Those were also placed by amateurs, so people that may not know where to put one so it "takes root". Trees are the anchors of a Econsystem. They are not some weed you can plant everywhere, they got very specific requirements.
- And they still need decades to grow to actuall tree-size. As Hazard151 said, what they made is at best shrubland.
Moving life, grown trees instead of planting sapplings is a utter luxury good. There are a number of companies for that, but they do not mention their prices publically.
Again, trees are not weeds. They need the right envionment. Preparing the target environment for them or picking the right tree for the target is almost as hard a moving the dang thing.
Just seedlings are unlikely to cut it. They usually need the underground Ecology and shade provided by the mature trees to sprout.
And if I want cover for my troops, I can just build bunkers. Worked with the Marginot line (until politics defeated the Belgian part of it). Worked with the Atlantic wall (until the sheer discrepancy in forces overcame it). Worked on every border worth guarding that ever existed*.
*That actually gives me a idea for the border defense issue.