Level 7: Eco Friendly
Build 1 million trees
You will probably have to delete the entire map and replace trees all over again. 1 million trees also requires a decent income.
First tree on free tile costs 22 GBP, 3 more are 44 GBP, so to plant one full tile costs 154 GBP, which is 38 500 000 GBP for 1 million trees.
Also cutting the trees again to plant more is costly.
Have fun with RSI :).
PS: Building on dense tree tiles is really expensive. In order to make other people enjoy the game once you finished the quest perhaps you can delete a lot of the trees again, If you still have any strenght in your fingers that is ^^. Ofcourse this is not obligatory.
Example Save Eco Friendly