@Lion.Kanzen, as we learned at school, people knew about planting trees yet when they were formed.
Some of them covered from danger by walking with the herds of cows, some of them noticed, that in places where they dropped seeds, plants began to grow, so they used plants as shield from outer danger, including, self-made. [4]
In ancient Egypt, for example, gardens could pass for kilometers from one garden to another. [1]
In ancient Greece they planted trees in gardens too (at first from vine and fruit gardens, till oaks and olive trees, sycamore, cypress, palms) [2]
Garden of ancient Rome Empire were created under the influence of ancient Egyptian, Persian, and Greek gardening equipment [3]
And etc.
So gardens have to be. But another problem - that no one civilization planted forests for wood. But those civilizations, who used wood, had enough wood and without planting. There was enough trees to reproduce themselves, without humans.
Another thing - game maps. They have no such sizes, so they have reduced distances between towns in comparison with real ones. And amount of trees is reduced too. This leads to that after some time in game we feel a lack of trees, of course, we could gain wood by other ways or win or loose quicklier. But enjoyment from the game is reduced because we have bare lands.
So as we have one inaccuracy with sizes and lack of trees, we could make another one, which will solve problem with lack of trees.
In other side, civilizations could plant trees if they wanted this. As player is temporary a leader of some union, he could make such a decision. That civilizations could not use cars and phones, but they could plant trees.
1. https://ru.wikipedia.org/wiki/Сады_в_Древнем_Египте
2. http://lektsii.org/2-3477.html
3. https://ru.wikipedia.org/wiki/Сады_в_Древнем_Риме
4. https://ru.wikipedia.org/wiki/Непослушное_дитя_биосферы