We plant far more crops like corn every year with machinery. I'm not sure the humans are even necessary, trees are pretty good at planting themselves. The bigg

We plant far more crops like corn every year with machinery. I'm not sure the humans are even necessary, trees are pretty good at planting themselves. The biggest issue is outcompeting other smaller plants growing in the same place. It takes a long time for an area to naturally become a forest. I believe human intervention with weeding and watering can speed it up significantly. But that's a bigger investment than merely planting them.