My main problem with planting trees is - you're not planting seeds, but seedlings. These seedlings were already growing elsewhere, you just burned some fuel to transport them to another place.
Additionally at least in my region whenever I leave a plot of land unattended for a few years - tree appear on it. If you leave it for more than a decade it becomes a forest.
So - it seems the important part is assigning land for forests and not bothering the trees that will grow there. The tree-planting is optional.
But it was growing already. The bottleneck is land allocated for trees, not the tree-planting procedure (in fact it's automatic once you have some trees in vicinity).
Are you sure? A forest grew on my grandpa's field automatically about 15 years after he stopped using it (mostly as a cow pasture). Nobody planted anything, we just left it be.
In fact if you don't mow a lawn for a few months saplings start appearing :) Forests are perfectly good at spreading by themselves, they evolved for millions of years to do that. If you want more trees you just need to let them grow.
Additionally at least in my region whenever I leave a plot of land unattended for a few years - tree appear on it. If you leave it for more than a decade it becomes a forest.
So - it seems the important part is assigning land for forests and not bothering the trees that will grow there. The tree-planting is optional.