First we'd have to stop the massive deforestation that's happening right now. A lot of that is for agriculture, so we'd have to find another way to feed those people.

We might also want to stop clearcutting vast forests in the Southeastern U.S. to ship wood pellets to Europe, which burns them.

Then we'd have to save the rest of our existing forests, many of which are struggling due to various effects of climate change, including heat stress, drought, topsoil loss, disease, and forest fires. Many large forests are net carbon emitters now.

After we've done all that and finally stopped losing trees, we can think about having more trees. We'll need space for them, so that means reducing agricultural area even more. We've been emitting about ten billion tons of carbon every year for quite a while, so it's going to take a lot of land.

We're also running out of fish in the ocean, so with that food source going away we're going to need more from agriculture than we get now.

"Just plant trees." Like many, perhaps even most purported climate change solutions, it sounds great until you work out the scale.