This is a new article from Skeptical Science that raises several important points, summarized below:
First is that even if you plant enough trees to cover the entire world without tearing down cities and farms, you’d only suck out a third of the carbon put out by human emissions over the last two centuries. Trees also need time to grow and mature, and often take years to do so, even decades and centuries for trees like the behemoth California redwoods out west. You also need to ensure these new forests you’re planting are biodiverse and filled with genetic variation, otherwise you wouldn’t get very far.
Planting trees is good, there’s no doubt about that, but the money being fronted towards this is better spent helping, say, indigenous peoples keep their lands in the Amazon, or preserving and restoring already existing ecosystems like the beleaguered pine forests of the Rockies.