the trouble with planting trees is that it's difficult to turn into a virtuous economic loop without government intervention because the cost of land is so high. nobody wants to buy a tree that is planted somewhere else, and very few individuals or companies are interested in purchasing trees for the creation of a carbon offset certificate or personal carbon offset. in other words, people don't value carbon sequestration, they value things that sequestration can produce downstream, like air that isn't jam packed with CO2.
in a vacuum, you can plant one tree for 20 cents of effort -- assuming you own the land you're standing on. but in the real world, land is expensive, and it has both up-front purchase costs as well as ongoing land tax costs. and trees aren't exactly productive assets unless the government assigns an economic value to carbon sequestration via a carbon tax or something similar.
all of these concerns occur well before you can even start to approach the idea of scaling tree-planting to the point where it makes a dent in the climate crisis. for the idea to work economically, it needs a lot of subsidization, at least under the present conditions.
of course, this is where philanthropy could step in and make a massive difference. if a billionaire or two would set aside a few billions to purchase deforested lands and replant them, we could make major progress. but they won't, so that leaves us hunting for a market capitalist solution to a problem caused by collective ignorance of externalities during economic activity.