Sometimes planting a tree is actually bad. We've introduced specific species into our environment for decorative or other purposes before and then ended up regretting it because it was a bad decision [1]. There's also the issue of when we mass-plant a specific tree species and then the trees all get devastated by disease (due to being a monoculture), creating a massive fire hazard [2].

So we need to plant more trees, but it goes beyond 'appropriate for the ecosystem', we have to consider very many factors to anticipate whether the trees we're planting will be a nuisance or even a hazard in 10 years and whether they will even survive to sink carbon. We need to make sure we plant a good mix of native species to ensure that they'll survive diseases and shifts in weather, but also deal with the fact that not every species grows at the same speed somehow. Forests are complex ecosystems and we basically have to build new ones from scratch to really have an impact here, and that's more complex than just planting some trees.

I used to be a big fan of Just Plant Some Trees but after seeing the impact of the two things I cite below I've developed some skepticism of the idea that we can just apply Tree Planting as a blunt-force instrument to compensate for all the damage we're inflicting on existing trees.

1: https://www.kqed.org/science/4209/eucalyptus-california-icon...

2: https://www.cbc.ca/news/canada/british-columbia/flexibility-...