Six-trillion tree claim

It would be of interest to know how they came up with the estimate, yes. The sources didn't provide that information; the abstract says something about it being based on projected tree densities, but honestly it's not clear what that means. What we know is that the researchers made such a claim, which we duly reported; it's both reasonable for us to report that they said that, and important that we attribute the claim — attribution being an important part of news neutrality.

Hm, well, maybe. A source does seem to indicate that they estimated forest cover and used the densities they'd come up with for this study.