Peter Smallidge shared this article with me on the potential carbon sequestration benefits of tree planting: https://www.sciencenews.org/article/planting-trees-could-buy-more-t...
The article claims that the 900 million hectares suitable for tree planting did not include "natural grasslands". My guess is that figure includes pasture land in predominately forest-covered regions like the Northeast US, but excludes rangelands used predominately for grazing like parts of the western US, Mongolia, or the Argentine Pampas. In any case, that number could likely be much higher if we considered the "win-win" opportunities for developing open pastures and rangelands (where appropriate) in to silvopastures.
I've always thought of the "trees in to pasture" (vs. the "pasture in to trees") direction of silvopasturing as the more challenging of the two. It ain't cheap or easy to plant trees in sod environments full of herbivores. If scratching your head why, refer to the article posted elsewhere here on the forum from Graze magazine.
That said, however, I feel that one of the uncalculated benefits of planting trees in silvopastures is that the trees are often going to be well-managed (they have to be if they're going to survive the establishment phase) and selected for a specific purpose. A prime example would be nut grove or orchard silvopastures.
The article in the link above also correctly points out that the benefits of tree planting to mitigate climate change go beyond just the carbon sequestered by the trees.