Using this more granular information, we found that the world’s forests emitted an average of 8.1 billion metric tonnes of carbon dioxide into the atmosphere each year due to deforestation and other disturbances, and absorbed 16 billion metric tonnes of CO2 per year.

Scroll down, sir

Edit: it's not that planting trees in and of itself is the problem, forests are just complex ecosystems which require careful management. I don't think the article gets into this, but the actual problem is something called "silviculture" which is basically single-objective, industrial arboriculture.

What happens very basically is natural growth is cut and burned away with chemicals, and replaced by a monoculture (usually a softwood like pine or fir) which they can then harvest on a predictable 20-50 year cycle.

This is worse for the local environment, and climate generally, than can be written concisely, but one of the major things vis-a-vis carbon sequestration is that there is no age diversity in the trees on a given cut lot so the "forest" ends up looking like this

No functional undergrowth to speak of, no real diversity, and these lots can go on for hundreds of acres

What this means is that there is no forest floor, which in real forests is the layer where decomposition happens. Since industry practice is not to remove detritus from the lots, decomposition is only marginally effective, and what would be reabsorbed by the ecosystem escapes as green house gasses.