How many trees are we really talking about? How much area will they take away?

The simple answer is: a lot of trees but they won’t take away from land that is currently adding value to a community. We aren't suggesting that people plant timber in their front yards or good pastures. No change of profitable practices is necessary. There is an unfortunate abundance of land that has been "used up" in almost every community. Those communities without poor land are either very lucky or someone has been practicing good land management.

The long answer begins like this:

For an example we will use the Loblolly Pine that has been extensively planted in the southern U.S.

One billion trees 1,000,000,000.00 trees
Planted at 300 per acre 3,333,333.33 acres
(741 per hectare)
(1,349,527.67 ha)
Equals 5,208.33 sq miles
  (13,495.28 sq km)

Puerto Rico covers 13,790 sq km. One billion trees will plant an area the size of Puerto Rico.

One Billion Loblolly Pine trees will use 15,000,000 tons (13.6 teragrams*) of carbon per year** for good growth. Over their 25 year harvest cycle, they will sequester 375,000,000 tons. (approximately 340 teragrams*)

*one teragram=1,102,311.3 tons
**conservative growth of 4.5 tons per acre per year

Each site has a different growth rate. Each climate zone and forest type grow and sequester carbon at different rates. The one common point is that forested regions receive more rainfall that deforested regions. The land in the same area that is used for agriculture will also benefit from this increased rainfall and feed more people on the same land already under cultivation.

It is not practical or desirable to plant forests on productive farmland. However, forests can become a part of crop rotations as was traditionally practiced by many indigenous people. Soil productivity becomes part of the planting equation that includes maintaining the heritage and traditions of forest people.