Because by placing trees down, they photosynthesise and produce oxygen and take carbon dioxide out of our atmosphere. We put carbon dioxide into the atmosphere via combustion, which is caused by chopping down trees and burning them to release the carbon from within. By planting roughly the same amount of trees we have chopped down, we make the combustion process effectively carbon neutral. This means that we are not putting to many hazardous gases into the atmosphere (let's not confuse ourselves with sulphur dioxides and carbon monoxides!).
Forests and jungles are important for carbon dioxide reduction, but also as many creatures use these areas as a place of sanctuary, by cutting them down we are single-handedly destroying species.
If we start planting trees, we can make soil that is considered infertile at the moment useful and tropical. This could also stop the process of desertification!
Watch out sahara!
Reforestation reclaims land that High Methane and CO2 industrys would otherwise take and use. Reforestation allows more individuals to contribute to the fight against global warming and spreads awareness of the issue on a grassroots level .Reforestation has the added bennift of re-establsihing an eco-system, which in it's self contributes to stabilizing the regional climate
As long as the tree remains healthy, it just keeps stealing carbon dioxide from the air and storing carbon in its cellulose. Carbon remains there even thetree becomes furniture or lumber. It si released only when the wood rots of is burned.
Carbon dioxide levels in the atmosphere have increased by a third since the start of the Industrial Revolution due mostly to the burning of coal and other fossil fuels. That bulidup has been linked to global warming.
Accordind to a neat calculator on the Web site of Casey Trees (http://www.Caseytrees.Org), a fair-size white oak tree with an 18-inch-diameter trunk would reduce atmospheric carbon dioxide by 622 pounds per year. A mature apple tree would lower carbon dioxide by more than 300 pounds.