Today I took part in a huge event where about 1000 volunteers planted 30,000 native trees. The area that is being reforested is a nature reserve and by planting native vegetation it is hoped it will encourage native wildlife back in to the area.
As well as providing a home for wildlife, planting growing trees helps remove some carbon dioxide from the atmosphere.
This is the area we reforested: http://waiwhakareke.co.nz/62/arbor-day.
If you want to help re-forest the world, there are local events in many places. I also really respect the work of The Nature Conservancy: as well as protecting many natural areas, they have a campaign to plant 1 billion trees, where each $1 donated gets 1 tree planted. Their website is http://www.nature.org.