Land-use is a key part of greenhouse gas cycling, with changes like deforestation and increased agriculture having strong effects on the size of GHG sinks and sources.
Big sinks for GHG include forestry and there is a lot of research on how much GHG forests can actually trap. However, they are probably not as easy an answer as they appear because recent studies have shown that although they might initially mop up elevated CO2, they soon after release this extra CO2 back to the atmosphere.
Still, each tree does provide a trap for carbon dioxide and they certainly can help. In fact an organisation called Future Forests will actually plant trees for businesses or individuals to offset their carbon emissions.