Trees remove carbon dioxide from the atmosphere as shown in the diagram below.
Carbon is absorbed through photosynthesis while the tree is growing, and it remains stored in the wood of the mature tree.
A plantation forest is thus a carbon reservoir, even if individual stands are continually harvested and replanted. When wood products are finally biodegraded or burned, the carbon absorbed during growth is released to the atmosphere, so the products are therefore carbon neutral.
However, fossils fuels are used to plant, harvest and process the timber, hence there are some net emissions of CO2 over the full life cycle of wood products. If waste wood is burned for energy instead of fossil fuel, there is a large reduction in CO2 emissions because the emitted carbon is only that absorbed during growth of the trees.