Just use logical first principles. Wood is basically a bunch lignin and cellulose. Those are just a bunch of C, H, and O in different configurations. The more wood that exists somewhere, the more C, H, and O are sequestered in it. All forests are seasonally more trap or sink. Look at global seasonal CO2 concentration maps (if those haven't been taken down from the EPA website). Forests grow and make leaves during some seasons. They tend to have forest fires during others. Finally, many shed leaves and those rot and release CO2 during other seasons. On the net, though, they trap carbon dioxide, which plants specifically consume in addition to water to make glucose during photosynthesis.