Trees rotting produces compost and over time soil, soil which ultimately contains organic carbon. Perhaps overtime the organic carbon from the original tree is

Trees rotting produces compost and over time soil, soil which ultimately contains organic carbon. Perhaps overtime the organic carbon from the original tree is entirely gassed into the air somehow, but in the meantime you've been growing a second, third, and forth. Growing and composting plants does contribute to the sequestration of carbon and more importantly prevents methane production.