You aren't accounting for the trees being living, growing beings. Asking how they capture CO2 is kind of like asking how we grab oxygen- it's in the air, we use it to fuel our growth and stay alive, and then it's CO2 when it leaves our bodies. It's the reverse for trees, I don't know if any significant amount of solid carbon would need to accumulate, as it forms the wood and the varies trees that grow and die.