They don't make true "wood" in the stems. It is very soft, has a low density of carbon compared to true trees, and that carbon decays very quickly back to the atmosphere. With true trees the carbon is trapped in the form of lignin which is very strong and very chemically stable.
There is a quite a bit of variability even with true trees. For example, our
wood shed currently contains a mix of both linden and beech. The beech logs are twice as dense as the linden, burn for more than twice as long, and have much more heat. Structurally they have more carbon in them.
The complication comes when you consider speed of growth as a factor. A fast growing tree stores more carbon per year; but they also tend to be short lived and the wood tends to have low density and decay more quickly.
Beyond that, unless the carbon is permanently removed from the biosphere - eg by making and burying
biochar - the stored carbon from any tree is only temporarily locked up. In geological terms it returns to the atmosphere very rapidly.
