AFAIK (I'm not a botanist) it's true of many larger trees that they use more oxygen than they produce and emit more CO2 than they consume. It's the biosphere that the large trees support that does a lot of the carbon sinking - mosses, ferns, vines, etc.

As a rule of thumb, the greater the ratio of woody mass to leafy mass the more the ratio tilts away from being a carbon sink, as the whole lifeform has to undergo aerobic respiration but only the leaves participate in photosynthesis.