That’s not true. Methane is a significantly more potent greenhouse gas than CO2, so if the tree is absorbing CO2 but the decomposing needles and leaves are emitting methane, it could easily be a net GHG emitter.
If a tree keeps growing, but also gets more successful at suppressing growth of it's neighbors, then it can be a net emitter overall even though it's a net absorber considered individually.
