Anecdotal evidence suggests to me that European spruce stores more CO2 than it sheds way over 80 years old as a spruce grows in mass significantly decades after reaching that age.

However here is a study that tries to bring some light to carbon pools on over-mature 167-213 year-old trees: https://www.mdpi.com/1999-4907/9/7/435/pdf

You don’t need a source. To be a net emitter the tree would have to be losing mass. That’s not what healthy trees do, even old ones.

I believe the article might have been trying to say is that trees tend to die and decompose (or burn) which makes them carbon emitters.

Wardle, P. (1991). Vegetation of New Zealand. Cambridge: Cambridge University Press.

If I have time tomorrow i'll find the page.