Genuine question, if old-growth forest do not work as carbon sinks, how all the petroleum we are extracting comes from? Is all from ancient marine environments?
Petroleum is mostly marine microorganisms. Coal is mostly old forests from a time before fungi developed the ability to break down lignin. Dead trees back then just continued to pile up without ever rotting.
The article doesn't say old growth forests don't work as carbon sinks, just that don't work as well as they initially thought.