Even if you let them fully mature they will eventually breakdown because that’s what trees do and then all that stored carbon will return to the atmosphere. This carbon capture is mostly fruitless as the amount of carbon they store is negligible compared to how much we are adding to the atmosphere but if they are turning it into “rock” which is likely just graphite that would take carbon out of the carbon cycle and actually sequester it. which we desperately need to do to offset the ridiculous amount sequestered carbon we are adding to the atmosphere