With 11 tonnes CO2 equivalent per hectare per year, the Ancient and Primeval Beech Forest of the Carpathians and Other Regions of Europe obtains a fifth place in the top 5 among the natural and mixed UNESCO World Heritage sites.
Together, the World Heritage forests form a carbon sink for approximately 13 billion tonnes. Between 2001 and 2020, they absorbed about 190 million tonnes of carbon dioxide from the atmosphere each year.
More details on how this was estimated can be found in the UNESCO report:
https://unesdoc.unesco.org/ark:/48223/pf0000379527.locale=en