It has been argued that cutting old-growth forests and replacing them with plantations of young trees would help alleviate the threat of global greenhouse warming. What important fact does this argument ignore?

Old trees provide a better environment for new trees. If you cut down old trees, you loose suitable conditions there.

One old tree is capable of providing huge amount of oxygen. One young tree (2 years old) is not. Nobody can guarantee all young trees will reach maturity in the future even under the best management practice. But old trees allow new trees to grow.

Clearcutting practice is especially dangerous. If this practice is selected, soil organic matter, that is necessary for new trees, as well as nitrogen and phosphorus, and other factors will most probably go away due to erosion.

There are many issues but to be concise I provided only some important facts here.

This argument ignores the fact that old-growth forests play a crucial role in sequestering large amounts of carbon dioxide from the atmosphere, and replacing them with young tree plantations would significantly reduce their capacity to mitigate greenhouse gas emissions.