How can that possibly be true? If forests were around for thousands or millions of years before humans started chopping them down, wouldn't the effective rate o

How can that possibly be true? If forests were around for thousands or millions of years before humans started chopping them down, wouldn't the effective rate of carbon in the air be near 0? Clearly carbon was getting back into the air somehow or there is only a limited amount of carbon forests could ever have stored on the face of the earth.