If all forests would be left alone, instead of actively managed like?
No. Managed forests should be better at sequestering CO_2 than natural forests. This is because trees sequester the most CO_2 during the peak growth period. Once trees are mature they sequester almost no CO_2 and if they die and decompose naturally most of the carbon is released back into the atmosphere and only a small amount is sequestered in the soil. So felling the mature trees and using the timber and planting new trees means that the forest always has new growth trees which are sequestering CO_2. Obviously, if your objective is sequestering you must not use the wood as firewood.