The duration is important because you can only grow and harvest a finite amount of wood each year, because trees can only grow so fast. So if you harvest the m

The duration is important because you can only grow and harvest a finite amount of wood each year, because trees can only grow so fast. So if you harvest the maximum amount of wood possible each year, W, and it lasts for 10 years, you have sequestered 10 x W units of carbon; if you use it in ways that lasts 20 years before it needs to be replaced, you have sequestered 20 x W units of carbon.