Do new forests or old ones capture more carbon dioxide from the atmosphere?

This week's question comes from Trip Kennedy in Victoria, British Columbia. He asks:

With all the recent attention being paid to climate change and decarbonizing our atmosphere, I am curious, which takes more carbon dioxide out of the atmosphere - 100 hectares of mature old growth forest, or 100 hectares of young forest?

Gregory Paradis, a forester, engineer, and assistant professor of forest management in the Faculty of Forestry at the University of British Columbia has an answer.

Trees capture carbon from the atmosphere by converting sunlight to cellulose through photosynthesis. When trees die and fall to the ground, they gradually emit most of this captured carbon back into the atmosphere. Young vigorous stands grow and sequester carbon at maximum speed. As stands get older, the tree canopy closes and individual trees begin to die off from self-thinning and other causes. 

Very old forest stands can reach a sort of carbon neutral equilibrium state where trees are dying and decaying at approximately the same rate as they are growing back. 

So, taking into account both growth and mortality, 100 hectares of young forest will generally speaking have a higher net carbon capture rate than older but otherwise identical stands.  

Paradis said that research has shown that the optimal landscape-level carbon sequestration policy may be to harvest and replant stands when they reach their peak growth rate. This is typically between 80 and 120 years old for most Canadian forest ecosystems, much younger than what is typically called old growth. 

Ideally we would use the harvested forest material — wood and fibre — to displace as much fossil fuel, steel, and concrete as possible, to reduce carbon dioxide releases.