Source: http://www.pacificforest.org/policy/forestcarbon.html

An acre of Douglas-fir forest at sixty years contains roughly 125 - 150 tons of carbon and accumulates carbon at 2.2 tons/year.  An acre of 10 year old Douglas-fir may contain roughly 50 tones of carbon and accumulate carbon at roughly 1.4 tons/year.  An old-growth forest of more than 500 years may contain more than 1,000 tons of carbon per acre, and accumulate at about a quarter-ton/year.