Forest carbon offsets are becoming an increasingly popular climate mitigation option.  However, many questions surrounding the role of forests as a carbon sink remain.  One of the most critical items to address is the maximum amount of carbon that can be potentially stored in a forest of a given type.  How much more carbon can be stored in an old-growth forest compared to an undisturbed second-growth forest, or a sustainably managed working forest?  We begin to address that question with a benchmark study that estimates carbon stocks in surface mineral soil (0-20 cm), forest floor, down dead wood, standing dead, and aboveground live biomass pools in hardwood and softwood old-growth forests in Maine, New Hampshire, and Vermont. All hardwood stands were northern hardwoods; softwood stands were either spruce, spruce-hemlock, or hemlock.  Twenty-four temporary plots were established (four hardwood and four softwood sites per state) and inventoried using standard methods.  Estimates of aboveground live and dead biomass carbon were corrected to account for decay and missing tops as noted during inventory.

Results/Conclusions

Carbon stock estimates were fairly consistent within forest types across the sites, even though the characteristics of each stand were different.  In hardwood forests, carbon in aboveground live biomass ranged from 94 t/ha in New Hampshire to 139 t/ha in Vermont, while estimated total stocks for all pools measured were 214-218 t C/ha.  In the softwood stands, carbon in aboveground biomass varied from 111 t/ha in Vermont to 143 t/ha in Maine.  Total carbon was significantly higher in softwood stands than in hardwood stands in all states, ranging from 234-292 t/ha.  Regardless of site, down dead wood was always a small carbon pool, averaging less than 8% of total stocks.  Carbon stored in the upper 20 cm of the mineral soil was fairly consistent, generally averaging between 63-72 t/ha, with no consistent differences between hardwood and softwood stands.