We measured greenhouse gas regulation in several ways. Using interpolated forest growth datasets, stand-level aboveground GHG sequestration was estimated from standing biomass by multiplying by a carbon:biomass ratio of 0.498:1 (Birdsey et al. 1992). This carbon stock is a measure of potential greenhouse gas captured by the growing forest and kept out of the atmosphere.
We also estimated a carbon use benefit using the methodology described by Lippke et al. (2011) as a sum of three terms: product storage, product substitution, and carbon emissions associated with harvesting. Product storage refers to carbon stored in long-lived wood products (LLWP) when wood is harvested and used for building materials, furniture, etc. This carbon is released back to the atmosphere slowly as LLWP are retired and eventually decompose; carbon in biomass harvested and utilized for LLWP forms a carbon pool with an 80-year half-life (Perez-Garcia et al. 2005). When harvested biomass is used as an energy feedstock, on the other hand, carbon stored in the biomass is released into the atmosphere almost immediately and, therefore, no product storage takes place. Product substitution is the benefit resulting from the use of relatively low-carbon renewable products (e.g. wood) in place of carbon intensive products (e.g. steel) and fossil fuels. In general, this substitution value has been found to be significantly greater when biomass is used for LLWP as compared to energy feedstocks (Figure 1 for Hubbard Brook). For example, LLWP have a carbon displacement ratio – the estimated carbon savings for every unit of carbon in biomass utilized in place of carbon-intensive alternatives - of 2.1:1, whereas ethanol produced by gasification has a carbon displacement ratio of only 0.38:1. It is important to acknowledge that the adoption of a product substitution term in this framework assumes that products DO substitute for other, more carbon-intensive products. In actuality, however, they might simply be additional to the economy (e.g. if increased availability of bioenergy products reduce overall energy prices, and therefore increase total energy demand) or might simply substitute for other products of similar carbon intensity (e.g. wood products substituting for other wood products). Assuming carbon substitution increases the value of the carbon use benefit (Figure 1 for Hubbard Brook). Finally, carbon emissions associated with harvesting are subtracted from the storage and substitution pools in order to come up with the final value of the carbon use benefit. In our analyses, carbon emissions from logging operations were assumed to be equivalent to 6% of the carbon contained in the pre-harvest stand at a site (Lippke et al. 2011).
Data on total belowground carbon flux did not exist for many watersheds. To illustrate our methodology, however, we took data on belowground fluxes of methane, nitrous oxide, and carbon dioxide from a short period (1997-2009) at two watersheds at Hubbard Brook and translated them into standardized measures of carbon dioxide equivalents using weightings (Figure 2) provided by the EPA Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts or TRACI (Bare et al. 2003).
FEST case studies involving this service include quantification of GHGR at Hubbard Brook and Turkey Lakes, as well as the inclusion of GHGR metrics in a tradeoff analysis at Hubbard Brook. These case studies include data on aboveground carbon only, and do not include product storage, product substitution, or emissions due to harvesting. The value of aboveground carbon mitigation was also estimated for managed northern hardwoods stands in the Adirondacks. In this case study, the full carbon use benefit was calculated using carbon displacement ratios of 2.1:1 for LLWP and 0.49:1 for energy feedstocks (assumed to be used as cordwood fuel in a small boiler). The monetary value of the greenhouse gas regulation service was also estimated in this case, using a value of $4.9 CO2-eq-1 (World Bank 2014).