The ratio of total to merchantable forest biomass and its application to the global carbon budget

Abstract

Records of merchantable forest volumes can be used to estimate rates of carbon storage or depletion using a ratio to convert merchantable weights to total forest biomass (T/M ratio). We present evidence that the T/M ratio used to estimate carbon storage in midlatitude forests has been seriously underestimated by neglecting carbon in trees of unmerchantable size and quality and in coarse and fine litter. Ratios for forest types and size classes in Virginia based on detailed plot-level analysis ranged from 2.1 to 5.0; the mean weighted ratio of 2.7 was 55% greater than a ratio currently in use. More general analysis indicated that the T/M ratio for Virginia was representative of forests of the East; forests of the western United States were comparable to those of the East when woody debris was included in the estimate of total biomass. Application of the weighted ratio to growth of United States forests during 1952–1977 yielded a per-annum accretion of carbon in biomass (excluding soil carbon) of 0.15 Gt C•year−1, about 10% of the 1.6–1.9 Gt C•year−1 computed for midlatitude forests. More complete studies of counterbalancing carbon losses from forests, particularly losses in litter and soils after forest harvest and conversion to agriculture, are needed before the source or sink nature of midlatitude forests can be determined with confidence.