Christine Goodale, Cornell University, addressed effects from direct human-induced changes in land use, forestry, and agricultural activities on terrestrial carbon stocks. She described the main land use states, including native vegetation (e.g., forests, grassland, savanna), cropland, pasture, wood harvest and recovery, plantation forests, and others (degradation, restoration, urban/suburban). A host of details affect the estimated carbon sink for each type of land use. For example, in croplands it is necessary to know the types of crops, crop rotation and duration, management regime, and soil amendments. Goodale suggested that sensitivity analyses with existing data and models be used to help discern whether and where such distinctions matter but noted that sufficient data might not currently exist over an adequate time horizon and on relevant spatial scales to make such a determination.
Carbon stocks primarily exist in living biomass, dead biomass, forest floor litter, soil, and wood products. Goodale stated that four key terms affect any estimation of carbon stocks and fluxes: (1) initial carbon stock of the system, (2) immediate or short-term changes in stocks due to land use change, (3) time required for the initial disturbance and for recovery, and (4) the area over which land use change occurs. Initial estimates of carbon stocks are influenced by a number of factors, such as the type, age, and state of an ecosystem. In northern systems this information tends to be relatively well known, but in the Southern Hemisphere the forest carbon stocks are not as well known. For example, Goodale reported that among seven estimates of total forest carbon stocks in the Amazon, the totals ranged from 39 to 93 Pg, and even among those with similar total estimates of carbon stocks, the spatial distribution of biomass varies considerably (Houghton et al., 2001). Knowing the spatial distribution of biomass is important to assess the effect of deforestation in different regions. Goodale suggested that comprehensive inventories of the tropical region and improvements in satellite remote sensing are needed.
Once the initial carbon stock is known, the effect of a particular land use change needs to be determined. Some questions to be answered include what fraction of the initial biomass is killed, how much is removed, how much is burned, and how much is converted into wood products. Inputs and assumptions for all these terms affect estimates of how land use change is altering carbon sinks. Some terms, such as soil carbon loss after cultivation, seem reasonably well constrained, as literature reviews consistently estimate this value at about 25 percent (Mann, 1985; Johnson, 1992; Davidson and Ackerman, 1993; Murty et al., 2002). The effects of forest harvest on soil carbon appear to vary, showing both increases and decreases depending on