Global carbon from vegetation

Figure 1. The global carbon cycle. Estimates of reservoir size and annual fluxes are from Post et al. (4), Vegetation carbon reservoir was estimated from latest carbon density estimates. All values except the atmospheric reservoir are approximate only. All values are in gigatons. Fluxes are next to the arrows and are in gigatons ear.

Several studies, based on models, examined the effects of land-use change on the global carbon cycle and conclude that there is a net release of carbon due to land clearing. However, the results and conclusions of these studies are based on assumed sizes of vegetation carbon pools which are inputs to the models. For example, Melillo et al. 24) concluded that boreal and temperate deciduous forests of the northern hemisphere are net sources of atmospheric carbon. Their analysis used values for carbon density derived by Whittaker and Likens 19) from work by Rodin and Bazilevich (27). Rodin and Bazilevich extrapolated results of small, unrelated studies in Europe and the USSR to estimate total biomass of Eurasian boreal and temperate deciduous forests. Their estimates have since been extrapolated to forests worldwide and are used often today. 

Most estimates of global vegetation biomass densities are extrapolations from studies never intended to represent large areas (e.g. 79, 36) or they were derived from questionnaires sent to botanists (57). These estimates are still used commonly in the examination and modeling of the global carbon cycle. Some of the earliest estimates were made when almost no quantitative data were available and the data or the estimates were largely speculative. Other estimates are... 

Most vegetation maps are derived from a variety of sources using different methods and made at different times. This can lead to an overlap between adjacent areas of interest, the exclusion of some areas, and the improper extrapolation of carbon densities, thus resulting in inaccurate estimates of reservoir size. We found that the biomass density of the southern North American boreal forest was over 2.5 times larger than the biomass density of the northern part of the boreal forest (55). Past estimates of boreal forest biomass density extrapolated southern biomass density values to the entire boreal forest, which in part accounts for the large overestimation (7). It is important that a consistent method be developed to map vegetation globally. 

Soil contributes to a greater extent to total carbon storage than do above-ground vegetation in most forests (Johnson and Curtis 2001). The total amount of soil organic carbon (SOC) in the upper meter of soil is about 1500 x 1015 g C (Eswaran et al. 1993 Batjes 1996), and the global atmospheric pool of CO2 is about 750 x 1015 g C (Harden et al. 1992). The CO2 emission from soil into atmosphere is about 68.0-76.5 1015 g C per year, and this is more than 10 times the CO2 released from fossil fuel combustion (Raich and Potter 1995). Variations in SOC pools and SOM turnover rates, therefore, exert substantial impacts on the carbon cycles of terrestrial ecosystems in terms of carbon sequestration in soil and CO2 emission from soil. 

Figure 6.3. Soil organic carbon inventory to 1 m depth by parent material category, for California and globally. Well-drained soils in California (white bars) are from the Soil-Vegetation Survey data set, n = 568, well-drained soils only. Worldwide data (gray bars) are from Zinke et al. (1984), n = 2995, which includes the California Soil-Vegetation Survey data, all drainage classes. Reprinted from Torn et al. (1997).

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