Changes in Biomass

Biomass (dry weight) is approximately 50% carbon, and the amount of carbon in terrestrial biomass (385650 PgC) is of the same order of magnitude as the amount of carbon in the atmosphere (^780 PgC) and in the surface layers of the ocean (^700 PgC). Thus, changes in terrestrial biomass have a significant effect on the concentration of CO2 in the atmosphere.

Houghton et al. Potter

Houghton et al. Potter

Figure 3 Three estimates of the distribution of forest biomass in the Brazilian Amazon. Reproduced from Houghton RA, Lawrence KT, Hackler JL, and Brown S (2001) The spatial distribution of forest biomass in the Brazilian Amazon: A comparison of estimates. Global Change Biology 7: 731-746, with permission from Wiley-Blackwell Publishing Ltd.

Table 4 Area, total living biomass, and mean biomass of the world's major terrestrial ecosystems

Area

Total biomass

Mean biomass

Ecosystem type

(106 ha)

(Pg)

(Mgha-1)

Tropical forestsa

1 850

350

190

Temperate forestsb

2 450

185

75

Arctic tundra

560

4

7

Mediterranean shrublands

280

34

120

Croplands

1 350

8

6

Tropical savannas and grasslands

2 760

160

57

Temperate grasslands

1 500

12

8

Deserts

2 770

20

7

Ice

1 550

0

0

Total

15 070

770

bIncludes temperate and boreal forests from Table 2 and Australia's forests (from Dixon RK, Brown S, Houghton RA, et al. (1994) Carbon pools and flux of global forest ecosystems. Science 263: 185-190). All other rows are from Table 1.

aFrom Table 3.

bIncludes temperate and boreal forests from Table 2 and Australia's forests (from Dixon RK, Brown S, Houghton RA, et al. (1994) Carbon pools and flux of global forest ecosystems. Science 263: 185-190). All other rows are from Table 1.

The carbon released to the atmosphere as a result of deforestation and degradation of tropical forests (1-2 PgC yr-1) is thought to account for 10-20% of anthropogenic emissions (8.4 PgC yr-1 were released in 2006 from fossil fuel combustion). In contrast, temperate zone and boreal forests are believed to account for a significant carbon sink in northern mid-latitudes 2 PgC yr- ).

Knowing 'changes' in biomass is crucial for determining terrestrial sources and sinks of carbon. However, the carbon sink in northern mid-latitude forests calculated on the basis of data from forest inventories (0.65 PgC yr-1) is smaller than the carbon sink inferred from the top-down approaches to evaluating carbon sources and sinks from atmospheric data and models 2 PgC yr- ). The difference might be explained by errors (both estimates have wide ranges of uncertainty), by ecosystems other than forests that are not systematically inventoried, or by the accumulation of carbon belowground (roots and soil carbon are not measured by forest inventories).

If the northern mid-latitude sink of —2.0 PgC yr-1 were evenly distributed over the forests of the region, the average annual increase in forest biomass would be 1.2 MgC ha-1 yr-1, or 2.7%. Such a change would be difficult to observe with an error of ±40%. However, the sink is not evenly distributed in space. Forest inventories indicate that Canadian and Russian forests lost living biomass -1990 (0.08 PgC), while forests in the US, Europe, China, and other northern regions gained it (a total of 0.28 PgC). The spatial variability raises the possibility that the major terrestrial sources and sinks of carbon may be much larger than the average sink. What if, for example, 90% of the net terrestrial flux of carbon occurs on lands where the annual changes are large, 12 MgC ha-1 yr-1, instead of 1.2 MgC ha-1 yr-1? At present, we do not know what fraction of the northern forests is growing and what fraction is already 'grown' (Figure 2). We do not know whether the terrestrial carbon sink is distributed over very large areas

(with small annual changes) or limited to areas characterized by rapid rates of change.

In the tropics, the general lack of repeated forest inventories makes it impossible to estimate the sources and sinks of carbon directly from observation. Rather, changes in land use have been used, together with the changes in biomass known to be associated with land-use change, to calculate sources and sinks of carbon. The calculated emissions from tropical deforestation and degradation vary between 1 and 2 PgC yr-1. Uncertainties in biomass contribute about as much to this variability as uncertainties in rates of deforestation. Moreover, estimates of average biomass are insufficient for accurate estimates of carbon emissions. Regional averages, even if accurate, do not necessarily correspond to the biomass of the forests actually deforested.

Carbon emissions determined from deforestation and degradation in the tropics are consistent with the net source of carbon inferred from top-down analyses based on atmospheric data and models. On the other hand, repeated measurements of biomass on small plots of undisturbed forests throughout the tropics indicate an increase in the biomass of (undisturbed) Amazonian forests (although not in tropical Africa or Asia). If the increase applies over large areas of Amazonia, it represents a significant carbon sink. The reasons for the increase in Amazonian biomass are unclear. Are the forests recovering from earlier disturbances, or are they responding to a changing environment (e.g., higher concentrations of CO2 in the atmosphere or less cloudiness)? Answering this question is important for understanding whether the accumulation of carbon in biomass is part of a steady-state system or a new carbon sink, and, if it is new, whether it can be expected to continue.

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