Quantities of Biomass

The quantity of biomass per unit area varies spatially and temporally. Living biomass ranges over 2-3 orders of magnitude, from averages of ^400Mgha_1 in tropical forests to averages of less than 10Mgha~ in treeless grasslands, croplands, and deserts (Table 1). Biomass

Table 1 Mean living biomass, area, and total living biomass of the world's major terrestrial ecosystems

Ecosystem type

(Pg)

Mean biomass (Mgha-1)

Tropical forests

1 750

680

390

Temperate forests

1 040

280

270

Boreal forests

1 370

110

83

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

14 930

1 308

87

Adapted from Saugier B, Roy J, and Mooney HA (2001) Estimations of global terrestrial productivity: Converging toward a single number? In: Roy J, Saugier B, and Mooney HA (eds.) Terrestrial Global Productivity, pp. 543-557. San Diego, CA: Academic Press.

Adapted from Saugier B, Roy J, and Mooney HA (2001) Estimations of global terrestrial productivity: Converging toward a single number? In: Roy J, Saugier B, and Mooney HA (eds.) Terrestrial Global Productivity, pp. 543-557. San Diego, CA: Academic Press.

also varies considerably within ecosystem types. Some tropical forests and forests of the Pacific Northwest in North America may attain values in excess of 600Mgha~\ In part this variability results from differences in environment (e.g., soil nutrients or the seasonal distribution of precipitation), and in part it results from disturbance and recovery. A recently burned forest has a living biomass of nearly zero, and as it recovers it reaccumulates carbon (Figure 1). Forests do not accumulate biomass indefinitely, however, because stand-replacing disturbances keep turning old forests into young ones.

Most of the biomass in forests is in the stems, or boles, of trees, with branches, roots, and foliage accounting for lesser fractions. The fractions vary with growth, however. As forests grow larger, a greater and greater fraction of total biomass occurs in boles.

The estimation of forest biomass depends, in part, on spatial scale. At the level of a forest stand, biomass varies through time as a result of disturbances (and recovery). At the landscape scale, biomass varies through space because the ages (since the last disturbance) of patches

r 300

K 200

5 50

r 300

K 200

5 50

--U-i

/—

/

A /

/

/I/

/

/

0 15 30 45 60 75 90 105 120 135 150 Years

0 15 30 45 60 75 90 105 120 135 150 Years

Figure 1 Changes in the living biomass of a forest (Mg ha-1) in response to disturbances (e.g., fire or logging) and recovery. Each disturbance removes living biomass, which subsequently reaccumulates as a consequence of growth.

vary across the landscape. Figure 2 shows the ages of forest stands in a ^450 km2 area in the province of Krasnoyarsk, Russia. The remarkable feature of this landscape is the degree to which the forest is a mosaic of different aged stands. The spatial variability in biomass is much greater than it appears from observations of the canopy from the air or from space and suggests that most forests are in the process of recovering from natural or human-induced disturbances.

The estimates of biomass shown in Table 1 suggest a global total of ~ 1300 Mg, and forests account for more than 80% of that total. But the uncertainty is high. Another estimate suggests a global terrestrial biomass of only 770 Mg (see below).

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