Measurement of Biomass

There are two aspects to quantifying biomass: measurements at specific sites and methods for extrapolating the results from such sites to large areas. Measurement at a site is, in principle, straightforward. It involves harvesting all plant material within a defined area, drying it to a constant weight, and weighing it. In practice, this type of destructive measurement becomes more difficult if the belowground portions are included and if the vegetation includes large trees. Sorting roots from the soil-root matrix is difficult. The size of the plot is also important: small plots will either over- or underestimate the average biomass if they include or exclude large trees, respectively. Perhaps, more importantly, selection of sites is critical. Sites need to be representative of existing forests, not representative of some idealized notion of a forest. Sampling a large number of sites may help minimize biased selection.

81-100 1201-220

Figure 2 Forest stands of different age in eastern Krasnoyarsk, central Siberia. Each cell is 500 m x 500 m.

81-100 1201-220

Figure 2 Forest stands of different age in eastern Krasnoyarsk, central Siberia. Each cell is 500 m x 500 m.

Because destructive sampling is time consuming (expensive), foresters and ecologists have developed indirect methods for estimating biomass. The most common approach uses allometric equations that allow the estimation of tree biomass from more easily measured properties, such as diameter at breast height (dbh), height, and wood density. Systematic sampling (forest inventories) with such measurements and equations allow the biomass (or commercial wood volumes) to be obtained over large areas.

Because temperate zone and boreal forests have been inventoried more than once, their biomass is reasonably well known (Table 2). Accuracies vary among regions and countries but they are generally high for wood volumes. In the Southeastern US, for example, the error calculated for the total volume of growing stock was within 1.1%. On the other hand, 'changes' in the stock had an error of ±40%. The much larger error associated with change results from magnitude of the change relative to the magnitude of the stocks. Small differences between large numbers are difficult to determine precisely.

Although forest inventories have been carried out in some tropical forests, large areas have rarely been inventoried. Nevertheless, estimates of the average tropical forest biomass exist (Table 3), based on one of three approaches: (1) dividing the forests into different classes (each type corresponding to an average biomass), (2) calculating biomass as a function of environmental parameters (e.g., mean annual temperature and the seasonal distribution of precipitation), and (3) 'direct'

measurement from remote sensing data. A comparison of seven estimates for the Brazilian Amazon revealed not only a wide range (greater than a factor of 2 between the lowest and highest estimates of total biomass), but also no agreement as to where the forests with the highest and lowest biomass existed (Figure 3). More recent investigations with radar and lidar show much more consistent results, and satellites to measure aboveground biomass directly are being designed by space agencies in Japan, Canada, Europe, and the United States.

Such satellites, in concert with field measurements, are urgently needed. A comparison of Tables 2 and 3 with Table 1 suggests an uncertainty in biomass that is a factor of 2, especially if the differences for forests apply to other ecosystems. Table 1 is a recent compilation of site-specific measurements in different types of ecosystems. In contrast, Tables 2 and 3 are based on forest inventories with a much larger number of measurements, presumably more representative of existing forests. Although some of the differences may be the result of changes wrought by human activity (natural forests degraded through use), most of them are probably the result of a biased selection of sites for Table 1 .

Table 4 shows an alternative estimate of global biomass, where the estimates for forests are derived from Tables 2 and 3 and the estimates for other ecosystems are the same as in Table 1 . Although the areas of forests (and land ecosystems) are similar in Tables 1 and 4, global biomass from this new compilation is approximately half of that in Table 1 (770 as against 1300 Pg). That is, global biomass is not known to even one

Table 2 Areas, total carbon stocks, and average carbon stocks in the biomass of forests and woodlands in the northern temperate and boreal zones in 1990

Forest

Other

Woodland

Average forest

Average woodland

area

woodland area

Forest living

living biomass

biomass

biomass

Region

(10e ha)

(106 ha)

biomass (Pg)

(Pg)

(Mgha1)

(Mgha1)

Canada

316

88

26

3.2

82

36

United States

212

86

27

6.6

125

77

Europe

149

46

15

0.4

103

9

Russia

821

66

67

1.2

82

18

China

119

39

9.2

1.2

77

31

Other®

92

16

9.4

n/a

102

n/a

Total

1710

340

154

12.6

90

37

Forest and other

2050

166.6

81

woodland

combined

aCountries included: Japan, North Korea, South Korea, Mongolia, Latvia, Lithuania, Estonia, and the Commonwealth of Independent States other than Russia.

Adapted from Goodale CL, Apps MJ, Birdsey RA, etal. (2002) Forest carbon sinks in the Northern Hemisphere. Ecological Applications 12:891-899.

aCountries included: Japan, North Korea, South Korea, Mongolia, Latvia, Lithuania, Estonia, and the Commonwealth of Independent States other than Russia.

Adapted from Goodale CL, Apps MJ, Birdsey RA, etal. (2002) Forest carbon sinks in the Northern Hemisphere. Ecological Applications 12:891-899.

Table 3 Areas and average biomass of natural forests in tropical regions®

Region

Forest area (10e ha)

Total biomassb

Average biomassb (Mgha 1)

Asia

264

37

140

Africa

636

85

134

America

952

225

236

Total

1852

348

188

aIn Africa, three nontropical countries, Lesotho, South Africa, and Swaziland, are included, while the six countries of northern Africa are not. In Latin

America, three nontropical countries, Argentina, Chile, and Uruguay, are included.

bValues of aboveground biomass were increased by a factor of 1.2 to include belowground biomass.

Adapted from FAO (2001) Global Forest Resources Assessment 2000. Rome: Food and Agriculture Organization of the United Nations.

aIn Africa, three nontropical countries, Lesotho, South Africa, and Swaziland, are included, while the six countries of northern Africa are not. In Latin

America, three nontropical countries, Argentina, Chile, and Uruguay, are included.

bValues of aboveground biomass were increased by a factor of 1.2 to include belowground biomass.

Adapted from FAO (2001) Global Forest Resources Assessment 2000. Rome: Food and Agriculture Organization of the United Nations.

significant figure. Similarly, estimates of the average biomass of forests vary by more than a factor of 2, a fact that is remarkable given that wood volumes are determined to within 1-2% in national forest inventories. Some fraction of the discrepancy results from scaling wood volumes to total biomass, including not only roots, leaves, and branches, but also understory vegetation, noncommercial species, and trees smaller than those generally inventoried. Much of the difference between estimates, however, is probably the result of site selection.

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