Environmental Engineering Reference
In-Depth Information
Table 11.2
Derivation of the Global Harvest of Woody Biomass in 2000 (in Gt of dry weight)
Category
Range of Estimates
Best Estimates
Fuelwood removal
1.6-2.0
2.0
With phytomass losses
2.6-3.3
3.0
Roundwood removal
(3.43 Gm
3
)
2.6
With illegal cutting
3.0
With phytomass losses
6.0-7.5
6.5
Fuelwood and roundwood
8.6-10.8
9.7
Including roots (+25%)
10.8-13.5
12.0
Including disturbances
11.8-15.0
13.4
In light of these multiple assumptions and corrections, it is not surprising that
the appropriation studies did not handle this in any uniform manner. A narrowly
dei ned removal account by Vitousek et al. (1986) ended up with 2.2 Gt/year during
the late 1970s. Rojstaczer, Sterling, and Moore (2001) added 0.9 Gt of fuelwood
and 1.5 Gm
3
of timber (the equivalent of 0.83 Gt, using their conversion mean of
560 kg/m
3
) to get about 1.75 Gt of removed phytomass during the late 1990s, and
multiplied the total 2.7 times to convert merchantable wood to total forest biomass:
that adjustment, if applied to all wood removals, would yield an annual appropria-
tion of about 4.75 Gt. In contrast, Imhoff and Bounoua (2006) put wood product
harvests (fuel, construction paper) at about 7 (4.84-8.53) Gt C in 1995, with fuel-
wood accounting for about 60% of that total. Assuming a 50% carbon content,
their aggregate appropriation comes to about 14 Gt of dry matter, a total that is in
an excellent agreement with my liberal estimate that includes all below- and aboveg-
round woody phytomass removed, lost, and degraded in the course of wood harvests
(see table 11.2).
Expressing wood harvests as a share of standing forest phytomass or as a fraction
of global forest NPP is subject to substantial errors. The forest inventories of rich
nations are fairly reliable, and hence the calculations of harvest shares will be close
to actual values. For example, the end-of-twentieth-century U.S. forest inventory
listed 23.65 Gm
3
of growing stock (about 14 Gt) in the country's timberlands and
an annual growth rate of 670 Mm
3
(USDA 2001), compared to nearly 490 Mm
3
(that is, 250-290 Mt, including all timber, pulp, and fuelwood) cut in the year 2000
(USDA 2003). U.S. wood removals were thus equal to 2% of the standing stock
in all timberlands and 73% of the annual growth of all merchantable timber. But
during the mid-1990s the total standing phytomass of all U.S. forests (timberlands,
