Environmental Engineering Reference
In-Depth Information
efi ciencies of fossil fuel combustion—the differences between simple wood stoves
and large coal or hydrocarbon-i red boilers, household gas furnaces, and various
internal combustion engines burning liquid fuels or natural gas are commonly two-
to threefold—biofuels contributed less than 5% of the world's i nal (useful) energy
supply.
Information about wood harvests for lumber and pulp has much smaller margins
of uncertainty. The FAO database lists roundwood consumption (a category that
includes all saw logs, veneer logs, and pulpwood) of about 1.7 Gm
3
in 1950, 2.88
Gm
3
in 1975, and 3.43 Gm
3
in 2000 (FAO 2011e). These volumes are for natural
(as felled) wood, and the conversion to dry matter would yield—assuming, as previ-
ously noted (Glass and Zelinka 2010), an average specii c gravity of 0.5 and an
average moisture content of 50%, that is, a weight of 750 kg of oven-dry weight
per cubic meter—nearly 2.6 Gt. Once again, these totals underestimate the actual
impact of harvesting because they do not account for woody phytomass that is not
included in the reported volumes of commercial harvests.
The i rst omission, and the one that is most difi cult to quantify, is wood obtained
by common illegal cutting. In 1999 it was estimated that as much as 27% of
German and 62% of British tropical timber imports could have been from illegal
sources (Forest Watch 2007). A very conservative assumption based on data,
reports, and estimates by Global Timber (2011) would be to increase the global
total by 15% to about 3 Gt. An omission that is easier to quantify is that of the
exclusion from harvest data of all small trees that are destroyed during forest
cutting: generally only live sound trees of good form with a diameter at breast
height of more than 12 cm and a 10 cm diameter at the top (measured outside
the tree bark) count.
U.S. statistics also exclude live sound trees of poor form: such cull trees usually
account for about 6% of the total forest phytomass (USDA 2001). More important
is the omission of all phytomass that does not become part of the merchantable
bole (stem, trunk): stumps and roots, branches, and treetops. Various formulas
convert merchantable stemwood volumes to aboveground phytomass (Birdsey 1996;
Penner et al. 1997). Species-specii c U.S. multipliers from timber to total phytomass
show a range from about 1.7 (southeastern loblolly pine) to just over 2.5 (spruce
and i r), with a median at about 2.1. An inventory of Canadian forests showed
multipliers of 2.17 in Alberta and 2.43 in British Columbia, and a nationwide
mean of 2.56 (Wood and Layzell 2003). Actual shares in large-scale harvesting
will depend not only on specii c tree composition but also on harvesting methods.
A conservative conversion would be to double the total harvested mass of 3 Gt
