Environmental Engineering Reference
In-Depth Information
to yield the annual roundwood harvest and removal of at least 6 Gt, and an upper
bound of adjustment would be to multiply by 2.5 and raise the total to 7.5 Gt of
dry weight.
This correction should also apply to at least a quarter of all fuelwood harvests
that are done in commercial fashion rather than by children and women collecting
woody litter; this would add 1-1.3 Gt of dry wood (2.0/4 = 0.5
×
2 = 1 Gt; 2/4 =
0.5
2.5 = 1.25 Gt). These adjustments would mean that overall harvest of
woody phytomass in the year 2000 would have been 2.6-3.3 Gt of fuelwood, char-
coal, and associated losses, and between 6 and 7.5 Gt of roundwood removed from
forests, destroyed during harvesting, or abandoned after harvests, for a grand total
of 8.6-10.8 Gt of absolutely dry weight. These totals should be enlarged by an
average of 25% to account for root biomass (Cairns et al. 1997), raising the global
woody phytomass appropriation to 10.8-13.5 Gt.
Further additions could be made by considering a variety of other human impacts.
Forest disturbances resulting from construction (road and dam building), settle-
ments, and logging will lead to reduced productivity that would have to be quanti-
i ed as a difference between potential and actual photosynthesis. In some areas this
impact may be of the same order of magnitude as tree clearing: an analysis of satel-
lite imagery found that between 1972 and 2002, 15% of Papua New Guinea's
tropical forests had been cleared and another 8.8% degraded through logging
(Shearman et al. 2009). A very conservative global estimate of such degradations is
at least 5 Mkm
2
, and the real extent may be twice as large. Even if the degradation
were to reduce average NPP by no more than 10%-15%, the annual productivity
loss would be on the order of 1-1.5 Gt of dry phytomass, pushing the annual grand
total to 11.8-15 Gt of dry mass in the year 2000. Consequently, a broad dei nition
of human claims on woody phytomass in the year 2000 would bring the most likely
total to about 13.5 (12-15) Gt of dry matter. Table 11.2 summarizes the derivation
of that annual rate.
More phytomass will be lost to anthropogenic i res as commercial logging
opens up previously inaccessible areas to settlement. And tree growth will change
(the shift can also be positive, at least for a period of time) as a result of the long-
term effects of atmospheric deposition, above all from soil acidii cation in some
regions and nitrogen enrichment (from the deposition of airborne ammonia and
nitrates) in others. The latter process, well demonstrated in temperate regions, is
now affecting even some tropical forests (Hietz et al. 2011). None of these effects
can be quantii ed without resorting to concatenated and uncertain assumptions, but
the inclusion of all of these factors could raise the aggregate claims by at least 10%.
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