Environmental Engineering Reference
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actually boosted such sites' NPP. But the overall global NPP had to decline, from
perhaps as much as 70-75 Gt C/year about 8,000 years ago (at the beginning of
the agricultural expansion) to less than 60 Gt C by the year 2000.
And although crop yields have risen, the only large cultivated regions with an
NPP greater than 10 t C/ha are Western Europe, East Asia (Japan, South Korea, and
eastern China), the central United States, and to a lesser extent parts of southern
Brazil and northern Argentina (Monfreda, Ramankutty, and Foley 2008). The peak
rates in excess of 20 t C/ha, made possible only by harvesting two or more crops a
year, are recorded in the most intensively cultivated areas of Atlantic Europe (above
all in the Netherlands) and in irrigated parts of the Middle East, the western United
States, Java, and India. Modern well-managed tree plantations yield mostly between
10 and 15 t C/ha.
Many regional and countrywide summaries of crop and wood production have
been available for decades in national statistics, but global assessments of phytomass
harvests and their comparison to the biosphere's total primary productivity began
only during the 1970s. This rel ected ecology's beginnings as an examination of
ponds and meadows and forest patches, small communities of plants, and het-
erotrophs. One of the great classics of the science, Lindeman's (1942) pioneering
examination of energy transfers among trophic levels, focused on Wisconsin's Lake
Mendota, and 15 years later Odum (1957) did a more thorough investigation of
a another freshwater ecosystem in Florida. The advent of new environmental con-
sciousness, satellite monitoring, and the emergence of global ecology shifted the
focus all the way to the planet as a whole in general and to the biosphere's global
production and carbon balance in particular (Lieth and Whittaker 1975; Bolin
et al. 1979).
The best expression of this approach to biomass harvests has been the studies of
human appropriation or co-option of NPP. There is no agreement on what should
be included in such appropriated or co-opted totals, nor is there any uniform cal-
culation procedure to follow. Moreover, NPP is a questionable choice for a common
denominator because it is a theoretical construct that cannot be measured directly,
whose annual rate l uctuates, and whose derivation, as much as it is now helped by
satellite monitoring and complex dynamic plant productivity models, is not highly
accurate. Global estimates used in different appropriation studies have differed by
about 40%, ranging from about 57 Gt C/year, used by Imhoff et al. (2004a), to
about 80 Gt C, used by Wright (1990): that is a substantial discrepancy for a value
used as the key denominator. As with most studies of this kind, it is necessary to
make entire chains of concatenated assumptions in order to aggregate the key sub-
