Environmental Engineering Reference
In-Depth Information
The only other cited pre-World War II estimates were of the right order of mag-
nitude: Noddack's (1937) total of about 300 Gt of carbon (600 Gt of dry biomass)
was only a small fraction of Vernadskii's original aggregate. Neither of these esti-
mates was constructed from an aggregation of major vegetation subtypes; the i rst
attempt to do so was made only during the late 1960s, when three Russian biolo-
gists reconstructed the Earth's potential terrestrial phytomass of the preagricultural
era by aggregating the phytomass of 106 major plant formations (Bazilevich, Rodin,
and Rozov 1971). Their total was 2.4 Tt, or about 1.2 Tt C. Shortly afterward
Whittaker and Likens (1975) used 14 ecosystem types to calculate global phytomass
for the year 1950 at 1.837 Tt (about 920 Gt C), nearly 25% below the Russian
estimate for the preagricultural world, a plausible difference given the intervening
conversion of forests and grasslands.
A major rei nement came when Olson, Watts, and Allison (1983) gathered most
of the available information on phytomass stores in various ecosystems and pre-
sented their global terrestrial phytomass estimate of 0.5
cells. Recognizing
many inherent uncertainties, they opted for a range of 460-660 (mean 560) Gt C
rather than for a single i gure, and concluded that any value above 800 Gt C should
be seen as unrealistically high, while totals well below 560 Gt C would not be all
that surprising. Most of the subsequently published terrestrial phytomass totals were
based on global carbon cycle models and have included a relatively low value of
486 Gt C (Amthor et al. 1998), and a 60% higher total of 780 Gt C (Post, King,
and Wullschleger 1997).
The Pilot Analysis of Global Ecosystems, an attempt to synthesize data from
national, regional, and global studies, underscored the continuing uncertainty
surrounding phytomass estimations by adopting an excessively broad range of
268-901 Gt C (Matthews et al. 2000), while Saugier, Roy, and Mooney (2001)
settled on 652 Gt C and Houghton and Goetz (2008), stressing the continued
uncertainties in our understanding of global phytomass, offered a nearly twofold
range of 385-650 Gt C. The latest appraisal of tropical forest phytomass ended
up with about 250 Gt C (Saatchi et al. 2011); assuming that tropical forests account
for 50% of the global total, the terrestrial phytomass would be just 500 Gt C.
Table 2.4 compares all of the reviewed estimates of global terrestrial phytomass.
It should be noted that differences among these values are much larger than the
totals published for the oceanic phytomass. Because phytoplankton has a very short
life span—its fast turnover means that its entire stock is consumed every two to
six days (Behrenfeld and Falkowski 1997)—its standing mass adds up to only 1-4
Gt C.
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