Environmental Engineering Reference
In-Depth Information
3.9 A segment of cellulose, the biosphere's most abundant
macromolecule, composed of about 3,000 units of glucose.
plants. Monosaccharides (40% C, 15.5 kJ/g) and disac-
charides (42% C, 16.5 kJ/g) are present in plants in rela-
tively limited quantities, and the bulk of the world's
phytomass is stored as cellulose, a polysaccharide (44%
C, 17.5 kJ/g) that makes up 66%-72% of softwoods
and 74%-80% of hardwoods. Long-chained cellulose,
consisting of about 3,000 units of glucose, is the most
abundant biopolymer in the biosphere (fig. 3.9). Lignin
(63% C, 26.4 kJ/g), the biosphere's most abundant
aromatic polymer (composed of varying ratios of cross-
linked alcohols), makes up about 20% of hardwoods and
30% of softwoods. Larger shares of energy-dense lipids
(39 kJ/g) are usually present only in seeds.
The dominance of cellulose and lignin is a structural
necessity because these polymers make it possible to
build stiff yet flexible vertical (trunks, stems) and hori-
zontal (cantilevered branches, boughs) supports that can
expose myriads of perennial or seasonal photosynthesiz-
ing surfaces (leaves, needles) to the Sun. Standard
conversion of dry phytomass to carbon is 0.45, corre-
sponding to about 17.5 kJ/g, but 0.48-0.50 is a more
appropriate rate for woody phytomass in temperate
forests. However, Savidge's (2001) analyses of 40 North
American tree species show that interspecific differences
matter for accurate conversions. C in hardwoods ranged
from 46.2% to 49.97%, in softwoods (conifers) from
47.21% to 55.2%. Lieth and Whittaker (1975) recom-
mended the following conversions for the world's major
biomes (in kJ/g): tropical rain forest 17.1, temperate
mixed forest 19.7, boreal forest 20.1, grasslands 16.7,
and cultivated land 17.1. Detailed chemical composition
analyses and energy values are available for many hun-
dreds of food and feed crops (Watt and Merrill 1963;
NRC 1971), and forestry literature contains similar data
on many tree and shrub species (Tillman 1978; NAS
1980; Davis et al. 2001).
Two major uncertainties complicate any large-scale
estimates of phytomass (in small areas its above-ground
components can be readily and accurately determined by
destructive sampling): unreliable data on the extent of
