The Carbon Quandary: Essential and Detrimental - Sustainable Design: The Science of Sustainability and Green Engineering - page 285

continuously, adding layers after each growing season. This process makes the
tree rings. Trees receive nutrients from soil via roots and from air via leaves.
The leaves also absorb light energy needed for photosynthesis. So the tree is
a system of living and dead tissue, both absolutely necessary for structure and
function.
All plants contain cellulose, but woody plants also contain lignin. Both
cellulose and lignin are polymers, which are large organic molecules comprised
of repeated subunits (i.e., monomers). Lignin is the “glue” that holds the tree's
biochemical system together. The monomers that comprise lignin polymers
can vary depending on the sugars from which they are derived. In fact, lignins
have so many random couplings that the exact chemical structure is seldom
known. One configuration of the lignin molecule is shown in Figure B7.2.
Lignin fills the spaces in a woody plant's cell wall between cellulose and two
other compounds, hemicellulose and pectin. Lignin accounts for the rigidity
of wood cells and the structural integrity and strength of wood by its covalent
bonds to hemicellulose and cross-linking to polysaccharides.
TREES AS AN ENERGY SOURCE
Both herbaceous and woody plants can serve as bioenergy crops , which include
annual row crops such as corn, herbaceous perennial grasses [known as herba-
ceous energy crops (HECs)] and trees. One of the most prominently men-
tioned HECs is switchgrass ( Panicum virgatum ), a hardy, perennial rhizomatous
grass that is among the dominant tall grass prairies species in the high plains of
North America. Bioenergy crops also include fast-growing shrubs and trees,
known as short-rotation woody crops (SRWCs), such as poplar. SRWCs typically
consist of a single-genus plantations of closely spaced (2 to 3 m apart on a
grid) trees that are harvested on a 3- to 10-year cycle. Regeneration is an im-
portant selection criterion for bioenergy species. HECs must regrow from the
remaining stubble, and SRWCs must regrow from the remaining stumps. * The
harvests can continue for two decades or more. Pesticides, fertilizer, and other
soil enhancements may be needed, but the farming does not differ substantially
from that typical of growing ordinary crops.
Both the cellulose and lignin have heat values; thus, these crops are known
as lignocellulosic energy crops . The feedstocks of HECs and SRWCs may be
used directly to generate electricity or can be converted to liquid fuels or
combustible gases.
* U.S. Congress, Office of Technology Assessment, “ Potential environmental impacts of bioen-
ergy crop production, ” Background Paper, OTA-BP-E-118, U.S. Government Printing Office,
Washington, DC, September 1993.