Paulownia - Bioenergy Crop Plants - page 672

and Paulownia. The United States uses approximately 140 billion gal of gasoline per year, and there
is a growing urgency to replace it with biological resources or biomass (US DOE 2006).
Biomass is a complex mixture of organic materials (such as carbohydrates, fats, and proteins)
along with small amounts of minerals (such as sodium, phosphorus, calcium, and iron). The main
components of plant biomass are carbohydrates (~75%, dry weight) and lignin (~25%) which can
vary with plant type. The carbohydrates are mainly cellulose or hemicellulose fibers which impart
strength to the plant structure, and lignin which holds the fibers together. A major advantage of using
biomass as a source of fuels or chemicals is its renewability. The net annual production of biomass
by photosynthesis has been estimated to be 10 times that of our current annual consumption of fossil
fuels (Miyamoto 1997).
It is easier to produce ethanol from sugar feedstocks (e.g., sugarcane, sugarbeets, sweet sorghum,
fruits, and other materials known as saccharides) or starchy feedstocks (e.g., corn, cereal grains,
sweet potatoes or cassava). However, the cost of production is prohibitive because both of these
groups are in the human food chain. For the production of cellulosic ethanol, residue including
postharvest corn plants (stover) and timber residues could be used. Therefore, the scientific
community is engaged in exploring other systems that are more efficient in breaking down the
cellular structures for cellulosic ethanol production.
Paulownia species are highly suitable to revalidate agricultural set-aside areas, to reclaim
mining areas, or to restore contaminated sites where major emphasis is on biomass production for
chemical or thermophysical processing. Because of the low cost, plentiful supply, and amenability to
biotechnology, carbohydrates appear likely to be the dominant source of feedstock for biocommodity
processing. In the case of starch, the advantage of enzymatic compared with chemical hydrolysis has
already been realized. In the case of cellulose, this has not yet been realized. Cellulose hydrolyzing
enzymes can only act effectively after pretreatment to break up the very stable lignin, cellulose,
or hemicellulose composites. These treatments are still mostly thermal, thermomechanical, or
thermochemical and require a considerable input of energy.
With a deep root system that is fed by underground water at a level below 2 m, Paulownia does
not compete with the roots of other crops. Intercropping with Paulownia improves microclimate by
reducing the effects of drying winds by 20-50% on average and increasing air moisture. This can
considerably increase the yield of some crops, such as ginger, winter wheat, and millet (Zhu et al.
1986). The tree benefits by recovering excess fertilizer that runs deep into the ground and the crops
benefit from the nutrients put into the topsoil by fallen leaves. Further, large green leaves are rich
in nitrogen and can be used for fodder and green manure. Therefore, the leaves are rich in protein
(16.2%), carbohydrates (9.44%), and minerals (Song 1988), making them ideal for animal fodder
and green fertilizer (a 10-year-old tree produces 80 kg of dry leaves/year). The fast growth rate of
Paulownia may be capitalized upon for agroforestry (Wang and Shogren 1992; Jiang et al. 1994),
biomass production (Song 1988), land reclamation (Carpenter 1977), and animal waste remediation
systems (Bergmann et al. 1997). Sufficient variation among P. elongata clones was revealed for
growth parameters and foliar nutrient concentrations to anticipate a benefit from the selection
of genotypes that are the most efficient for the remediation of animal waste. The data show that
P. elongata has potential for use as a swine waste utilization species (Bergmann et al. 1997). This
particularly suits the southeastern region of the United States where there is high a concentration of
swine and poultry industry. Swine industry effluents and chicken litter are available at comparatively
lower prices, replacing the need for synthetic fertilizers.
27.2 hIstory oF PauloWnIa In the orIent and the unIted states
Paulownia is known as kiri in Japan, specifically referring to P. tomentosa . The name kiri comes
from the word kiru (to cut) because it was believed that the tree would grow better and more quickly
when it was cut down regularly. It was once customary in Japan to plant a Paulownia tree when a
baby girl was born and then make it into a dresser as a wedding present when she gets married.