Environmental Engineering Reference
In-Depth Information
In the U.S. Northwest, R.F. Stettler of the University of Washington and P.E. Heilman of
Washington State University collaborated on a hybridization and cloning program of
P. tricho-
carpa
that has had a major influence on poplar biomass production in the United States and Canada
(Heilman and Stettler 1985; Stettler et al. 1988). The new
P. trichocarpa
×
P. deltoides
hybrids
from that research and development program have been studied widely throughout the region and
world and with industrial collaborators has led to extensive poplar commercialization in the Pacific
Northwest of North America (Stanton et al. 2002).
Early studies conducted at 1.2 × 1.2 m spacing compared the new hybrids with select native
black cottonwood clones. Average biomass production of the black cottonwood varied by clone
and was from 5.2 to 23.1 mt/ha per year. However, the new hybrid
P. trichocarpa
×
P. deltoides
clones were impressive and yielded from 15.6 to 27.8 mt/ha per year. These high yields prompted a
series of further research studies on the productivity of the new clones (Heilman and Stettler 1985;
Stettler et al. 1988). Heilman and Stettler (1990) studied coppicing of the new hybrids compared
to native black cottonwood. Mean yields were 11.3 mt/ha per year for the coppice and 12.6 mt/ha
per year for the initial harvest. However, many clones had higher yields after coppice, and there
was significant clonal variation in coppice yields that suggested that superior coppicing clones can
be selected. D. DeBell and coworkers at the U.S. Forest Service conducted a series of field studies
with the Stettler/Heilman clones in the 1990s in western Washington State. They found that poplar
clones had uneven yields in monoclonal versus polyclonal blocks at close spacing (i.e., 0.5, 1.0,
and 1.5 m spacings). Biomass yields for individual clones ranged from 11.7 to 18 mt/ha per year
for these spacings. Monoclonal blocks yielded more biomass than polyclonal blocks. The highest
biomass was produced in the closest spacing (i.e., 0.5 m) (DeBell and Harrington 1993; DeBell
and Harrington 1997). DeBell et al. (1993) also studied the so-called “wood grass” concept versus
more conventional spacings. Two hybrid poplar clones were planted at 0.2 and 0.3m spacing and at
wider spacings (i.e., 0.5, 1.0 and 2.0 m). The fast growing Stettler clone, H 11-11, harvested annually
produced 7.0 mt/ha per year over 5 years. The yield of the same clone at wider spacing was 18.8
mt/ha per year. During the 5 years, H 11-11 produced over 30 mt/ha per year for a single rotation.
They concluded wider spacings and longer cutting cycles outperformed “wood grass.” DeBell et
al. (1996) also found biomass differences in the two aforementioned clones planted in monoclonal
blocks for 7 years. Clone H 11-11 averaged 18.2 mt/ha per year at 1m spacing for 7 years. DeBell
et al. (1997) also studied four clones including black cottonwood and H 11-11 monoclonal blocks
at 1 × 1 m spacing. Biomass productivity ranged from 11 to 18 mt/ha per year after 5 years. Hybrid
poplar H 11-11 out-produced the native black cottonwood, but production peaked at 4 years at close
spacing. Ceulemans et al. (1992) and Scarascia-Mugnozza et al. (1997) also studied the productivity
of two Stettler hybrids compared to parental
P. trichocarpa
and
P. deltoides
clones at 1 × 1 m spac-
ing. Clonal ranking for biomass production of 4 years showed the hybrid clones outperformed the
parental clones. H 11-11 produced over 35 mt/ha per year whereas the parental clones yielded 16 and
14 mt/ha per year, respectively, for the
P. trichocarpa
and
P. deltoides
clones
.
Operational plantings are not as productive as field research studies because of soil heterogene-
ity, weather, and pests and diseases. DeBell et al. (1997) have reported biomass production rates for
operational poplar plantings of 12-17 mt/ha per year in western Washington. Industry researchers
Stanton et al. (2002) reported commercial yields of improved poplars averaging 13.8 mt/ha per year
with selected hybrid poplars yielding over 20 mt/ha per year in 6 years. These results highlighted
the importance of genetic improvement in short rotation poplar culture. When clones from the early
stages of hybridization and cloning program produce over 20 mt/ha per year in commercial planta-
tions, even higher yields can be expected when future improved clones become available. These
results suggest a ”green revolution” of short rotation biomass production is possible with poplar. Just
30 years ago Cannell and Smith (1980) had calculated in their review and appraisal of close spaced
hardwoods in temperate regions that the maximum theoretical working production was between
just 10-12 mt/ha per year.
