Forest Trees - Bioenergy Crop Plants - page 367

In the United States, the forest products industry is currently the largest producer of bioenergy,
much of which is in the southeastern United States. The approximately 82 million ha of forestland
in the Southeast produce 18% and 25% of the world's roundwood and pulp, respectively (FAO
2004). Loblolly, P. taeda , and slash, P. elliottii , are the most important southern pines because of
their broad natural ranges that constitute the majority of the approximately 15 million ha of planta-
tions in the Southeast (Peter 2008). The main products for loblolly and slash pine plantations have
been pulpwood, wood composites, sawtimber, and poles/pilings. Loblolly pine and slash pine are in
closely related clades (Dvorak et al. 2000). Loblolly pine is a model pine because of its economic
importance and well characterized reproduction and genetics (Lev-Yadun and Sederoff 2000).
Silvicultural intensity for loblolly and slash pines varies depending on plantation objectives and
initial investments. Silvicultural treatments and genetic improvement greatly enhance tree growth
and stand productivity (Fox et al. 2004, 2007) and shorten rotations. Seed orchards provide >95%
of the seed for commercial nurseries, and aggressive breeding and genetic testing are underway.
Tree improvement has focused on growth, stem form, and disease resistance. Improving volume
growth and yield has been emphasized (White et al. 1993, McKeand and Bridgwater 1998). In the
first two breeding cycles of loblolly pine, gains of 30-40% in stem volume per cycle were achieved
(Li et al. 1999). Resistance to fusiform rust and pitch canker may also be improved in loblolly and
slash pines (Kayihan et al. 2005). Loblolly and slash pines each have only one breeding zone each
because genetic by environmental interactions are not significant (McKeand et al. 2006). Southern
pines can be clonally propagated by rooted cuttings and somatic embryogenesis (Nehra et al. 2005).
Varietal lines of elite germplasm selected and propagated by somatic embryogenesis have been
developed, and loblolly pine varieties are now being commercially deployed in the Southeast.
Although wood properties are important in the traditional utilization of southern pines (Peter
2007; Peter et al. 2007) and presumably also for bioenergy, improvement programs are not actively
breeding for these traits. Wood density is under moderate to high heritability in loblolly and slash
pines. Strong correlations between juvenile and mature wood suggest that early selection can be
used. In loblolly pine, it may be more difficult to improve both density and growth simultaneously.
Significantly less is known about genetic control of wood chemical composition in loblolly and slash
pines, even though these traits are significant for both chemical pulp production and bioconversion
to ethanol. Loblolly pine wood chemical composition is under weak genetic control in juvenile and
mature wood, and was not correlated, or only weakly so, with growth (Sykes et al. 2006).
Loblolly pine's economic importance, genetic material, and easily studied wood characteristics
have stimulated significant biotechnology research, e.g., Sewell et al. (1999, 2000, 2002), Brown
et al. (2004), Kirst et al. (2003), and Lorenz et al. (2006). A large loblolly pine resequencing project
discovered single nucleotide polymorphisms (SNP) in 8000 unigenes (Neale 2007), which will
identify gene candidates that control disease resistance and wood properties, potentially leading to a
genome sequence. Genetic engineering methods can genetically transform loblolly pine. Transgenic
plants have been derived from an organogenic method starting with mature zygotic embryos (Tang
et al. 2001) and from somatic embryos (Connett-Porceduu and Gulledge 2005).
Advanced generation southern pines in the Southeast have high bioenergy potential (Peter 2007).
Tree improvement programs and management systems coupled with clonal propagation, genetic
engineering, and genomic research customizing trees for bioenergy and chemicals make southern
pine even more promising for bioenergy production through (1) integrated forest biorefineries that
produce bioenergy and biofuels in addition to pulp and paper (Van Heiningen 2006; Chambost et al.
2007a, 2007b; Towers et al. 2007) or (2) co-firing, wood pellets, biofuels, or gasification. Overall,
harvesting and transportation account for approximately two-thirds of the total delivered wood
costs (Peter et al. 2007).
Although harvesting and transporting small diameter trees is a significant cost barrier to grow-
ing and using southern pines for bioenergy, slash and sand ( P. clausa ) pines have been evaluated as
SRWCs with more than 4000 trees/ha and harvests within 8 years (Campbell 1983; Campbell et al.
1983; Frampton and Rockwood 1983; Rockwood et al. 1983, 1985; Rockwood and Dippon 1989).