Page 374 - Biosystems Engineering

Evaluation of Transgenic Wood for Pr oductivity & Quality 351

products falling under the purview of more than one agency. In the
case of transgenic trees, the agency responsible is the USDA; the EPA
is also involved when modified traits may have an environmental
impact. The USDA regulates novel plant release through its Animal
Plant Health Inspection Agency (APHIS). APHIS permits for release
into the environment are usually issued or denied within 120 days,
and during that time state officials will inspect facilities to determine
their security and operating conditions. Permits for field trails are
renewed annually.

12.3 Productivity and Quality of Transgenic Wood
Wood quantity is the total amount of wood and can be measured in
diameter, length, volume, weight, or any combination. Breeding for
fast growth may lead to poor wood quality. Wood quality is defined
in relation to a norm, which depends entirely on the end product, for
example, structural lumber for construction requires a high wood
density, whereas low-density trees (aspen and pine) are more desir-
able for oriented strandboards and paper manufacturing. There is no
absolute measure for wood quality. Quality assessment is multifac-
eted and depends on the intended application. Quantity and quality
may not be treated as independent factors. Quality control should
form an integral part of most breeding programs.

12.3.1 Growth Rate
Magnussen and Keith (1990) developed six selection strategies aimed
at genetically improving volume production and wood quality fac-
tors such as density, heartwood content, and stem taper. Selection
indices were computed under various assumptions about economic
values of the traits under selection and with constraints on the mag-
nitude and direction of expected genetic gain. Stem taper, wood
density, and heartwood content were under strong genetic control;
however, the low phenotypic variation of wood density limits its
potential for genetic improvement. Heartwood content emerged as a
trait amenable for rapid genetic improvement.
A strong correlation between wood density and growth rate is
often found. There are a number of reports of a negative relationship
between growth rate and wood density in several genera such as
spruce (Zobel and Jett 1995). Factors affecting growth rate are envi-
ronment, age, and heredity. The evolution from juvenile to mature
wood is the expression of age effects. Foresters can influence growth
rate with silvicultural practices, such as initial spacing or thinning.
Most studies on the influence of initial spacing or thinning intensity
conclude that these factors have little effect on the wood density of
Sitka and Norway spruce. Stand fertility seems to have more influ-
ence on wood density through growth rate than initial spacing