Biomedical Engineering Reference
In-Depth Information
were widely exploited, first during a shortage of pulp for paper, card and packaging
and then as a wood fuel.
The propensity for fast growth of willows is now exploited in the shrub species
(primarily from subgenus
Vetrix
) by growing them as coppiced stools. Commercial
short rotation coppice (SRC) willow plantations for biomass typically consist of a
mixture of specially developed elite varieties, grown in dense plantations of
15,000-17,000 plants per hectare. Planting mixtures of varieties enhances and
extends plant breeding efforts by exploiting genetic diversity to limit pest and
disease development. Pesticides are virtually excluded from production systems
by the physical size of the crop and the impracticality of application. SRC willow
plantations are harvested on a 2-4 year cycle and will normally remain in place for
more than 20 years. Coppicing reinvigorates the growth of a plantation and only
minimal fertilizer inputs are needed as the willows redistribute nutrients during
their perennial cycle
Willow can grow well in environments where the alternative land uses are
limited, such as cooler and wetter areas of northwest Europe and North America
and the high clay content soils of those and other areas of the temperate latitudes. In
these environments, because alternative land uses are limited, the production of
SRC willow is financially very competitive. Here, we summarize the main activities
that have led to improved willow varieties, including brief coverage of the germ-
plasm available and the main target traits for breeding. We finish by discussing
prospects for further crop improvement and expansion of SRC.
Taxonomy and Domestication
The family Salicaceae is commonly considered to consist of only three genera;
Salix
,
Populus
, and
Chosenia
, although more recently the Angiosperm Phylogeny
Group have added a further 52 genera.
Chosenia
is an unusual genus, containing
only one species,
Chosenia arbutifolia
, sometimes considered as
Salix arbutifolia
Pall.
Populus
is a well-known and widely studied genus and of similar interest for
bioenergy as
Salix
across much of the world.
The authoritative taxonomy of
Salix
has been conducted by representatives from
the major centers of biodiversity, G. W. Argus in North America [
1
,
2
], A. K.
Skvortsov in Russia [
3
], and Zhenfu, Shidong, and Skvortsov in China [
4
]. Argus
and Skvortsov agree upon the division of the genus into four subgenera;
Salix
,
Longifoliae
Andersson,
Vetrix
Dum, and
Chamaetia
Nasarow (Table
4.1
). The
authorities disagree upon a total species number, placing the sum between
350 and 500.
Subgenus
Longifoliae
comprises only a few New World species with specific
morphological traits such as
S. exigua
with stomata on the lower and upper leaf
surface. The subgenus containing the greatest number of species and of greatest
interest in commercial breeding is
Vetrix
; they are the species best adapted to rapid
growth rates and coppicing in cultivation. A few species of subgenus
Salix
have
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