Breeding Willow for Short Rotation Coppice Energy Cropping - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 70

as planting in distinct rows at high density ensured that the stems were upright and
branch-free to be easily processed mechanically. Today large-scale harvesting
machines can cut and chip stems with diameters at the base of 120 mm, the
maximum encountered on a 3 or 4 year coppicing cycle, while on the move.
Lower-cost, lower-capacity machines are restricted to 80 mm diameter stems. For
combustion, larger stems are preferred as the wood-to-bark ratio is increased (see
section “ Quality traits ” below).
Deliberate breeding of willows for commercial purposes began at a similar time
to mechanization, during the late 1970s, at which time some more classical domes-
tication was undertaken. Biomass for pulp or bioenergy was considered best
produced on a 3-year cutting cycle rather than the annual cycle that many had
traditionally used. This was best achieved by planting at lower density and allowing
stem diameter to develop over the period in between harvests. Growth in larger
commercial plantings also necessitated greater pest and disease resistance and the
selection of growth forms that inherently produced multiple upright, branch-free
stems.
The most recent phase of domestication has involved selection for characters
suitable for the different energy conversion processes including minimizing the
inorganic component of the wood (ash), and especially the alkali metals, for
thermochemical conversion and maximizing sugar release from the cell wall
(lignocellulosic) fraction for biological conversion.
Areas of Production
Willow is a temperate genus, and the use of willow as a crop is confined to the
temperate areas of the Northern Hemisphere. Most northern European countries
have an interest in the crop, but further south in Europe, other species take
precedence, particularly poplar. In North America, the greatest interest in willow
cropping is in the Northeastern United States and across southern Canada. China,
Japan, and Russia and other parts of more northern Asia hold important genetic
resources in the genus ( S. sachalinensis, S. miyabeana, S. rehderiana ), and many of
these areas have increasing interest in growing willow.
In the UK, the calculation of area planted is complicated by the differing systems
employed by the devolved administrations. England represents the greater area of
farmed land in the UK, but Northern Ireland probably contains the greatest density
of willow plantings per unit of farmland. Eason et al. [ 7 ] summarized the data
available in 2009 as 7,400 ha willow in the UK. This may have fallen in more recent
years as the improvement in agricultural commodity prices and the associated
removal of a compulsory set-aside requirement on EU farms favored more arable
cropping. Many earlier plantings had exploited set-aside land where food crops
could not be grown as a supply control measure.
A positive policy framework in Sweden between 1991 and 1996 resulted in rapid
increases in crop area. Planting grants worked in conjunction with environmental