Shrub Willow - Bioenergy Crop Plants - page 688

and can be produced locally. However, wood chips have not been widely adopted as a fuel for
small-scale heating in this region for reasons of delivery and storage logistics, difficulty of use, and
cost. Pelletizing woody biomass greatly simplifies the logistics of delivery and makes conversion
more reliable because of uniform size, composition, and handling properties. With recent spikes in
the cost of heating oil, many consumers in the Northeast have switched over to using wood pellet
stoves or boilers, resulting in dramatically greater demand for wood pellets. Wood pellets for heating
have, in the past, been made from clean residues collected from the forest products industry, but
reduced supply and increased demand for those residues has forced pellet-makers to seek alternative
feedstock sources and incorporate wood from forest harvesting operations. With the tightening
market for forest product residues, dedicated perennial energy crops could provide a reliable, local
source of biomass that can be incorporated into the feedstock mix used to make pellets.
Shrub willow ( Salix spp.) represents a proven, high-yielding perennial crop that can be grown on
underutilized or marginal agricultural land, especially poorly drained sites unsuitable for food crops,
and could contribute significantly to the mix of regionally optimized biomass commodity crops.
Research conducted or directed over many years by the U.S. Department of Energy at Oak Ridge
National Laboratory has concluded that willow has superior properties as a perennial energy crop for
the Northeast and Midwest United States (Tolbert and Schiller 1996; Tolbert and Wright 1998). Shrub
willow crops have a short harvest cycle, low incidence of pests or diseases in improved varieties,
adaptability to a wide range of site conditions, high yield of biomass with low input of fertilizer,
efficient recycling of nutrients in leaf litter, and great potential for genetic improvement. For over
20 years, academic research and development in New York State and Canada has demonstrated the
potential for shrub willow crops in North America, drawing from a wealth of study and experience
in Sweden, the United Kingdom, and Denmark, where willow bioenergy crops are planted on over
20,000 ha (Kuzovkina et al. 2008). Sustained efforts in optimizing agronomic and management
techniques, adapting planting and harvesting technology, and demonstrating conversion methods
have lead to commercial deployment of shrub willow crops on the rural agricultural landscape
in New York since 2005, with over 300 ha in cultivation and dramatic expansion of commercial
plantings being proposed to take advantage of nearly 800,000 ha of underutilized agricultural land
in New York alone (Volk et al. 2006). According to the National Biofuels Action Plan, willow
is characterized as a “third-generation” feedstock crop designed for fuel production. Ongoing
breeding and selection of willow as an energy crop has generated a diverse array of varieties that
display incremental improvements in yield and are adapted to regional conditions of climate, soils,
pests, and diseases. Genomics-assisted breeding of willow will further enhance its sustainable yield
potential on vast areas of underutilized agricultural land in the northern United States, northern
Europe, and elsewhere.
28.2 dIversIty and ecoloGy oF SAlix
Willows are classified in the genus Salix which includes at least 350 species worldwide. Willows are
further grouped into three to five subgenera on the basis of morphological characteristics, as described
by willow taxonomists for North America, Eurasia, and China (Argus 1997; Skvortsov 1999;
Zhenfu et al. 1999; Dickmann and Kuzovkina 2008). The proper identification and classification of
willows is notoriously difficult because many of the foliar characteristics are highly variable within
a species depending on developmental stage, genotypic variation, and responses to environmental
conditions. Considering the potential for variability, these characteristics, such as leaf and stipule
size, shape, and pubescence, may be similar across one or more species. To settle on a definitive
identification, it is necessary to conduct careful observation of distinguishing floral, stem, bud, and
foliar features at many stages of growth. According to the scheme of Argus, species are placed in
the subgenera Protitea (Ohashi), Salix , Longifoliae (Andersson), Chamaetia (Dumort.), and Vetrix
(Argus 1997; Dickmann and Kuzovkina 2008). Generally, the tree-form species, including Salix
nigra and Salix amygdaloides are in Protitea Ohashi or Salix , whereas Longifoliae includes only a