Short-rotation woody crop supply systems in the United States: what do we know and what do we need to know?
Introduction
Short-rotation woody crop (SRWC) supply systems were first described in the late-1960s and early 1970s as a means of rapidly producing lignocellulosic fiber for use in the wood products industry and for energy.1, 2, 3, 4Shortly after SRWC concepts were proposed, the U.S. Department of Energy (DOE) embraced this technology as a way of supplying biomass feedstock for the conversion to liquid transportation fuels.[5]By definition, SRWC supply systems involve appropriate site selection and preparation (i.e. the selection of agricultural quality land of moderate to high fertility with moderate to well-drained soils and complete ground cover control 6–9 months prior to planting), in combination with the use of improved clonal planting stock, extensive weed control (i.e. mechanical or chemical control of weed competition through the first 2 years of growth), fertilization as required, based on the use of plant-based fertilizer prescriptions for micro- and macro-nutrients, pest control, including both the use of resistant planting stock and chemical treatment of infested plants, and efficient harvesting and post-harvest processing.
Over the last 15 years, DOE, through the Biofuels Feedstock Development Program (BFDP) and its partners, has funded research directed toward creating SRWC supply systems for many regions of the U.S. Initially research efforts concentrated on species-site trials within potential production regions. From these efforts model species were selected for regions of the country that contained a land base large enough to support a national biomass-based energy industry.[5]These regions primarily included the north-central and southeastern U.S., with secondary regions in the Pacific Northwest and northeastern U.S. The selected model species were poplars (Populus spp.), sycamore (Platanus occidentalis L.), silver maple (Acer saccharum Marsh), and hybrid willow (Salix spp.), with poplar being the principal candidate for SRWC throughout most of the U.S. Genetic improvement programs, silvicultural studies and basic research projects were initiated and continue today in the Pacific Northwest, north-central, and southeastern U.S. for members of the Populus genus.[6]In addition, preliminary selection projects are ongoing for silver maple in the north-central region and for hybrid willow in the northeast.[6]
As a result of DOE-funded, university- and federal-based research, several forest products companies have adopted SRWC supply systems for production of fiber for pulp. In the Pacific Northwest, there are ca 20 000 ha of land in SRWC production, with an estimated 50 000 ha scheduled for establishment in the next 5 years.7, 8There are fewer acres in SRWC production in other regions of the U.S. (e.g. 12 000 ha in the southeast and 4000 ha in the north-central regions), though projections for new SRWC plantings over the next 5 years in these regions are high (e.g. 28 000 ha in the southeast and 18 000 ha in the north-central regions). Likewise, the acreages in SRWC for use as feedstock for biomass-based energy are projected to increase substantially over the next 5–10 years.[9]These projected SRWC energy plantings, along with the residuals from SRWC planted by the forest products industry (estimated at 1.0–2.0 dry t ha−1 yr−1), represent a considerable biomass-based energy resource. This shift toward SRWC supply systems correspondingly represents a substantial change in land use patterns and environmental impacts within the agricultural sectors.
In an effort to summarize current understanding and existing implementation of SRWC supply systems in the U.S. and to identify areas where further research is needed, the objectives of this paper are to:
- 1.
provide a historical reference point for the state of the science in SRWC supply systems, with particular reference to earlier predictions; and
- 2.
identify unresolved questions or technical barriers to further implementation of SRWC supply systems, with particular reference to environmental issues.
The remaining portion of the paper will be separated into three main sections: current status; technical barriers and future research; and overview. Within 2 Current status, 3 Technical barriers and future research, the discussions will deal with the following topics: silviculture; breeding and genetic improvement; integrated issues; and harvesting and handling.
Section snippets
Current status
In 1987, Ranney et al.[5]published a synthesis article summarizing 10 years of BFDP SRWC research. In this article recommendations were made regarding potential production regions, suitable species and suggested silvicultural practices by region and species. Potential benefits from traditional breeding and biotechnology were also presented. As a means of establishing a historical reference, this earlier article will be used to provide comparisons to prescriptions as they exist today. It should
Silviculture
The most unresolved silvicultural issues related to SRWC supply systems relate to the use of fertilization and irrigation. That is, will fertilizers, used in combination with irrigation, increase growth rates and reduce rotation lengths? Are there economic advantages to supplemental vs continuous irrigation? Are there situations where spray or furrow irrigation is preferred to drip irrigation? Can sludge be used to supply nutrients as well as water? Are nutritional amendments needed to maintain
Overview
The biological, economic and environmental potential of SRWC supply systems are coupled. Producing fiber as quickly as possible and as efficiently as possible may or may not maximize environmental benefits or minimize environmental impacts. As noted above, growing genetically-improved clonal planting stock at 1200–1400 stems ha−1 under 6–10 year rotations with competition control during the first 2 years and fertilizers and pesticides used on an as-needed basis will maximize productivity with
Acknowledgements
This research was sponsored by the Biofuels Systems Division of the U.S. Department of Energy under Contract No. DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp.
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