Hybrid and Native Willows Growing on Marginal Lands in Central Minnesota - Biomass production and potential ethanol yields
Why Willows for Energy?
Willows have been regarded to have a high biomass production potential for energy. As a Short Rotation Woody Crop, willows are being developed as a sustainable source of biomass for the production of bioenergy, biofuels, and bioproducts. With the potential increase in demand of biomass for energy in the future, there is a need to diversify and expand its source. Willows have been widely employed as they are fast-growing species with high biomass yield potential, low nutrient inputs, and are adaptable to a wide variety of site and climatic conditions.
Despite their general plasticity, adoption of willow as a bioenergy production system remains a challenge. A series of willow yield trials show yields ranging 5.3-27.8 ton ha-1 (oven dry) (Volk et al., 2011), however, these results do not cover a wide range of site conditions characteristic of marginal lands in central Minnesota and elsewhere.
This study evaluated and compared the growth performance, biomass production, energy content, and potential ethanol yields of shrub willow hybrids and native willow varieties in areas not yet captured in existing willow yield trials such as the marginal lands in central Minnesota in order to provide a regional perspective of their potential for bioenergy.
Ten willow species, which comprised of eight shrub willow hybrids and two native willow accessions, were used in the study. Shrub willow hybrid cuttings were obtained from the State University of New York through its controlled breeding program while cuttings used for native willow varieties were collected naturally. The study was conducted in Wadena County in central Minnesota, considered a marginal land dominated with grasses and poor soil conditions. The site was sprayed with glyphosate - a non-selective herbicide, and rototilled 1 month thereafter.
Willows were established and planted in a high-density twin row coppice system resulting in 5989 plants/acre. In June 2008, dormant willow cuttings (8-inches in length) were manually planted. To promote multiple stem sprouting and growth, plants were coppiced in November 2008 and harvested in March 2012 after three more growing seasons representing one rotation period. Coppicing served as a regeneration technique to promote stem production.
Survival rates were determined three times over the course of three growing seasons. Numbers of stems produced by each plant and stem diameter at 30 cm height were measured at the end of the third growing season (October 2011). Willows were harvested manually using a chainsaw five centimeters above the top of the stool. Aboveground fresh mass was determined in the field using a portable weighing scale. After fresh mass had been determined, sub-samples were collected to determine biomass production. Sub-sample materials were oven dried at 70oC to constant mass before being weighed again; hence, biomass is reported in oven-dry basis. Potential ethanol yield was determined from chemical composition contents of (cellulose, hemicellulose, and lignin) of willows and was then calculated using the USDA National Research Energy Laboratory (NREL) online theoretical ethanol yield calculator.
Summary of Results
Survival Rate and Biomass Production
At the end of one rotation, shrub willow hybrids outperformed native willow accession species, with average survival rate of 95% and 63%, respectively (Fig 1). Biomass production of shrub willow hybrids was consistently greater than the native willow varieties ranging from 7.42 - 16.0 ton ha-1, and 0.69 - 1.83 ton ha-1 oven dry, respectively (Fig 2). It was observed that while some species had high survival rates and generated more stems per stool due to coppicing, they did not necessarily produce a high amount of biomass.
Fig 1. Survival rate of shrub willow hybrids and native willow varieties planted in marginal lands in central Minnesota, USA after one rotation consisted of three growing seasons. Numerical value above the bar Ranking of survival 1 year after coppicing (1 highest, 7 lowest). Asterisk Native to Minnesota.
Fig 2. Total above ground biomass production (oven dry) of shrub willow hybrids and native willow varieties planted on marginal lands in central Minnesota, USA, harvested after one rotation (=3 growing seasons). Asterisk Native to Minnesota.
The cellulose and hemicellulose contents between the hybrids and native willow varieties did not differ. In contrast, lignin content differed significantly among hybrids, but not across native varieties (Fig 3). The chemical composition (mass fraction % on dry basis) of willow hybrids and native willow varieties generally contained 39% cellulose, 19% hemicellulose, 25% lignin, 1.4% ash content, 17,500-18,500 kJ kg wood-1, and 385 L Mg-1 of theoretical ethanol yield. Given its lower lignin content, willow has a greater potential for biofuel production from biomass sugars and can be used in either thermochemical or biochemical processes for producing energy.
Fig 3. Chemical composition (mass fraction % on dry basis) of shrub willow hybrids and native willow varieties planted in central Minnesota, USA. Asterisk Native to Minnesota.
Potential Ethanol Yield
Energy content and potential ethanol yields did not vary between shrub willow hybrids and native willow variety on a dry weight basis (Fig 4). However, on an area basis, the amount of biomass produced per unit of land influences potential ethanol yields among hybrids. Thus, it is suggested that shrub willow hybrids offer promising potential as an alternative source of biomass.
Fig 4. Potential or theoretical ethanol yield of shrub willow hybrids and native willow varieties harvested after on rotation (=3 growing seasons) as calculated based on oven-dry basis. Calculation was based on the U.S. Department of Energy-National Energy Research Laboratory theoretical online calculation tool. Asterisk Native to Minnesota.
Four shrub willow hybrids S. purpurea (clone ID 94001), S. purpurea 'Fish Creek', S. miyabeana 'SX64', S. sachalinensis x S. miyabeana 'Sherburne' 9871-31 and S. x dasyclados 'SV1') showed greater potential for use as biomass energy crops on marginal lands in central Minnesota than the native willow varieties, hence, there is a need to test their potential at a larger scale. Results of this study indicate the need to understand the biomass production potential of willow in different regions under varied growing conditions for its wider adoption for bioenergy.
Diomy Zamora, firstname.lastname@example.org, 612-626-9272
Gary Wyatt, email@example.com, 507-389-6748
Note: Information presented in this factsheet series was taken from a peer reviewed article:
Zamora D, Wyatt G, Apostol K, Tschirner U. 2013. Biomass yield, energy values, and chemical composition of hybrid poplar in short rotation woody crops production and native perennial grasses in Minnesota, USA. Biomass and Bioenergy. Vol. 49. pp. 222-230.
Funding was provided by the Productive Writing Conservation Lands Program of the Tree Rivers RC&D