Hybrid Poplar in Minnesota - Biomass yield, energy values, and chemical composition
Why Biomass (Hybrid Poplar) for Energy?
Plant biomass of woody crops such as hybrid poplar offers potential to meet the requirement of the USA 2007 Energy Independence and Security Act mandating the production of at least 60% of renewable fuels from cellulosic feedstocks. Hybrid poplars are ideal feedstock for energy because they can generate significant amount of biomass and provide ecological services such as carbon sequestration and wildlife habitat improvement. Moreover, interest in non-grain biomass is promising because it is creating investment and entrepreneurial opportunities in many rural communities.
Information on hybrid poplar biomass production is insufficient as past research had focused more on the clonal production of hybrid poplars. We conducted a study to examine the biomass production, chemical composition, and energy content of selected hybrid poplar commercial clones at harvest maturity.
Hybrid poplars were planted in Rosholt Farm in Westport, Minnesota in the spring of years 1995 and 1997, containing 59 and 79 clones of hybrid poplars respectively. Both hybrid poplar sites totaling 2.82 ha were irrigated. The hybrid poplar cuttings (10 inches length) were planted at a 3.05 m x 3.05 m (10' x 10') spacing resulting in a stem area density of 1075 ha-1 (435 plants/acre).
The 1995 and 1997 hybrid poplars plantings were harvested using logging harvesting equipment in Winter 2008. Data collection focused on commercial clones such as NM6 (Populus nigra x P. maximowiczii), D105 (Populus deltoides) and DN34 (P. deltoides x P. nigra), which are preferred by landowners in Minnesota. Prior to harvesting, height and diameter of these clones were determined. Diameter at breast height (DBH) was measured to the nearest 0.1 cm on these clones. Collected diameter data were used to estimate biomass following destructive aboveground harvest of multiple hybrid poplar genotypes across numerous sites. Individual-tree biomass data were multiplied by the stocking rate (i.e., stem area density) and their product was divided by the age of the trees to calculate biomass per unit land area per year. Wood samples were collected from base, middle and slash (composed of branches and tops) for energy content analysis; hence a total of three samples per tree.
Hybrid Poplar potential ethanol yield was determined from chemical composition (cellulose, hemicellulose, and lignin) and was then calculated based on chemical composition analysis using the USDA National Research Energy Laboratory (NREL) online theoretical ethanol yield calculator.
Summary of Results
The amount of biomass produced between clones planted in 1995 and 1997 differed significantly. At harvest maturity (end of the 13-year growing season), NM6 had significantly greater biomass production than D105 and DN34 with a total biomass production of 11.46 Mg ha-1 (Fig 1). The higher growth rate of clone NM6 compared with DN35 and DN105 may be related to its particularly rapid early growth and its exceptional early rooting.
Fig 1 - Biomass production of commonly used hybrid poplar clones in Minnesota planted in 1995 and 1997. Means with the same letter within a specific year means biomass production between clones is not different.
The cellulose and hemicellulose (low heat, high level of oxidation) content of D105 and NM6 were similar (Fig 2); however, NM6 had significantly 8% times higher lignin (high heat, low level of oxidation) content than D105. The chemical composition (mass fraction % on dry basis) of hybrid poplar clones generally contained 39% cellulose, 21% hemicellulose, 27% lignin, 1.3% ash content, and about 17,900e18,031 kJ kg-1 of dry wood.
Fig 2 - Biomass of hybrid poplar clones in Minnesota planted in 1995 in Westport, Minnesota, USA. Means with the same letter within chemical composition category are not significantly different at α=0.05 level of significance.
Potential Ethanol Yield
- Comparing results from samples planted in 1995 and 1997 showed no variation in total energy and ash contents among clones; there was also no variation of theoretical ethanol yield for hybrid poplar in 1995, but there was a variation among clones planted in 1997 with NM6 having greater ethanol production than DN34 (Fig 3). This result indicates that hybrid poplar could be harvested as early as the end of the 12-year growing season, or even earlier, if the sole purpose of the planting is for sugar based biofuels.
- Comparing theoretical ethanol production among tree components, there was significant variation in samples planted in both 1995 and 1997 with the base (tree trunk) and the slash (branches and tree tops) having the greatest and least theoretical ethanol yield, respectively (Fig 4). Conventional harvesting operations leave a large amount of slash (residues) that constitute a significant source of biomass (theoretically 375-390 L of ethanol for every ton of biomass) that could be utilized for bioenergy.
Fig 3 - Theoretical ethanol yield that could be generated based on the amount of biomass produced of commonly used hybrid poplar clones in Western, Minnesota, USA. Means with the same letter within the year are not significantly different at a α=0.05 level of significance.
Fig 4 - Theoretical ethanol yield that could be produced based on tree parts in each year of planting across clones. Means with the same letter within the year are not significantly different at a α=0.05 level of significance.
Photo: Dean Schmidt
Though harvesting tops can also be utilized for energy, from an environmental perspective, leaving logging residues on sites are particularly important in sustaining multiple trophic level food web interactions, nutrient cycling and soil sustainability. Management goal or long term commitment of landowners to conservation should be given full consideration before harvesting prior to operational practice.
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