Hybrid Poplar (Populus spp)
Due to its widespread geographic adaptation, rapid growth, ease of reproduction and coppicing ability, Populus species are ideal candidates for SRWCs. Populus is a widely researched tree genus and is the most studied genus among all SRWC taxa (Tuskan et al., 2006). Populus spp. and their hybrids (i.e., poplars) are a significant component of the total biofuels and bioenergy feedstock resource in the USA, Canada, and Europe. They are capable of producing in excess of ha-1 yr-1 by age six even in the harsh climate of the North Central region of the United States (Riemenschneider et al., 2001; Zalesny et al., 2009).
Biology and Production System
Bernie McMahon, Univeristy of Minnesota-Duluth
Fig 1. Two-year old hybrid poplar trees planted in a loam soil in central Minnesota, USA
There are 29 recognized Populus species worldwide, with 12 native to North America. They possess abundant genetic variation, occurring at the genus, sectional, species, and clonal levels (Berguson et al., 2010; Zalesny et al., 2011). Features that make them suitable for biomass production include their ease of rooting and vegetative propagation, quick establishment and fast growth, and ability to resprout following repeated harvesting (Fig1)
Poplar plantation establishment uses unrooted, hardwood stem cuttings approximately 20-30 cm in length. The use of vegetative propagation allow for easy and inexpensive planting stem cuttings from dormant material. Once harvested, cuttings are stored under refrigeration, and then planted in the field when soil temperatures reach levels appropriate to specific regions and genotypes.
Plant spacing of poplars can vary widely depending on the intended end product. In situations where debarked wood is required (e.g., paper, oriented strand board, potentially biochemical liquid fuels), lower plant populations ranging from 750 to 1700 stems ha-1 are used to produce large diameter trees which facilitate debarking, typically on a ten to twelve year rotation. However, plantations oriented toward repeated coppice production of woody biomass for energy applications may be planted at higher densities ranging from 5,000 to 10,000 stems ha-1 (Al Afas et al., 2008). Plantations require intensive weed management during the first one to three years, until canopy closure is reached
Management and Harvesting
Poplars can be managed in a number of ways, depending on the desired end product, management system, and target rotation age. More closely spaced systems have the potential advantage of reaching canopy closure more quickly and producing higher biomass yield. However, this advantage is partially offset by higher planting costs, thus affecting economics (Berguson et al., 2012). Populus fiber production systems are harvested every 7-15 years (Zalesny et al., 2011). Populus bioenergy plantations require shorter rotation periods, although more research on bioenergy rotation periods, as well as how yields from these compare to longer rotation periods, is needed.
The most important drivers of poplar SRWC system productivity include proper species or hybrid selection for site conditions, correct site preparation, pest/disease resistance (Hansen, 1994; Berguson et al., 2010; Zalesny et al., 2011,) and weed control at establishment (Hansen, 1994; Kauter et al., 2003). Typical operational procedures may include tillage, fertilization, and irrigation (Sartori et al., 2007). Thinning is sometimes conducted at higher planting densities to provide a supply of biomass at intermediate points in the rotation.
Bernard McMahon, Univeristy of Minnesota, Duluth
Fig 2. Twelve year old hybrid poplar plantation in a single-stem, single-harvest management system in Minnesota, US ready for harvest
Populus species are typically harvested after several years, with a rotation period of five years or less for bioenergy crops. Rotation lengths for poplar are expected to range from 3-5 years in coppice regeneration as opposed to 5-15 years in more traditional single-rotation, single-stem plantations (Hansen et al., 1994; Berguson et al., 2010, Zalesny et al., 2011) (Fig 2). The means through which trees resprout after harvest, or coppicing, varies with species. Sprouting characteristics affect ease of harvest and machinery requirements as species that are restricted to stump sprouting retain the original planted row orientation. The ability for many Populus species to resprout after harvests defrays the cost of replanting in successful plantations.
Breeding and Genetic Improvements
Genetic variation among both pure-species collections and hybrids is very high in Populus, which presents opportunities to improve biomass yield and nutrient efficiency. There are many possible breeding strategies that can be applied to the development of a hybrid poplar woody biomass crop (Riemenschneider et al., 2001). Yet, all breeding strategies derive from the need for a commercial variety to possess several attributes simultaneously such as an adventitious root system, rapid growth, and resistance to pests (US DOE, 2011). Elsewhere, interspecific hybridization may be necessary.
This need for an aggregate genotype possessing all required commercial attributes gives rise to the several breeding programs found throughout North America (Table 1) and elsewhere in the world.
Commercial genotypes have most, if not all, of the important traits affecting production. However, the number of commercial genotypes in use today is relatively low, and diversification, as well as yield improvement, is a goal of breeding programs (Berguson et al., 2010; US DOE, 2011; Zalesny et al., 2011). In the US, genetic improvement research varies considerable across the country (Table 2).
Biomass production is one of the criteria of assessing feasibility of Populus clones for bioenergy and bioproducts application. The sprouting ability of these species is also necessary to understand yield potential. It is currently understood that over 20,000 stems ha-1 of Populus can resprout after first-year thinnings from a combination of stump and root sprouts depending on the species. Populus species have great potential for biomass production. Certain genotypes of the Eastern cottonwood (P. deltoides) and its hybrids, for example, are capable of producing yields ranging from 9 - 20 Mg ha-1 yr-1 in the US (Wright, 2006; Berguson et al., 2010; Zalesny et al., 2011, Zamora et al., 2013) and other parts of the world (Table 3) with a wide range of environmental conditions suitable for their production, which differs based on climate, soil properties, and cultural practices.
Table 1. Currently active Populus spp. breeding programs in North America.
|Company||Location||Region of concentration||Area||Focus|
|Greenwood Resource Institute||Boardman, Oregon, USA||USA||9,300 ha|
|Alberta-Pacific Forest Products||Alberta, Canada||Canada||8,000 ha|
|ArborGen||USA, New Zealand, Australia and Brazil||Worldwide||University of Minnesota. Duluth-Natural Resources Research Institute||USA||USA|
Table 2. Genetic improvement of Populus spp in the United States based on geographical locations.
|Location||Genetic improvement focus||Authors|
|Upper-Midwest||DN34, DN5, and NM6 were found to be disease resistant and fast growing under a range of conditions. Large breeding program has produced new hybrid to increase growth rate, genetic diversity and disease resistance||Hansen et al., 1994; Berguson et al.,2010|
|Southern U.S. (Lower Mississippi Alluvial Valley (LMAV)||P. deltoides perform best on those sites that are high in nutrient availability and have consistent rainfall. Unlike pure P. deltoides, limited testing of hybrid poplars in the South has not determined the incidence of Septoria canker, a potentially serious disease of some poplars.||Berguson et al., 2010|
|Pacific Northwest||Identify superior clones of P. trichocarpa and produce hybrids of P. deltoides and P. trichocarpa for commercial production||Stanton et al., 2002|
Table 3. Biomass yield of Populus spp. across the globe.
|Country||Varieties||Rotation||Biomass (Mg ha-1)||Reference|
|Ghent, Belgium||Populus trichocarpa x deltoides-Hoogvorst||annual||3.5||Vande Walle et al., 2007|
|Nanjing, China||P. deltoides Bartr. cv. 'Lux', P. euramericana (Dode) Guinier cv. `San Martino', P. deltoides Bartr. cv. `Havard'||annual||10.3||Fang et al., 1999|
|Jiangsu Province, China||P. deltoides cv. "35"||annual||8.8-15.2||Fang et al., 1999|
|Bihar, India||P. deltoides G3 intercropping system||annual||12.1||Das and Chaturvedi, 2005|
|Sweden||P. balsamifera L., P. maximowiczii x P. trichocarpa, P. trichocarpa x P. deltoides, P. trichocarpa||4 years||141.9||Johansson and Karačić, 2011|
|Quebec, Canada||2 clones of P. maximowiczii x P. nigra (NM5 and NM6)||4 years||66.5||Labrecque and Teodorescu, 2005|
|Hungary||P. trichocarpa x deltoides||annual||16.3||Marosvőlgyi et al., 1999|
|Scotland||P. balsamifera var. Michauxii (Henry) x P. trichocarpa var. Hastata (Dode) Farwell||annual||5.5||Proe et al., 1999|
|US Iowa||7300501||5 years||16.8||Riemenschneider et al., 2001|
|US Wisconsin||80X00601||5 years||8.5||Riemenschneider et al., 2001|
|US Minnesota||D121||5 years||6.8||Riemenschneider et al., 2001|
|US Iowa||Eugenii||7 years||17.0||Goerndt and Mize, 2008|
|US Iowa||Eugenii||7 years||5.4||Tufekcioglu et al., 2003|
|US Iowa||Cradon||10 years||30.0||Goerndt and Mize, 2008|