Plant Crops Bioenergy Research UK
Poplar (Populus spp.)
The genus Populus, which includes poplars, cottonwoods, and aspens, is a member of the Salicaceae family that also includes the genus Salix (willow). Poplars are native to wide areas of the Northern Hemisphere and are among the most abundant woody plant genus in temperate forests, representing some of the fastest growing temperate trees. Populus are single-stemmed, deciduous (or semi-evergrees) trees that mostly spread clonally by means of rootborne suckers. This genus also reproduces sexually and is usually dioecious and wind pollinated. This leads to extensive interspecific hybridization and high morphological diversity posing difficulties in identifying species. Currently approximately 30 species are assumed (Taylor, 2002) but this could be as high as 70 or more. Two species are considered native to the UK, the aspen P. tremula and the black poplar P. nigra var betulifolia although others may also be native such as the grey poplar P. x canescens.
The haploid chromosome number is 19 and all members of the genus are diploid (2n = 38), therefore hybrids are fertile and can generate F2 and backcross progenies that segregate for a wide range of traits. Breeding can be carried out in a greenhouse on detached female branches. Within 4-8 weeks each pollination can yield hundreds of seeds. Seeds germinate within 24h and give rise to 1- to 2-m tall seedlings by the end of the same year (Bradshaw et al., 2002). Vegetative propagation of poplars allows hybrids to be immortalized as clonal material. The rapid growth rate of the juvenile phase, identity of vigorous hybrids and ability to respond positively to coppicing leads to short rotation coppice (SRC) as a means to optimize yield from poplar when planted as a dedicated energy crop.
These deciduous pioneer trees continue to grow after bud burst in the spring by initiating, expanding, and maturing leaves throughout the growing season. This indeterminate growth habit allows continued productivity under favorable conditions, until photoperiod initiated bud-set in the autumn. The separate male and female trees flower before leaf emergence in spring after reaching reproductive maturity following approximately 5 -6 years growth. In Europe cultivated poplars can reach heights of between 30 and 35 m with some hybrids (P x canadensis) reaching 50 m. Poplars have an optimal growing temperature of between 15 C and 25 C (range 5 C - 40 C) and cannot survive below - 30 C.
Management characteristics
Populus grown as a SRC plantation constitute densely planted high-yielding varieties harvested on a 2-5 year cycle (commonly 3 year) following coppice of the first establishment year's growth so forming a multi-stem plantation. The productivity of the stool that remains after coppice determines the life-span of the crop, but plantations are commonly viable for at least 30 years. Poplar will grow in most soils. Deep fertile medium textured well aerated soils offer maximum yield, and water-logged, very dry, or gravelly quick draining soils are best avoided. In the UK poplar plantations are generally confined to the South, Midlands and the East of England. Optimum soil pH is 6.5 and the range is pH 5.5 - 7.5 although some varieties can tolerate outside of this range. For successful root development the site may require sub-soiling in addition to deep ploughing (25-30 cm). Poplars can consume as much as 4.8 mm of water per day, and if yearly precipitation at the site is below 600 mm then irrigation should be considered.
In the UK cuttings should be taken when buds are fully dormant (between December and March). If not immediately planted cuttings should be stored in sealed plastic bags (to avoid desiccation) and kept between -2 C to -4 C. Cuttings should be between 20 - 25 cm long and 1 - 2 cm in diameter with a healthy bud approximately 1 cm below the angular top cut. The rigid requirements for poplar cuttings mean that mechanization of planting is as developed as it is for willow (DEFRA, 2002).
Freshly cut material, or stored cuttings following an overnight submersion in water, should be planted in early spring (February - March), although cold-stored material has been successfully planted as late as June. Cuttings are planted at a density of between 10, 000 - 12, 000 cuttings ha-1 this is generally at a lower density than planting of SRC willow.
During establishment poplars are intolerant to weed competition. A broad spectrum contact herbicide should be used to remove all perennial weeds prior to cultivation, and residual soil-acting herbicides can be applied post planting (Tubby & Armstrong, 2002). No fertilization should be applied during the establishment year and when compared with most arable crops poplar has a low demand for additional nutrient inputs. The crop is generally cut back at the end of the establishment year to encourage the formation of a multi-stemmed plant in the following years. Poplar, having a stronger apical dominance than willow, generally produces less shoots per stool. Therefore canopy closure and shade suppression of weeds may not be as rapid requiring additional herbicide treatments.
Protection from browsers particularly rabbits, hares and deer is important especially during crop establishment. Poplars are hosts to a range of leaf-eating insects and fungal pathogens. The most important insects are beetles (Phratora spp.) and Melampsora rust is the most important fungal pathogen particularly in the UK Melampsora larici-populina (Tubby & Armstrong, 2002). In 1994 a new pathotype of M. larici-populina (pathotype 'E4') arrived in the UK and infected all commercial varieties that were previously totally rust tolerant. New varieties 'Hoogvorst' and 'Hazendans' were resistant to E4 but susceptible to the recent pathotype E5 (Tubby, 2005). With no approved pesticide for use against poplar rusts in SRC, site location of at least 500 m away from the alternative host (Larix spp.) is recommended. Further recommendations are to plant the greatest practicable array of genetic material including the approved varieties from P. trichocarpa x P. deltodies and those from P. deltodies x P. nigra crosses (Lonsdale & Tabbaush 2002). Current advice is varieties 'Ghoy', 'Gaver', 'Gibecq' (all P. deltodies x P. nigra) and 'Trichobel' (a P. trichocarpa variety) show the greatest rust tolerance (Tubby, 2005).
Harvest and yield
The dry matter content of winter harvested material is approximately 50 %. A number of technologies including modified forage harvesters are available for direct on-site cut and chip, the most common form of harvesting. These higher moisture content chips will quickly deteriorate and the microbial activity will reduce the calorific value of the chips (Defra, 2002). Harvested wood chips should either be directly combusted or dried. Whole shots or billets can be and dried through the summer reaching a dry matter content of 70 %.
Despite trial yields of 35 ODT ha-1 y-1 for selected P. trichocarpa x P. deltodies varieties in Washington state USA (Scarascia-Mugnozza et al. 1997), and up to 24 ODT ha-1 y-1 for the highest yielder of a interspecific pedigree derived from P. trichocarpa P. deltoides grown in the UK (Rae et al., 2006) commercially grown UK plots generally yield an average maximum of 8 - 10 ODT ha-1 y-1.
Broad chemical composition and energetics
The broad chemical characteristics of hybrid poplar on a percentage basis are: Cellulose 42-56 %, Hemicellulose 18-25 % and lignin 21-23 %. Hybrid poplar has a net calorific value of around 18.5 GJ t-1 dry weight.
Production
Under the Woodland Grant Scheme the combined total plantings of poplar for England, Scotland and Wales was 3242 hectares.
References
Bradshaw HD Jr, Ceulemans R, Davis J and Stettler R. (2002). Emerging Model Systems in Plant Biology: Poplar (Populus) as a Model Forest Tree. J Plant Growth Regul., 19, 306-313
Rae AM, Pinel MPC, Bastien C, Sabatti M, Street NR, Tucker J, Dixon C, Marron N, Dillen SY and Taylor G. (2007). QTL for yield in bioenergy Populus: Identifying GxE interactions from growth at three contrasting sites. Tree Genetics and Genomes. In press
Taylor G. (2002). Populus: Arabidopsis for forestry. Do we need a model tree? Annals of Botany, 90, 681-689.
Scaracia-Mugnozza GE, Ceulemans R, Heilman PE, Isebrands JG, Stettler RF, and Hinckley TM. (1997). Production physiology and morphology of Populus species and their hybrids grown under short rotation. II. Biomass components and harvest index of hybrid and parental species clones Can. J. For. Res, 27, 285-294
