The main bioenergy crops for regions with a temperate climate.
The production of energy by burning biomass (
Biofuels constitute a very broad category of materials that can be derived from sources including municipal by-products, food crops (
|Cereals||Barley||Herbaceous||Annual||1st gen. ethanol / Solid biofuel|
|Starch crops||Jerusalem artichoke||Herbaceous||Perennial||1st gen. ethanol|
|Dedicated bioenergy crops||Kenaf||Herbaceous||Annual||Solid biofuel / 2nd gen. ethanol|
|Reed canary grass|
Oilseed crops have long been grown in rotation with wheat and barley to produce oil for human, animal or industrial use. Today, these crops primarily provide feedstock for biodiesel. Biodiesel is produced by chemically reacting a vegetable oil with an alcohol such as methanol or ethanol, a process called transesterification. Cereals and starch crops, whose main economical use is for food and fodder, can also be transformed to produce biofuels. For example, the starch in the grains of maize (
The category of dedicated energy crops notably includes all lignocellulosic (mostly perennial) crops grown specifically for their biomass and used to produce energy. Such crops include herbaceous (
We shall limit our description to woody species, because they constitute the focus of our research.
Woody crops for energy production include several silvicultural species notably sharing the following characteristics: fast growth and high biomass yield, potential to be managed as a coppice and high management intensity (highly specific needs with regard to fertilization, irrigation, etc).
A recent review of the literature revealed that about ten different terms are used to refer to the silvicultural practice of cultivating woody crops for energy production: short-rotation woody crops, short-rotation intensive culture, short-rotation forestry, short-rotation coppice, intensive culture of forest crops, intensive plantation culture, biomass and/or bioenergy plantation culture, biofuels feedstock production system, energy forestry, short-rotation fiber production system, mini-rotation forestry, silage sycamore, wood grass . The same author suggested adoption of standard terminology based on an earlier work  that had defined this cropping system as “
2. Willow short-rotation coppice in Quebec
2.1. A brief history
Scientific interest in short-rotation bioenergy willows in Canada dates back to the mid-1970s’ oil crisis, which stimulated the use of biomass for energy production. The Federal government’s 1978 ENFOR (ENergy from the FORest) program, coordinated by the Canadian Forest Service was part of a federal interdepartmental initiative on energy research and development to promote projects in the forest bioenergy sector. Scientists from the Faculty of Forestry at the University of Toronto pioneered the investigation of willow’s potential for bioenergy in Canada, convinced that willows could produce high annual yields in temperate zones [18-19] Louis Zsuffa's (1927-2003) work on selection and breeding of poplars and willows through genetic trials on small surfaces inspired the next generation of researchers, including one of his graduate students, Andrew Kenney, who implemented short-rotation intensive culture technology on the first prototype energy plantations in Canada . As well, Gilles Vallée, of the Quebec ministry of Natural Resources, investigated the genetic improvement of hybrid poplar and willow with the aim of developing clones adapted to the shorter growing seasons of boreal forest locations. Our own
2.2. Site selection
Several environmental factors can potentially influence a willow short-rotation coppice plantation and all should be evaluated prior to plantation establishment to maximize success. Ecologically, the majority of willow species are common in cold temperate regions and are adapted to mesic-hydric habitats. However, most riparian species require well-aerated substrate and flowing moisture, whereas non-riparian species have less exacting soil aeration requirements . Moisture availability is an important factor determining native distribution in natural environments, successful plant establishment and high biomass yield. On average, willow coppice requires more water for growth than conventional agricultural crops  and consequently highly moisture retentive soil is an essential prerequisite. The lower St. Lawrence Valley, where most willow plantations in Quebec have been successfully established over the past two decades, is characterized by a temperate and humid climate with an annual average temperature of 6.4°C, average growing season (May-October) temperature of 15.8°C and a mean total annual precipitation of 970 mm. The period without freezing is on average 182 days and the total number of growing degree-days (above 5°C) is 2100.
Soil composition is another important factor for ensuring willow crop establishment and yield. In general, willow can be grown on many types of agricultural land. However, since this species is more water-dependent than other crops, particularly dry land should be avoided. On the other hand, although willow has been shown to be a rather flood-tolerant species compared to other woody energy crops , permanently submerged soils also constitute unsuitable sites. Ideally, willows should be grown on a medium textured soil that is aerated but still retains a good supply of moisture. Most willows grow best in loamy soils, with a pH ranging from 5.5 –7.0, although to a certain extent suitable soil types may range from fine sands to more compact clay soils. Several studies have shown that heavy clay soils are not very suitable for willows . Most abandoned agricultural lands in Quebec are thus highly suited to growing willows, being situated in temperate regions and often adequately fertile. Other pre-establishment considerations are linked to the location of the plantation. Economical (and ecological) benefits can be maximized when high production levels of willows are achieved in combination with low input requirements, which result in high-energy efficiency and low environmental impact. For this reason, choosing the right location is crucial for achieving a sustainable energy production system. Normally, the plantation should be situated as close as possible to the end utilisation point (
2.3. Choice of planting material
Willow yield varies greatly depending on both environmental and genetic factors. The genus
In Quebec, the first trials for evaluating willow biomass potential began on small plots in the early 1990s with two species, one indigenous (
New selected planting material has also been made extensively available by several willow growers interested in development of willow cultivation in Quebec and operating jointly with researchers. Agro Énergie (www.agroenergie.ca) was the first large-scale commercial nursery in Quebec to produce diverse varieties of willow and has continued to expand its willow plantations across Eastern Canada. For the joint project between our research team and Agro Énergie, we provide scientific expertise in terms of plantation layout, species selection, cultivation methods and management practices. The 100 hectares of land provided by Agro Énergie represent an opportunity to scale up experimental technology, perfect techniques and evaluate costs and yield, using the high performance agricultural equipment necessary for large-scale commercial production.
|Black willow||North America||S05*|
|Almond-leaved willow||Eurasia||Noir de Villaines+, P6010+,|
|White willow||Europe, Africa, & west Asia||S44*|
|Heart-leaved willow||North America||S25*, S546*|
|American pussy willow||North America||S365*¥|
|Schwerin willow||East Asia|
|Common osier or basket willow||Eurasia||SVQ*, S33*,5027*, Jorr+|
|Miyabe willow||East Asia||SX64*, SX67*|
|Purple willow or purple osier||Northern Africa & Europe||Fish Creek*|
|Pointed-leaf willow||Eastern Europe||S54*|
|Bjorn+, Tora+, Torhild+, Sven+, Olof+|
2.4. Land preparation and weed control
Appropriate soil preparation is essential to ensure good plant establishment and vigorous growth. This is particularly true when willows are to be established on soil with low fertility or marginal land. The main goal of any land preparation operation should be to eliminate weeds, aerate soil and create a uniform soil surface for planting. Once the planting site has been chosen, the first operation to be performed is preparation of the land much as for any other agricultural crop. The productivity of trees under short-rotation intensive culture is strongly influenced by herbaceous competition. One of the first trials conducted by our research team in the early 1990s showed that weed suppression was essential to willow establishment . On Quebec’s generally well-drained lands, the most common weeds are broad-leaved annuals such as white goosefoot (
2.5. Plantation design and planting
Willows can be planted according to two different layouts. In most North European countries (Sweden, UK, Denmark) and in the US, the most frequent planting scheme is the double row design with 0.75 m distance between the double rows and 1.5 m to the next double row, and a distance between plants ranging from 1 m to 0.4 m, corresponding to an initial planting density of 10,000 - 25,000 plants ha−1. The most common plantation density in these countries is currently around 15,000 (1.5 x 0.75 x 0.59 m) plants ha−1 . This rectangular planting arrangement is used to facilitate field machine manoeuvres through the plantation site. Tractors overlap the double row and the wheels run in the wider strips between those rows . In Quebec, a simpler willow planting design, similar to that used for poplar in short rotations, has been in use since initial trials with only minimal modifications. It consists of a single row design ranging from 0.33 m between plants on a row and 1.5 m between rows (20,000 plants ha-1) in the very first plantations, to 0.30 m on the row and 1.80 m between rows (18,000 plants ha-1) in newer willow plantations. Theoretically, this design facilitates weed control during the establishment phase (the first three years), and consequently willow rooting and growth. In fact, the design choice depends mostly on machinery available for planting and harvesting, since it has been clearly demonstrated that planting design has less impact than plant density and cutting cycle on the yield of
Planting material consists of dormant willow stem sections, either rods or cuttings, depending on the planting machinery to be adopted. In some countries, for example in the UK and in the USA, ‘step planters’ are the most commonly used machines. Willow rods 1.5-2.5 m long are fed into the planter by two or more operators, depending on the number of rows being planted. The machine cuts the rods into 18-20 cm lengths, inserts these cuttings vertically into the soil and firms the soil around each cutting. Step planters have been calculated to cover 0.6 ha/hr in a UK study. . In Quebec, the most common planting machine is a cutting planter that uses woody cuttings (20-25 cm long) and may operate on 3 rows simultaneously (Figure 1).
Normally, a cutting planter inserts cuttings into the soil at a depth of about 18 cm. Based on empirical experience, this equipment can plant 3,600-4,000 cuttings per hour (1 ha of willow every 3-4 hours), although the duration of this operation may vary depending on several factors (site topography, soil type, plot shape, etc.). Planting material in Quebec is prepared by harvesting one-year-old stems (about 3 m long) in the autumn (
If cuttings are left in temperatures above 0°C, a break in their dormancy will occur, adventitious roots will develop and the buds may burst. This will lead to a reduction in water and nutrient content and consequently reduced viability. Thus, it is very important to plan the planting operation carefully in advance, calculating the number of cuttings that can be planted.
The time of planting varies according to meteorological and soil conditions. Planting should be undertaken as soon as possible in the spring, to allow plants to benefit from the high soil water content following snowmelt, and then to establish quickly and take maximum advantage of a long growing season. In addition, a late willow planting is also more subject to failure due to drought if a dry summer should occur. However, there are several additional factors that play an important role in determining the planting date. In order for soil preparation (
2.6. Crop management
2.6.1. Establishment year
All operations carried out in a willow stand during the first year are aimed at promoting plant establishment and a high survival rate, thereby ensuring the on-going productive life of the plantation. Weeds are the main problem encountered in willow crop, and they may still colonise fields despite pre-emergence treatments. It was established decades ago that during the first year after planting, vigorous weeds reduce willow growth by between 50% and 90% . Most of these invasive species have higher growth rates than young willow shoots, and compete with them mainly for light , and to a lesser extent for water and nutriments, leading to high plant mortality within the first few months. Hence, great care should be taken to control weed development in the field in the weeks following planting. On most willow plantations in Quebec, one to three passes with a rotary tiller cultivator between rows are needed to control weeds during the establishment year. In case of a severe weed problem, manual weeding may be required between plants within each row.
There is much evidence that most newly-established willow plantations profit immensely from being cut back at the end of the first growing season (Figure 3).
Not only does cutback encourage established cuttings to produce vigorous multiple shoots the following spring, it also helps reduce competition by weeds, thereby reducing the need for continued chemical weed control . Furthermore, cutback facilitates entering the field at the beginning of the second growing season to fertilize and till soil between rows. Cutback is normally performed in the fall by cutting all newly-formed shoots at ground level using conventional agricultural equipment, such as reciprocating mowers for large surfaces or a trimmer/brush-cutter for small plots.
For many reasons, fertilization is a controversial aspect of short-rotation plantation, subject to fluctuations in practice. Our review of the historical evolution of willow short-rotation forestry in different countries suggests that the initially highly favourable attitude toward using chemical fertilizers has tended to attenuate over time, mainly because other issues beyond the biomass yield (both economical and environmental) have arisen. Different perspectives on this topic have also arisen out of legislation that in some countries has favored more environmental-friendly management (
However, it is an irremediable fact that, due to high biomass yields, most willow energy crops grown in short-rotation and intensively managed and harvested remove nutrients at a high rate, though evidence varies somewhat (Table 3).
|Annual nutrient removal (kg tDM-1)||Reference|
Some authors have highlighted that N fertilization in willow plantations at the beginning of the cutting-cycle, excluding the year of planting, is generally a very efficient way to enhance plant growth [45-46]. On the other hand, willow nutrient requirements are relatively low, due to efficient recycling of N from litter and the relatively low nutrient content retained in biomass (stem). Therefore, much less nitrogen fertilizer should be applied than is typical with agricultural crops, although dosage should also be based on formal soil chemical analyses performed prior to plant establishment. Several authors have indicated that no nitrogen is required in the planting year for short-rotation coppice [39-47]. This also reduces the competitiveness of weeds that would take advantage of fertilizer application. Economical considerations are yet another factor to consider when determining the dose of fertilizer to be used, since fertilizer constitutes a significant percentage of the financial cost involved in the production of willow biomass crops. A recent study conducted in New York State showed that fertilizer represents up to 10–20% of the cost of production over several rotations . The average dose generally recommended in Quebec ranges from the equivalent of 100-150 kg N, 15 kg – 40 kg P and around 40 kg K per hectare per year after the establishment year. Because it is not possible to introduce heavy equipment into the field after plantation establishment, fertilizer application is normally performed one year after planting and after any harvest, when tractors can circulate freely in the field.
An interesting alternative to mineral fertilizers are biosolids and other industrial and agricultural byproducts, which have been tested in many countries since the early 1990s. These include municipal wastewater , wastewater from the dairy industry, landfill leachate , diverted human urine , industrial wastewaters such as log-yard runoff , as well as solid wastes like digested or granulated sludge  and pig slurry . In fact, the majority of these products contain high levels of nitrogen and phosphorous, elements that might constitute a source of pollution for the environment but at the same time represent a source of nutrients for the plant. Thus there are many advantages to using such products in willow plantations:
recycling of nutrients, thereby reducing the need for farmers to invest in chemical fertilizer;
conservation of water;
prevention of river pollution, canals and other surface water, into which wastewater and sewage sludge would otherwise be discharged;
low-cost, hygienic disposal of municipal wastewater and sludge.
Willow cultivated in short rotation is a very suitable crop for fertilization with these products for several reasons. First, it has been determined, both by measured and estimated models, that this crop has high evapotranspiration rates and thereby consumes water quantities as high as any other vegetation cover, which allows significant wastewater disposal over each growing season [24-55-56]. Furthermore, willow short-rotation stands have been shown to be able to uptake large amounts of nutrients present in this waste . Last but not least, willow coppice is a no food no fodder crop and, if properly handled, any possible source of human or environmental contamination is strongly reduced . In some early trials carried out in Quebec to test the possibility of using sludge in willow short-rotation culture, it was found that a moderate dose of dried and palletized sludge (100-150 kg of “available” N ha-1) might constitute a good fertilizer during the establishment of willows, especially on clay sites [53-59]. Today, the recommended dose of derived wastewater sludge fertilizer in Quebec ranges between 18-21 t ha-1of dried material, which corresponds to 100-120 kg available nitrogen per hectare. Fertilization is performed in spring of the second year after planting with ordinary manure spreading machines. Another recent project investigated the effect of the use of pig slurry as fertilizer on the productivity of willow in short-rotation coppice (Figure 4).
The results showed that pig slurry is good fertilizer for willow plantations . In fact, very high biomass yields were obtained over two years, and even made it possible to predict that typical three-year rotation cycles could be reduced to two years, under the proper production conditions. This means that even though nitrogen in slurry may be less efficient than that in a mineral fertilizer, a significant reduction in the production costs of willow-based biomass as well as recycling of a greater quantity of slurry can be achieved simultaneously .
2.7. Pests and diseases
Although there are a great number of insects feeding on willows, three main species are of concerns for willow short rotation coppice in Quebec. The first is the willow leaf beetle (
The other predominant insects found feeding on willow trees and shrubs are two aphid species: the giant willow aphid,
The giant willow aphid. is one of the largest aphids ever recorded, measuring up to 5.8 mm in length . It feeds almost exclusively on willow, but has very occasionally been recorded on poplar (
The black willow aphid,
Other less damaging insects have been found on willow plantations in Quebec.
Willow can be injured by several diseases . Willow leaves may be sensitive to
2.8. Harvesting and yields
Willow should be harvested at the end of each rotation cycle (2-5 years), normally in fall, after leaf shedding. All willow stems should be cut at a height of 5 - 10 cm above the soil surface in order to leave a stump from which new buds will form sprouts the following spring. Essentially, there are three ways to harvest willows, the choice largely depending on the final destination of biomass and the equipment available. When willows are grown to produce rods to be used in environmental engineering structures such as sound barriers, snow fences and wind breaks along highways and streets [72-73] or to produce new cuttings, plants are harvested with trimmer brush-cutters. Whole willow rods can also be stored in heaps at the edge of the field and chipped after drying.
Another option involves the use of direct-chip harvesting machines (
A third harvest system recently developed in Canada, mainly adapted to willow short-rotation coppice, is a cutter-shredder-baler machine that performs light shredding and bales willow stems , producing up to 40 bales hr-1 (20 t hr-1) on willow plantations (Figure 6).
The main advantage is that, since bales can be left to dry before being chipped, the risks linked to handling wet biomass are reduced . In Quebec, willow biomass harvest is usually done in fall after leaf shedding.
As with any other agricultural crop, biomass yield of willow short-rotation coppice depends on many co-occurring factors including cultivar, site, climate and management operations. Soil type, water availability, and pest and weed control also affect yield. Data from existing commercial sites in the UK suggest that average yields of around 8-10 odt ha-1yr-1 are representative of plantations using older cultivars, whereas biomass yields as high as 15-18 odt ha-1yr-1 can be obtained by using selected genetic material . In other northern European countries, an average annual growth of 15–20 odt ha-1yr-1 has been observed in early experiments , although more recent figures suggest that an average of 10 odt ha-1yr-1 is more realistic . Experimental yields of short-rotation willow ranging from 24 to 30 oven dry tonnes (odt) ha−1 yr−1 have been measured in the US and Canada [43-44], although typical yields are more often in the range of 10 to 12 odt ha−1 yr−1 .
Long-term trials show that under southern Quebec’s pedoclimatic conditions, short-rotation willow coppice can provide high biomass yields over many years, although results vary according to variety. In one clonal test for instance, at the end of the third (3-years) rotation cycle, the most productive willow cultivars were SX64 (19 Odt ha-1 yr-1) and SX61 (17 Odt ha-1 yr-1) (Figure 7). Also, indigenous (
|Rotation||Average biomass yield|
|Total (Odt ha-1)||Annual (Odt ha-1 yr-1)|
3. Perspectives for future research: The use of willows in phytoremediation
In Canada, it is estimated that millions of hectares of arable land lie uncultivated. These so-called marginal lands tend to be less productive, less accessible, poorly drained, or even contaminated . Willows have been successfully used to capture leached nutrient and heavy metals from soils [54, 59, 80, 81]. The various species of
The main types of contaminants found in Quebec soils are petroleum products and heavy metals . In many urban areas, past industrial activities have resulted in thousands of contaminated sites that require decontamination prior to any further utilization. Estimates by the province’s ministry of environment have shown that, in the region of Montreal alone, there are over 1350 contaminated sites of which only 54% are in the process of being rehabilitated by traditional methods . Current decontamination methods imply the excavation of the contaminated soils, transport to a landfill treatment facility followed by chemical cleaning, vitrification, incineration or dumping; these steps are extremely expensive . Plant-based
Phytoremediation using willows is becoming an increasingly popular alternative approach to decontamination, and several studies and pilot projects are underway. Willows have been used successfully to treat highly toxic organic contaminants such as PCBs, PAHs, and nitro-aromatic explosives . Similarly, willows, in particular
Although the fast-growing perennial habits of short-rotation coppice willow planted at high densities result in a low concentration of metals accumulated in biomass after one year of growth, the high biomass production of
3.2. Genetic improvement of willow for phytoremediation
Historically, most genetic selection to improve willow germplasm has been oriented toward increased capacity for biomass production , adapted to temperate climates and resistant to pathogens. However, in the context of phytoremediation, the ideal willow genotype must also: i) be adapted to specific pedo-climatic conditions; ii) be fast growing; ii) produce a large root biomass; iv) be resistant to a variety of contaminants; v) have a high concentration factor of contaminants; vi) be easy to establish, maintain and collect. The exceptional diversity of the genus
To our knowledge, one of the rare efforts to understand the genetic and genomic bases underlying the potential of willow for phytoremediation is the three-year Genorem project (www.genorem.ca) launched by research teams at the Université de Montréal and McGill University (Project Leaders Dr. B. Franz Lang and Dr. Mohamed Hijri, both of the
Phytoremediation as a decontamination technology can be applied to large surface areas, causes less environmental disturbances and represents a significantly cheaper approach than traditional methods. However, treatment is lengthy (several years), and the methodologies appropriate for each type of contamination require refinement. While the biomass produced in the context of a phytoremediation project may potentially be contaminated, this does not affect its utilization as a product outside the food chain. Moreover, the highly concentrated ashes resulting from conversion of the biomass to fuel facilitate disposal and treatment of the contaminant, particularly for a large, diluted volume of contaminated soil. Hence the decontamination by means of phytoremediation is a less intensive technique.
Eastern Canada is one region where willow short-rotation coppice has been the focus of numerous research projects over the last 15-20 years. Most experimental data published during this period concerning Quebec have found a high biomass potential, due to a combination of several factors, including the very high biomass yield of certain willow varieties, favourable pedoclimatic conditions and the very low incidence of severe pests and diseases. These high biomass yields have encouraged some growers to choose willows as an alternative agricultural crop, leading to a dramatic expansion of land devoted to willow short-rotation coppice in the province, especially over the last five years. However, the future evolution of this crop’s production will most certainly be influenced by the development of an active market for such biomass, which would encourage farmers to grow willow over a much larger surface area. In particular, developments in the technology of feedstock transformation and marketing issues related to product potential both merit further study. The high potential of willow for bioenergy production and environmental applications, including phytoremediation, in the Quebec context has been clearly demonstrated.