Short rotation coppice

Short Rotation Coppice (SRC) is grown as an energy crop for use in power stations, alone or in combination with other fuels such as coal.

Species used
SRC uses high yield varieties of poplar and willow. Typically the willow species chosen are varieties of the Common Osier or Basket Willow, Salix viminalis. Poplar is generally planted for visual variation rather than being a commercial crop, although some varieties can outperform willow on suitable sites. Species are selected for their acceptance of varying climate and soil conditions, relative insusceptibility to pests and diseases, ease of propagation and speed of vegetative growth. To combat pests such as brassy and blue willow beetles, as well as the fungal pathogen Melampsora (a rust), planting a carefully selected mix of varieties is recommended.

Planting
SRC can be planted on a wide range of soil types from heavy clay to sand including land reclaimed from gravel extraction and colliery spoil. Where used as a pioneer species the SRC yield may be smaller. Water availability to the roots is a key determinant for the success of the SRC. Saplings are planted at a high density, around 15,000 per hectare for willow and 12,000 per hectare for poplar. Planting takes place around March to take advantage of the high moisture of the soil in the spring and the amount of sunshine in the early summer. The most efficient planting machines plant four rows at a time and can plant a hectare in around three hours. Saplings are left to grow for a year and then coppiced. The first three years are part of the establishment phase and do not yield much dry matter.

Harvesting
After four years the plantation will be ready for harvest. Harvests take place on a two to five year cycle, shorter for willow and longer for poplar, and are carried out in winter after leaf fall when the soil is frozen. The established root system and the nutrients stored in the roots and stumps guarantee vigorous growth for the shoots. A plantation will yield from 8 to 18 tonnes of dry woodchip per hectare per year. A plantation can be harvested for up to thirty years before needing to be replanted.

When willow or poplar shoots are harvested as whole stems they are easy to store. The stems can be dried for combustion in a pile outdoors; the moisture content of the wood will decrease to about 30 % on average until the next autumn. The stems can be cut further into billets that may not need to be chipped depending on use.

Where wood chip is being produced it is most efficient to use direct-chip harvesters. These are heavy self-powered machines that cut and chip the shoots on a loading platform. Some can be attached to a normal tractor and a hectare can be harvested in around 3 hours. Direct chipping reduces costs as a separate chipping in the store will not be needed; however, the wood chip needs to be well stored to avoid it composting. Harvesting Poplar requires heavier machinery as it produces fewer and heavier stems.

The price of dry willow as a heating fuel is currently around 45 euro per tonne in most of Europe. This is not a relatively high-return crop, but it is low-maintenance and is a way of utilising difficult fields. Small scale production can be combined with the production of material for wicker work. Correctly managed there is little need for pesticides or treatments.

Greenhouse gas impact
SRC has a low greenhouse gas impact as any carbon dioxide released in power generation will have been sequestered by the plantation over just a few years. Some carbon may also be stored in the soil, however this is dependent on the carbon content of the soil to begin with.

The carbon costs associated with SRC are: the planting, farming and chipping of the SRC plantation, generally done with fossil fuel powered machinery; the crops require herbicides during establishment, fertiliser throughout growth, and occasional pesticide treatment - these chemicals require substantial amounts of energy and potential fossil fuel usage through manufacture.

Electricity or heat from SRC provides between three and six times the CO2 reduction per pound that can be obtained from bioethanol from cereal crops. However, the reduction in CO2 emissions is slightly lower than grass energy crops such as Miscanthus grass due to higher maintenance costs.

Environmental impacts
Good conservation management encouraging biodiversity can reduce the reliance on pesticides. Biomass crops such as SRC willow show higher levels of biodiversity in comparison with intensive arable and grassland crops. SRC has a higher water consumption than agricultural crops. The root systems of SRC have a lower impact on archaeological remains than forestry but greater than agricultural crops such as wheat.

Energy generation
A power station requires around 100 hectares (1 km²) of SRC for 1 MW of power capacity. The primary barrier to establishing plantations is the cost as there is no financial reward for four years from a large initial investment. The current nature of the power industry generally requires flexibility in energy supply which is incompatible with the long term commitment SRC requires; however, there is much interest in SRC due to the need to reduce fossil carbon emissions. Grants may also be available in some jurisdictions to further this type of land-use.