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Take a bough: introducing the wonder of willow

Martin Wolfe believes in the potential of willow in agroforestry (WRENmedia)
Martin Wolfe believes in the potential of willow in agroforestry
WRENmedia

Looking out at his two-hectare alley-cropping system, Martyn Wolfe is a proud man. Organic wheat and oats are thriving on these experimental plots at Wakelyns Agroforestry in eastern England, which are flanked by natural windbreaks of willow (Salix spp). But the trees offer more than just protection from the elements; they are part of a complex food production system with an important by-product: fuel.

The biofuel boom has reignited interest in willow. With global food price surges at least partly the result of the production of fuel from food crops, research into the next generation of biofuels derived from cellulose and algae is already underway. Many believe willow will also be a crop to watch.

Energy by accident...

Willow talk
Willow has a long tradition of use by humans, from fence-building and hedge-laying to the production of baskets, boxes and cricket bats. The trees thrive in temperate, wet conditions and produce strong, light, durable wood. Salicyclic acid, found in the leaves and bark of the willow tree, was the precursor to the anti-inflammatory painkiller asprin.

For Wolfe, willow is a multipurpose tree, providing fast-growing "production hedges" that are crucial to the sustainability of his cropping system. The willow alleys are planted in lines 12 metres apart, running north-south to ensure maximum sunlight and protection from wind. This helps create a microclimate ideal for crop production. As a deciduous tree, leaf drop in autumn replenishes the soil, transferring nutrients from the depth of the root zone to the soil surface where the crops benefit. Wolfe's willows have no need for inputs such as nitrogen fertiliser, pesticides and fungicides.

"These trees provide all sorts of services in terms of biodiversity, nutrient cycling, shade, shelter, and habitat for useful organisms," he enthuses. "Even if we didn't use them for fuel, they would still be providing something useful."

Willow is a fast-growing source of green energy (Rothamsted Research)
Willow is a fast-growing source of green energy
Rothamsted Research

But the wonder of willow is that when coppiced, chipped and fed into biomass boilers, the branches are highly-efficient feedstock. The trees also sprout quickly after harvesting, so that the CO2 released by burning is re-absorbed during the following season's growth: the epitome of clean, green, renewable energy.

"We're producing fuel but it's a kind of residue; a leftover," Wolfe continues. "The main objective is sustainable food production."

...And fuel by design

The UK-based Rothamsted Research centre - one of the oldest agricultural research institutions in the world - is leading the way in working with willow. The organisation is home to the National Willow Collection and scientists are using DNA fingerprinting to unlock the secrets of willow from the 1,300-strong repository. They are also improving varieties for use in heat and electricity generation, with plans to study ways of producing liquid transport fuels from willow. Willow chips are already fuelling power stations in the UK and Sweden.

Rothamsted's scientific director, Angela Karp, is a firm believer in the benefits of willow. "Some species can grow extraordinarily fast in a very short space of time and produce large amounts of woody biomass," she explains. "Although they use more water than a crop like wheat or sugar beet, it's no more than normal woodland."

Willow trees can be grown easily from cuttings (WRENmedia)
Willow trees can be grown easily from cuttings
WRENmedia

But, as well as its use as an agroforestry tree or as a fuel crop in its own right, willow has another valuable trait. "One of the most important things is that willow will grow on poorer soils," Karp continues, "So it does not compete with the best farmland." This means it clears the first hurdle in the race to become a second generation biofuel.

While productivity of fast-growing grasses like bamboo is head-and-shoulders above willow in warmer, tropical areas, they are also much thirstier and unable to tolerate temperate regions. Willow is well-placed to fill the energy gap in agricultural systems in Eastern Europe and upland areas of China and India, where new sources of fuel are essential. The availability of small woodchip boilers would enable smallscale farmers to produce their own fuel - with the option of selling surplus chips.

Burning desire

Rothamsted's Willow Breeding Programme, funded by the UK Department for the Environment, Food and Rural Affairs (DEFRA) and the Biotechnology and Biological Sciences Research Council, has already released several fast-growing, high-yielding varieties for use as fuel. The organisation's "Willow Power" exhibit, focusing on the importance of different varieties of these trees in the fight to limit climate change, won a gold medal at the 2008 Chelsea Flower Show. It is hoped that the much-needed publicity will help boost interest in willow from growers and users alike.

Whether grown for fuel or as part of a holistic cropping system, willow is likely to become an important part of the drive towards sustainable fuel. This means Wolfe's agroforestry system and Rothamsted's ongoing research could be promising glimpses of a brighter future.

Date published: July 2008

 

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