Trees & Agriculture: These Farmers Can See The Forest For The Trees

November-December 2007

IT SOUNDS TOO GOOD TO BE TRUE. But experiments at Guelph University have resulted in double the crop yield in drought years. A farming practice that results in better soil, more earthworms, much higher capture of carbon dioxide, less nitrogen runoff, more birds and insects, and double the crop yield in drought years – it sounds too good to be true. Yet this is exactly what experiments at Guelph University are suggesting. The most intriguing conclusion is that if farmers adopted the practice throughout the 455,000 square kilometres of marginal or degraded land now being farmed, Canada could reduce its annual CO2 emissions by well more than half of what’s currently required to meet its Kyoto commitment.”

The practice is called intercropping – planting crops between rows of trees. At Guelph, the rows of trees are 12.5 metres to 15 metres apart, and this year the crop is soybeans.

The experiments have been going on for several years, using different crops and the results have been compared to identical crops grown in open fields.

Andrew Gordon, professor of forest ecology and agroforestry, has been supervising the experiments, assisted by Naresh Thevathasan, manager of agroforestry research and development at the Ontario Agricultural College in Guelph.

Thevathasan explains that the trees take up about 10 per cent of the intercropped area. Yet in drought years, the yield has been double that in the open area, because the trees helped retain moisture in the ground. In normal and wet years, the yield in the actual growing areas has been identical, but because of the space taken by trees, the yield for the total area, trees included, has been about 10 per cent lower.

Accompanied by two graduate students, Thevathasan gave me a tour. The soybean plants were ripening to a buttery yellow and a row of poplars stretched as far as I could see over a gentle rise. The trunks were a good 40 centimetres across at hip height and the trees were roughly 13 metres high.

Fast-growing poplars can reach this size in 12 years, Thevathasan says. A couple of years ago, some were sold for $100 to $150 a tree.

In this experiment, he adds, there are 111 trees per hectare. They can be used to make plywood, low-quality lumber, chipboard, pulpwood, even pellets for heating.

They also produce a lot of leaves each year, which are shed where the crops are planted, making the soil richer in organic matter.

Earthworms in the leaf litter number 125 to a square metre, compared with two worms per square metre in the open field. Ten different kinds of birds foraged in the intercropped area, Thevathasan says, compared to four in the open area.

In addition, there were a lot more parasitic insects in the intercropped area, which might be good news for pest management.

Nitrogen added to the soil through decomposing leaves reduced significantly the amount of fertilizer required for crops.

And, says Thevathasan, leaching of nitrates from the soil, which causes nutrient overloading in waterways, was reduced by 50 per cent in intercropped areas, because tree roots take up nitrogen.

It was Rachelle Clinch, one of the graduate students, who put her finger firmly on the issue raised by the experiments.

“We haven’t changed our thinking about agriculture since North America was colonized,” she said. “It’s always been focused on clearing land to grow crops, with no place for trees.

“If only we would learn to use trees to help farming, instead of seeing them as a hindrance, we’d see many, many benefits.”

In the struggle to control global warming, there’s a simple, three-word memo government ministers of the environment and agriculture should be issuing to their staff. “Check this out,” it should read.

— reprinted with permission by Cameron Smith, The Toronto Star, Sept. 15, 2007

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