Agroforestry to the rescue!

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Soil washing off a ploughed field and clogging up road drains
A field of grain ripening in the summer sun is a familiar sight, the promise of food on the table, a symbol of our ingenuity in conquering nature. And yet if we return to that field in the winter we’ll see a very different picture. I live in a little valley close to the coast in Devon. We’re surrounded by mixed farming, some pasture grazing, some arable, and when it rains in the winter the stream that passes our cottage turns brown with the soil being washed off the bare fields.
A significant part of the problem is the ploughing of hillsides, which are extremely vulnerable to erosion –the niche that the ancestors of modern grains naturally inhabited was relatively flat valley floors where water moves much more slowly. This problem was exacerbated when we switched from pulling ploughs with animals to using tractors. Animals can plough on contour, creating a series of mini ditches across the slope that catch rain and allow it time to soak in. Erosion then was fairly minimal. Tractors, by contrast, cannot plough on contour on slopes beyond a certain angle, because they tip over, often killing the drivers. They do however have much more power, so instead they plough up and down the slope – a process that requires vastly more energy to perform – they climb the hill hundreds of times instead of just once – but they don’t tip over. In doing so they create channels that run from the top of the hill to the bottom, which direct water straight down the slope, allowing it to pick up speed and carry much more soil away. Around here we see very little soil left on the tops of ploughed hillsides, something that’s really obvious from a distance on the chalk downs. We really shouldn't be ploughing slopes, but what can we do? Whether we’re designed to eat grains or not, we certainly seem, for the moment at least, to be addicted to them. We’re not going to stop ploughing anytime soon, so how can we make things better?
I have one word for you. AGROFORESTRY.

So what is agroforestry?

Newly planted hedgeAgroforestry is the practice of incorporating trees into more conventional farming systems. It’s not new – in the tropics people have been growing food under trees for a very long time, but in the higher latitudes like Britain we have significantly less light – about an eighth of the intensity compared to the tropics. Growing carbohydrate-rich crops here under trees is not possible. We can however still incorporate trees into our current farming systems in carefully designed ways to give us many benefits, including both increased productivity and the regeneration of the landscape.
There are a number of different ‘flavours’ of agroforestry, from relatively simple to quite complex and diverse systems, food forests in the tropics and forest gardens, their temperate landscape equivalents, both being examples of the latter. They’re not the focus of this particular article however.
Perhaps the most familiar (for those of us in Britain anyway) might be the hedging that we already see surrounding many fields, although, a few decades ago, subsidies encouraged many famers to remove theirs ‘to increase the area of productive land’. Anyone who thinks that something as abstract as money couldn’t shape our landscape should look at the farming subsidy system! More recently, thankfully, farmers have been paid to put hedges back in, though a young hedge is not nearly as beneficial as an ancient one. Hedges may seem to be just a thin sliver of woodland edge, but there may be upwards of 500,000 miles of them in Britain and the benefits they bring are many-fold, especially for wildlife.
Riparian buffers will be familiar to many people in upland river valleys. They are a strip of trees and other vegetation that border a river, providing habitats, protecting the bank from erosion by animals and trapping any material that comes down the river in a flood. They also shade the water, which is important for fish and other aquatic life – colder water is denser and carries more dissolved oxygen.
Riparian buffer strip in the Lake District
Silvopasture is the incorporation of well-spaced large trees into grazing land. They provide shelter for animals from the sun, wind and rain, while also benefiting the pasture by pumping up nutrients and water. These ultimately feed the grass and in turn the animals. They connect the pasture system to a larger source of fertility beneath the ground. Silvopasture is most likely to have evolved as the result of overgrazing woodland. Animals browse what they can reach, so as older trees die there are no young saplings to replace them. Horse browsing treeEventually you are left with the last of the big trees, surrounded by grazed areas that have returned to pasture. You can identify silvopasture easily because the animals browse as much as they can reach of the tree (the leaves provide extra nutrients as the tree roots more deeply that the pasture), leaving the base of each canopy flat. The distance between the ground and the canopy is a clue to the animals that graze the field.
But weren’t we talking about arable farming? Ah, yes, we were. That brings us to alley cropping. But how can we grow grains with trees if the grains don’t like shade? Well it’s all to do with spacing. There’s a clue in the name – the system involves creating a series of ‘alleys’, effectively like long thin fields, which brings the benefits of the trees closer to the arable area, but without throwing too much shade. Orientation of the rows is determined by the shape of the landscape. On a slope, where water erosion is more problematic, we’d likely plant on contour as that provides some stability to slopes and allows some degree of terracing. In a flat landscape, where wind erosion might be more likely we’d orientate the rows to give some protection from prevailing winds. In Britain that also allows a north-south orientation which minimises shade on the arable crop when the sun is at its highest.
Alley croppingThe rows of trees can be simple monocultures, or more diverse mixtures. They can be grown as short term biomass crops or for long term timber. Either way they provide several benefits to the arable crop – shelter from the wind and what it can bring (pollutants, diseases), the pumping of nutrients and water from underground, passed on to the topsoil in seasonal leaf fall and root return. The trees can even offer some shelter to young seedlings from heavy rains. Each year, regardless of how well the annual crop performs, the trees grow and provide a yield, even if it may not be harvested until the tree is fully grown. When trees are grown for long term timber, the lower branches are often removed, to both allow more light to the arable crop and to minimise knots in the timber, increasing its value. Where the trees eventually throw too much shade, the system might be succeeded by silvopasture. Mark Shepard's restoration agriculture provides a fine example of how to farm while working with nature's tendency to succession.
Perhaps the most challenging aspect to the adoption of agroforestry in cool temperate landscapes is the low number of working examples. In my mind there is no doubt that these systems can improve current farming practices, but they do make them more complex. Farmers will have to learn how to manage them, they may need some extra equipment, they will need to find markets for their additional produce. Thankfully, there are pioneers who are out there doing this research and showing the way. The French have been particularly active and the film ‘Agroforesterie’ is a good overview of what they have been learning. In East Anglia we have an especially great example of what alley cropping can achieve. Martin Wolfe at Wakelyns Farm has much to teach us, but there's no room for that here – his fascinating research will be the subject of my next blog.

Tags: agroforestry alley cropping silvopasture farming arable trees

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