Getting in to Agroforestry
by Peter Aspin, Shropshire Agroforestry Project
My personal interest in silvopastoral agriculture (i.e. the integration of trees into permanent pasture) developed from two angles. Firstly, the observations over several decades of the interaction of cattle with trees in terms of using them for browsing - cattle are most definitely not simply grazing animals, and shelter, be it sun, rain, wind or cold. Secondly, becoming fascinated by the concept of forest gardening, the main practitioner of which in Western Europe was a man from the south of this county, Robert Hart.
As I developed a small forest garden, I became more fascinated by the concept of agroforestry throughout the world and began to develop a system here almost fifteen years ago, initially simply using ash and walnut trees but as my interest and knowledge and curiosity developed, planting many other species which I felt had a relevance and requirement of further research.
The integration of tree crops into pasture or arable crops goes back hundreds, if not thousands of years. Most people know of the age-old practice of growing cork oaks in pasture in the Iberian peninsula, known as “dehesa” in Spain and “montado” in Portugal. In the seventeenth century, the diarist and arboreal advisor to Charles II, John Evelyn, commented on the management of walnut trees in wheat fields in northern France and how little they affected the yield of corn.
Agroforestry systems are well-developed in much of Asia, especially China, but these are largely silvoarable in nature. The Holy Grail in agriculture has for many years, and continues to be, perennial corn and I often wonder why no independent farmers in the UK are prepared to research the likely candidates. But the ultimate perennial crop is the tree, with so many myriad uses of all its component parts.
How it started here
Climate was the spur to the research here. I sold the dairy herd here in 1996 and since then reared beef cattle bought in as calves. About half were black, either Limousin or Angus crosses. On hot summer days they were very uncomfortable in open fields and many spent large parts of the day in the buildings, which is obviously not good for liveweight gain.
We’ve probably all observed cattle in the shade of trees or hedges. The hot summer of 2013 was a perfect example of how not to keep cattle in large open fields without shade, resulting in many losses and production problems. We are told that the average temperature will increase by 2-3 degrees centigrade by the end of the century. But the mean figure inevitably hides the extremes.
Much work has been done in both the UK and the USA about the use of trees to moderate urban temperatures, none as far as I am aware in terms of crops or livestock, and this continues to be part of the research here. As well as providing actual shade, agroforestry also produces convection currents as the warmer air away from the trees rises, drawing in the cooler air from beneath the canopy. And trees in leaf produce copious amounts of oxygen as they separate the components of carbon dioxide.
How it works on my farm
Planting here is in rows aligned north-south at five metre intervals between individual trees, with slightly larger distances to the headlands for machinery operation. There is a single permanent electric fence either side of the row to prevent damage by livestock. Spacing between rows is twenty metres, which I felt was the absolute minimum where cattle are grazing, without resulting in a “corridor” effect where the cattle spend too much time and effort going up and down the alley, and not enough foraging. Therefore on this farm the cattle are allocated a length of, say, 100 metres or 150 metres, depending on forage growth, multiplied by the width of the alley, 20 metres.
The most efficient shape to graze is the square, maximum area, minimum length of boundary. If this had been a larger farm, then inter-row spacing would have been 30 to 50 metres, but as I needed to get as many trees as possible (500 plus) into the area (20 acres), options were limited.
As mentioned earlier, bovines are to a considerable degree browsing animals. The main browsing tree in the past has been the English Elm, but since its demise its place has been taken by the ash. However, virtually any deciduous tree or hedging plant (especially at leaf emergence) will be readily consumed. A recently retired veterinarian I know jokes that he has the healthiest and quietest flock of sheep because of the salicilin (aspirin) they intake from the abundant willow in the hedgerows.
Indeed, livestock often prefer tree foliage to ground forage. And so many species of trees are grown here to provide forage for the cattle, and though the medicinal effects are vague and unproven, the very fact that there is developing such a varied diet as the trees grow, will manifest itself in healthier animals. The diet of so many farmed animals today is so frighteningly narrow.
We have all been made aware recently of the degradation of our soils, fertility and structure, that has taken since the last ice age to build up, being lost at an alarming rate. Virtually every time I am in the countryside, I see appalling land management which takes no account whatsoever of the long term damage to the land.
The finest soil conditioner of all is leaf litter and there are two methods of leaf fall in autumn. Wind induced defoliation disperses the leaves over a wide area. Frost induced defoliation concentrates the leaves below the tree, which can result in damage to and destruction of perennial crops. Some years in early winter harrowing is essential beneath mature oak, and especially sycamore, trees along the farm boundaries to prevent this. On the whole, the small leaves or leaflets of trees such as elm, robinias (black locust) or gleditsias (honey locust) are quickly absorbed . The larger leaves of trees such as walnut, sweet chestnut or sycamore take much longer to degrade or be taken below the surface by invertebrates. But generalisations are not possible.
The all-important tree of Chinese agroforestry, the foxglove tree (Paulownia spp.) has very large leaves but these degrade rapidly and are not problematic. The high calcium content of deciduous leaf-litter also tends to neutralise the soil (increase the pH), whilst, conversely, the leaf-litter of coniferous trees acidifies the soil over time.
Trees grown at Shropshire Agroforestry Project include:
Black and Honey Locust
As far as I am concerned, the plough is an instrument of last resort, a means to repair the result of poor management. Three alleys (6 acres) where the grass/clover components had become unproductive, owing to such poor management and excessive leaf fall damage, were reseeded in the autumn of 2014. It is ten years since any of the land was last ploughed and I foresee no need for it in the future. If any sward becomes short of an ingredient, perhaps clover, then I broadcast that just before grazing so that the cattle tread the seed in, so long as the ground is not too dry. This alley is then taken out of the grazing rotation and cut for silage seven to eight weeks later to enable the new seedlings to emerge and consolidate. I also use the traditional method of reseeding hay meadows whereby the sward is allowed to go to seed before cutting. The resultant tedding of the drying grass distributes the seed-heads across the pasture which the machinery wheels then incorporate into the surface layer.
The effect of this system of farming on drainage is very interesting. A few years ago a water mains burst in the main road running alongside the farm exemplified this. The alleys which had just been grazed were flooded owing to the recent soil compaction, but the rows where the trees were planted absorbed the water very quickly and the flooding came to a very sudden end by the third row. This outcome has also been observed after heavy thunderstorms. In the dry summer of 2014, narrow fissures running several inches deep along the rows of trees were conspicuous. Conventional thinking concerning field drainage is probably not appropriate in agroforestry.
The forage is conventional long- term organic seed mixtures with a fairly high proportion of white clover. Some herbs have been included in the past but the only persistent one has been yarrow.
In the first few years after establishment, tree pruning to shape is essential. The ideal is to have a clear stem of about two metres with branches radiating out from this height. Much below this figure and ground shading would become a major problem affecting yields of the crop beneath. The usually accepted critical figure is one third canopy cover, below this level crop production is barely impacted, but above it production loss begins to become significant.
For similar reasons late leafing trees are necessary in agroforestry systems. In Asia the aforesaid foxglove trees (very fast growing producing timber for construction) are often grown with wheat, which invariably reaches its full height before the trees come into leaf, so all the corn then has to do is ripen as shading intensifies. Here, about forty varieties of walnut have been established, and some do not come into full leaf until the second half of June, so grass production is barely affected.
All crops grown under shade tend to become “leggy”, that is tall and thin. Here, this is especially apparent on the southern boundary where an existing mature row of trees shades the ground all year round. But this accidental shelter belt has benefited the rate of growth of trees planted within its influence. Conventional high-yielding ryegrasses prefer to grow in full sun whereas fescue grasses grow well in some shade. The reseeding last autumn included a percentage of Fest(ul)olium which is a ryegrass/ fescue hybrid in order to try to maintain high growth rates under conditions of some shade.
As well as walnuts, other nut trees established include sweet chestnuts, hazels, almonds, hickories, gingkoes and monkey puzzles (araucaria); browsing trees include a number of species of ash and elm, several varieties of hackberry (Celtis), black and honey locust trees (both also leguminous),timber trees include hornbeam and sweet gum (liquidambar) and Liriodendron (tulip trees) but there are also trees for rubber and cork production, even root beer. However, all trees by their very nature are multi-purpose: walnuts not only provide nuts and timber and edible sap but the smell of their foliage deters biting flies and when the cattle begin to browse the astringent leaves you know they are hungry and need moving on to the next rotation block.
However, there is so much more research that needs to be done concerning the integration of trees and ground cover crops. I had hoped to establish a curvilinear system on another twenty acre field, to discover at what angle to due north yields were highest. Also which would depend on latitude, prevailing wind direction, strength of morning to afternoon sun etc., but that is for others now. On sloping land, contour and spiral contour strategies need to be assessed. If temperatures are to rise then wind velocities will become more problematic making shelter belts essential. And, of course, these landscapes/treescapes must be established decades before they are required. Finally, and in purely economic terms, it is usually accepted that production from an established agroforestry system has a fifty per cent higher total output than the same area of land devoted separately to conventional cropping and woodland.
Shropshire Agroforestry Project