Energy from the Agriculture-Forestry Interface

(c)Dr Roy H W Johnston, Techne Associates, July 2010

(comments to


This outline project(1) develops the implications of the 'linear forestry' process suggested by the planting of trees in high density along the borders of motorways(2). This suggests a need to develop a mechanised method for managing and harvesting these linear plantations, and this would be adaptable to a practice possible in association with agriculture, based on the development of the hedge into a linear plantation system by expanding it laterally with coppice and pollard species, to an extent capable of being harvested mechanically from the adjacent fields. The proposal explores also the implications of this for the agriculture in the enclosed sheltered field-space, and for the socio-economic organisation of the supply of the harvested biomass energy to a viable local energy conversion system, typically an electricity generation system in a nearby local town. The socio-economic organisation of this specialised production and marketing process, involving a number of owner-occupied farms, constitutes an opportunity for the regeneration of the co-operative movement, as initiated in the 1890s by Horace Plunkett and others with the IAOS, and remaining currently in existence as ICOS. Any initial funding should therefore be put into ICOS for setting up a development service.


The following outline proposal is divided into steps, dealing first with some problems arising in the management of existing hedge systems, then the development of a piloting process for hedge management and lateral expansion. The piloting of the output marketing system is then considered, based on an estimate of the pilot production; this introduces the assessment of the social organisation dimension, and the interface with local government and the urban market in the local town. The process for harvesting and drying the product is then considered, and interfaced with the problem of managing the linear plantations along the motorways. Finally an outline model of a production system, on a scale of about 10km square (10,000 ha) in the hinterland of a rural town, is suggested, and the implications of this for the electricity grid are considered, as well as the implications for agricultural organistion.

Existing Hedge Systems: Pilot Development

There has been little management applied to the current hedge system, despite the efforts of a handful of activists concerned with the role of the hedge as an ecological reserve. At best there is crude mechanical trimming.

In the event that one or more farms can be identified to pilot this proposal, the first thing to do would be to survey the hedge system, identify among the embedded trees those which are mature and due for felling, perhaps occasionally for quality hardwood, but more likely for firewood, and fell them. The next step is to identify species in the hedge showing promise for managed development, either to full maturity or in pollard mode, in either case being a potential source of quality hardwood (eg ash for hurls, currently it seems imported).

Species showing promise of healthy growth in the local environment will yield cuttings for propagation in the process of lateral expansion of the hedgerow. Gaps in the hedgerow may also need to be dealt with, by the usual hedging practice, so as to maintain the hedgerow as a barrier.

The next step is to propagate the cuttings of the promising species, in the initial case for planting elsewhere. The first pilot linear-forest expanded-hedge, in the interests of rapid results, however should be developed with saplings of the identified preferred species, bought in from a supplier. The selected species should be such as to thrive despite periodic cutting back to ground level (coppice) or to an accessible height (say 2m or so, pollard). The advantage of pollard is that it give a regular yield of biomass for energy (firewood or chips) while also yielding some quality hardwood eventually. This process is known as 'short-rotation forestry' (SRF).

Assessing the Local Market

Let us consider quantitatively a possible pilot farm of (say) 100ha; 1ha is 100m square, so such a farm would be a square km in area. In an area of land 10km square, seen perhaps as the immediate hinterland of a rural town, there could be 100 such farms, though in practice there would be a distibution, all shapes and sizes. We just want to get an order of magnitude to estimate roughly the energy potential. In what follows, the symbol ~ implies 'approximately'.

Our 100ha pilot farm would have ~4km of boundary hedges, and it might perhaps be criss-crossed by ~4 internal hedges, perhaps giving a system of ~9 fields, each about ~10ha. The boundary hedges, as as seen from the farm, have one side accessible, so in effect we have ~6km of accessible linear hedging. Let us assume we extend these laterally to a total width of ~10m, so we have ~60,000 square metres or ~6ha of linear forestry, on this crude assumption. This could be increased to ~10ha if the average field size was reduced; let us assume one way or another we can adapt the pilot farm to have ~10% of its area under linear forestry by expanding laterally the hedges.

According to the Teagasc energy group the minimum area for economic mechanical harvesting is ~6ha, but this refers to open field SRF, and we may need to explore this further.

From the Teagasc Farm Energy manual it seems that we can assume an annual average SRF production in the region of ~10 tonnes per hectare per annum of dry matter (DM), so that the annual output of the pilot farm system is ~100t/ha DM. The overall production cost of this is in the region of ~€50 per tonne DM. At a sale price of ~€120 per tonne this represents ~3.4 cents per kWh, the thermal output of a tonne being in the region of 3500 kWh. This is substantially less than natural gas or oil at current prices. The overall resultant farm income in the pilot situation is likely to be over ~€5000, which for 1/10 of the farm area is not unreasonable.

Note also that the expanded hedgerows, being harvested on a 4-year cycle partially from alternate sides, will always provide shelter to the enclosed fields, and this is said to increase their productivity, enabling them to be used for crops requiring shelter, and providing shelter for animals when under grass. This aspect needs to be explored quantitatively.

Harvesting and Drying

Harvesting in pilot mode would need to be done with a tractor, chain-saw and mobile chipper, pending the development of a special-purpose harvesting system, which could develop as a result of the need of the motorway verge linear forestry for cost-effective management. It is perhaps possible to conceive a system with the saw at the end of an extensible arm up to say 5m, having an associated conveyor-belt system feeding the mobile chipping system. Or perhaps a skilled harvester with a hand chain-saw feeding a basic mobile conveyor system would be better. These options need to be explored; my feeling however is that skilled human selection is probably more productive.

It is also appropriate to consider whether some of the pollard or coppice materials might harvest better as firewood-size log, with the chipping process reserved for the smallwood; in this case the tractor trailer would need to be in two compartments, engineered with handling in mind when delivering to the sheltered drying location (see below).

Local Neighbourhood Network and Social Organisation

This aspect requires active local promotion by an expert used to interacting with farmers in a forestry context. The scheduling and organisation of the pilot project would need to be done correctly, and a market obtained locally for the pilot production output, with records kept of all costs and revenues. This information could then be used in a local campaign to generalise the experience, if it turns out to be positive. If it does not, the reason why needs to be determined, and the ultimate feasibility evaluated.

When the time is ripe, the idea of a systematic managed co-operative approach to the local development of productive linear forestry needs to be promoted, and developed in terms of co-operative organisation with many participating farmers. The resulting co-operative could handle the harvesting and the logistics, as well as the drying and the marketing of the output. It would make sense to have the drying process near to where the wood supply is used, typically in an electricity production system, on the 'combined heat and power' (CHP) model, in the town of which the productive area is the hinterland. This enterprise would need to be planned as a co-operative enterprise by citizens, or as a local government enterprise.

The drying area would need to be planned as a feeder for the fuel store of the generator, and should be under shelter from rain, but open to drying by the wind. Typically wood-chips cut in the winter at ~50% moisture dry to ~25% moisture over the following summer. If heaped however they tend to heat up, with rapid fungal decay, and this would need to be prevented by providing forced air circulation within the heaps. A simple wind-driven circulation system with perforated pipes is perhaps worth a trial, as an alternative to a fuel-driven system; low-grade waste heat from the generator could also perhaps be used.

One way or another, the management of the interaction between the producing farms and the market(s) for their produce is a co-operative movement opportunity in a traditional mode where it has been successful in the past, mainly with milk. The opportunity to expand the co-operative service to all aspects of agricultural production (livestock, food products, horticulture etc), to increase the local added value, and to provide stronger bargaining power with the urban retail market generally, suggests itself as an obvious follow-up process, but initially the concentration should of course be on ensuring that value is added to the harvested logs and wood-chips and a reliable supply of biomass energy is made available to the local urban market.

A Possible Local Planning Model

In a fully developed situation, for example, a 10km square with ~100 farms could if it wished organise to supply 10,000 dry tonnes of wood chips annually to an energy conversion system in the local town. To get a feel for the order of magnitude of this energy source, a tonne of dry wood-chips is enough to supply a kW about half of the time throughout the year, so we are talking in terms perhaps of a CHP system of 10MW or more, but this aspect needs to be quantified, in load factor terms, in each specific situation, and adapted to the urban domestic and industrial thermal and electrical demand, and the relationship between the latter and the electrical grid.

For this to happen, some urban local government planning would be required, and the deficiencies of the present local government system would need to be remedied perhaps by a co-operative citizens' approach to developing some sort of bottom-up local government, beginning perhaps with a district heating system, and developing in the direction of electricity production subsequently.

The fuel input cost for the system would be of the order of €1M and the added value of the output products could be of the order of a factor of 2 for domestic hot water, and 5 for electricity. The price received for the latter would depend on the time of day, so the system would require intelligent management by skilled staff, and perhaps a storage aspect for the thermal output; all this would need to be allowed for in the planning.

The Grid, the National Market and the Food Market

If the grid were to be supplied by many (perhaps 100s) small-scale local wood-energy dependent CHP generating systems, these could perhaps be adapted to serve as a 'spinning reserve' capable of filling the supply gaps in a variable wind-energy supply system. A sophisticated localised despatching system would need to be devised to handle it, within the constraints imposed by the availability of pumped storage and capacities of the links to the UK and EU grid systems. The development of wind therefore needs to be phased and managed in a manner which takes into account the opportunities presented by the development of localised bio-energy systems as outlined above.

There is a further option relevant to the despatching problem, namely the availability of quick-start liquid-fuel systems, supplied by bio-diesel, rape-seed oil or whatever. This constitutes an additional market for a valuable annual crop in a rotation providing mainly food and fodder. Note that oil-producing crops also produce fodder as a by-product. The adaptation of agricultural production to the requirements of the food market within the energy constraints is another aspect of the overall sustainability development problem. It is likely that the fields sheltered by the linear forests will need to adapt to something along the lines of the practice which was developed in Britain during WW2, known as 'ley farming'. This approximates to what was 19thC best practice, with food crops, fodder crops and livestock in a localised managed rotation system, and the manure used in support of intensive horticulture. Such a system will increasingly have to depend on additional fertiliser from digested urban waste biomass, including sewage, and this problem will need to be addressed, given the long-term unsustainability of the supply of artificial fertilisers. The system is therefore likely to evolve in an organic direction.


I am submitting(1) the above not as a detailed quantified plan, but as a target achievable by a pro-active co-operative development agency, in response to the need for an organised socio-economic approach to the current requirement for renewable energy. I am suggesting that the resulting socio-economic organisation might usefully take on board also the maximising of local added value of food production, and improving the proportion of the value of the urban food and energy market that accrues to the primary producers.

It represents perhaps the taking up of where the co-operative movement would have gone, had it not been disrupted by the effects of the 1914-18 war and related events. I get the impression that prior to 1914 agricultural co-operation was about to develop in this direction. The trigger for a current revival of this process, I suggest, could be the opportunity presented for all farms to get into energy production via a process of co-operative management of linear forestry, which also shelters their fields and renders them more productive.

RHWJ 17/02/2010


1. The foregoing document was submitted in the context of the 'your country your call' project early in 2010. It did not achieve any recognition in that context, but it contains a basic concept which needs to be evaluated if it is to be promoted as a development path. I am open to critical feedback, and look forward to further exploration of feasibility. RJ August 2010. A WP version is available to download.

2. I feel now I should re-write the foregoing, as a proposal to the NRA for initiating a systematic experiment, such as to optimise the management practice for getting the most out of a strip forest, by an annual winter harvesting of chipped smallwood, firewood and eventually quality timber for a sub-set of optimal species showing signs of positive response to managed trimming etc.

Reflecting again recently (July 2011) on the look of the Belfast road, it seems to me that it should be feasible to define 1km strips and set them up for a systematic planned experiment, aimed at getting an optimal mix of coppice, pollard and full-grown timber, such as to support a minimum-cost DIY harvesting procedure suitable as a winter farm-fringe pastime.

Your harvester would need a tractor with a chipper mounted on the rere, with a trailer having space for chips and a separate space for firewood material in say 2m lengths. He would need a portable chain-saw, and a long-handled snips or other suitable smallwood cutting tool. In the planned experiment by the motorway, it would need to be a 2-person job, with someone keeping a record of the initial and final state of each defined experimental km length, in accordance with an experimental design.

The design would involve selective cutting of coppice, selection of pollard-worthy trees, and selecting of timber-worthy trees, according to a knowledge-based rule. Perhaps the coppice / pollard / timber ratios might be 70/20/10, or 60/30/10, or 70/25/5, or whatever, and in the coppice areas the cull might be biennial or triennial. Or would an annual cull and trim be best?

Perhaps the ratios would emerge as a result of knowledge-based decision-making by the operator, and they would take shape in the analysis of the records of successive chunks of strip forest, linked with some measures of soil condition, drainage etc, and records of the planting (if any?).

The smallwood would be hand-fed to the chipper, and the firewood logs chopped to 6-ft lengths, with some sort of time measurement on the operations, so as to get a feel for the cost of the necessary level of selective quality-oriented management. The aim should be to get a feel for how much time and effort a one-man operation would take, for a km of strip of standard width. in annual / biennial / triennial winter harvesting operations. In other words, is my concept, of a 100ha farm with say 10ha of expanded hedging developed into strip forest, manageable?

I had been hoping someone might help me develop the draft into a proposal to put to the NRA, or perhaps to a local authority interested in developing local enterprise. Might a presentation along these lines be suitable for your October event? It would need to be someone with hands-on experience, and I would be interested in helping with any subsequent analysis.

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Copyright Dr Roy Johnston 1999