25 October 2022

 

Dr David Cutress: IBERS, Aberystwyth University.

 

  • Agroforestry is receiving a lot of focus for its role in achieving net-zero globally
  • Agroforestry offers multiple integration options but it is important to consider what trees to grow where in combination with which livestock or crop                
  • Strategies like mixed-species stands and the incorporation of nitrogen-fixing trees appear to add extra benefits to overall system sustainability and resilience to risk

 

Agroforestry

To begin with, let’s ensure that we are all considering the same concept when discussing agroforestry as its definition can either be highly specific (as is the case in scientific literature) or it can be expansively broad (often in media and political communications). A recent review highlighted that the term agroforestry can be used in different ways which it referred to as ‘the good’ (true agroforestry), ‘the bad’ (agrobizforestry,) which focused on equity rather than environmental benefits and ‘the ugly’ (agrodeforestry), where the term agroforestry is used as a tool to legitimately allow deforestation, degradation and change of current primary or seminatural forests. At its core agroforestry involves the combination of woody vegetation (e.g. trees and hedges) at the same time as crops and/or animals within the same landscape. As such agroforestry falls within the category of mixed farming. Terminology within agroforestry includes;

Traditional agroforestry – Long-established woody crops alongside livestock or crop components to produce food.

Innovative agroforestry – Multifunctional systems combining food and non-food biomass (bioenergy for example) and balancing resources alongside environmental conservation.

Silvopastoral/Silvopasture – The integration of trees with livestock grazing.

Silvoarable – The integration of trees with arable crop growth.

Agrosilvopastoral – The integration of mixed farming crops and/or livestock with trees.

Riparian buffers – Specific use of vegetation (usually woody) in proximity to waterways.

Hedgerows – Barriers made of closely aligned trees or shrubs usually for physical separation or to mark ownership boundaries.

Shelterbelts – Barriers made of trees and shrubs to protect crops or livestock from environmental factors like high winds and to reduce erosion.

Many of the benefits of agroforestry have been discussed in a previous article but, in short, these include the impact on sustainability, biodiversity, climate change and soil health. For example, agroforestry is suggested to reduce soil erosion and help balance the available nitrogen and phosphorous in soils to the value of £4 - £15 per ha per year in the UK when comparing silvoarable to arable alone. If even half of the current cropable area could be converted to silvoarable that would lead to UK savings of around £24 - £91 million a year. Agroforestry is at the forefront of discussions about being a low-risk, low-investment, high-carbon sequestering land management practice and has several other ecosystem service benefits as noted in the diagram.

Taken from Lawson, Dupraz and Watte, (2019) The ecosystem services of agroforestry – NTFPs = Non-timber forest products, LER = Land equivalent ratio GHG = Greenhouse gas

 

This article aims to explore some of the options available across agroforestry systems and the research on how these perform. The soil association’s handbook on agroforestry offers a lot more advice for individuals considering agroforestry including this useful acronym to help with how to approach these systems - PAMASAL

  • Purpose: Where do you want to get to?
  • Advice: Where will you get advice and support?
  • Measures of success: How will you measure efficiency, effectiveness and impact?
  • Agroecology: How will you capitalise on agroecology and let nature do some of the work?
  • Starting points: Where do you want to start from? (e.g. pasture, arable or an orchard)
  • Adaptive management: As things develop, what tools will help you to adapt?
  • Layout: What varieties or species, spatial arrangement and sequencing will you use?

 

Tree considerations

Different tree species have different functions and considerations for best growth conditions within agroforestry with some being quick growing, some slow growing, some producing fodder or alternative revenue food source and others being more compatible for renewable energy production. A general rule of thumb is to plant at 100 stems per hectare as higher numbers could reduce benefits. Though different species in different environments may function best at higher or lower numbers.  The table below presents some broadleaf species researched in Europe and their general functionality.

 

Generally, conifer species' functional use in agroforestry is their sheltered environment for livestock outwintering, frost protection of any understorey forage and their long-term timber production (generally 40-60-year rotations depending on species). Specific information on which conifer and broadleaf species can work in which environments can also be found online from the Scottish Farm Advisory Service (FAS).

Mixed-species stands are reported to offer benefits to ecology as they are associated with a broader array of interacting species, they also act to fill and create different niches within an ecosystem distributing resources more efficiently. Mixed species systems also have benefits to resilience as they ensure events such as pests and pathogens do not eliminate all of a system in one fell swoop, though species choice is important here to avoid creating conditions that increase disease outbreak risk. Further to these benefits, there are suggested disadvantages of utilising monoculture stands such as reduced soil productivity and fertility, impacts on how water is utilised within systems and higher risks of loss to events such as storms and fires. Whilst the potentials of these systems are well noted the actual instructions and design guidance available for what species to combine successfully are somewhat limited and need further research.

Increasing interest has been seen in tree species with the ability to fix nitrogen toward improving soil nutrients and thus improving growth and biomass. In some studies, the production increase of these trees, mixed into stands of other species, has been as high as 27%. These include species such as alder, Scottish laburnum and acacias, with nitrogen fixation strategies suggested to work best on lower productive lands and in temperate conditions. What should be avoided across agroforestry as a whole is introducing non-native exotic species, as whilst on paper they can technically enhance biodiversity, there is much suggestion that they do so at the cost of shifting ecosystems from what works for the native environment.

 

Silvoarable considerations

Alley cropping arable fields with fruit trees can lead to increased farm income following initial (>7 years) negative cash-flows, with the carbon sequestration achieved being demonstrated to mitigate between 47-88% of these cash-flow costs incurred. This suggests that fruit trees can be beneficial due to their co-production benefits particularly if systems carefully consider which trees will yield well on the specific land in question and achieve good sales prices. Furthermore, alley cropping of trees is an easy way to ensure that machinery can still operate within these alleys and can reduce the complexity of the needs for labour and bespoke machinery. Other integrated forms of planting include the incorporation of lower bushes/shrubs like blackcurrants and raspberries around the edges of the field where they can benefit from the light and provide different shelter considerations.   

 

Some studies suggest that silvoarable systems benefit crops where the leaf of the crop is the “economic part” as in a shaded region of trees these plants will naturally allocate more biomass to leaves over roots to capture as much of the available light as possible.

Selecting arable and tree crops with complementary harvesting times can spread the labour and risk of difficult harvesting season windows for example later ripening apple varieties work well with cereals as the cereals can be harvested first. Some intercropped plants even have the potential to be used for pest management and additional production yields. For example, aromatic plants including basil have been shown to reduce aphid populations and also improve N release in soils available when intercropped with apple trees.  

 

Silvopastoral considerations

Whilst canopy coverage in silvopasture systems can lower the peak pasture growth rates in key production periods it has been demonstrated to make up for this by protecting growth during normally less favourable cold periods. Silvopastoral systems have also been shown to play interesting roles with regard to drought conditions, where tree species perform more favourably regarding moisture stress than equivalent forests and can reduce water loss from the soil systems and swards caused by evapotranspiration. These benefits to water stress in droughts are also bolstered in agroforestry systems as these are known to improve water penetration into soils and plant roots tend to gather nutrients and water lower down than grasses reducing direct competition. These features make year-round outdoor systems with lower stocking values more feasible. These would have added ecosystem benefits, to vitally important dung beetles for example, and have the potential to save costs and emissions associated with indoor housing and supplemental feed.

Figure 2 Taken from Jose, Walter and Kumar (2019)

Using pigs to forage woodlands has been a traditional practice of old as they can make use of multiple sources of nutrition including acorns (oak trees), and masts from beech trees all of which can reduce the feed requirements and boost animal welfare. Pigs also work well with fast-growing bioenergy crops (like miscanthus, willow and poplar) which are known to be efficient in taking up nitrogen, this is important as pigs, due to their excretory behaviour, are associated with producing high N hotspots where grazing. Though consideration and control of stocking density were shown to be of high importance for efficient systems.

Results from a section of the AGFORWARD project suggested that when utilising agropastoral systems site-specific grazing regimes could increase soil seedbanks and species richness of the pasture understorey compared to strategies that excluded grazing. Specifically, it could boost nitrogen-fixing legume fractions in swards. It was also noted that certain trees provide higher digestibility fodder options with ash leaves showing increased digestibility compared to grass. When considering leaves for fodder options it is important to consider management practices to either allow direct browsing (low hanging branches) or to give animals access to graze branches following coppicing via strategies like pollarding. Alternatively preserving materials from incorporated fodder trees is another option via drying, silage and pelleting, with different species having different processing considerations and edible regions. Such strategies can be important for sustaining livestock in periods of critical grass shortage such as seasonal droughts. But it is heavily noted that research on specific digestibility and preserving methodologies of tree products are lacking and would need further research and optimisation.   

Much study has been performed on the benefit of free-range systems and specifically wooded systems for poultry production. One of the benefits that were found by increasing the cover of woody areas in free range systems was it reduced the presence/interaction of poultry with ‘high risk’ avian influenza carrier bird species. This presents an interesting biological disease control consideration alongside the other welfare and natural fertilisation benefits noted with such systems. Though practically in such systems it is important to prune lower branches and allow sufficient light to encourage hen laying behaviours with enough shade to benefit naturalistic behaviours for positive animal welfare. Importantly, however, the selection of animals may change your consideration of tree planting as many animals will trample, graze or otherwise damage young trees. In several systems, this can involve the need to plant nurse species which improve the survival of the target tree species during initial growth. Despite these detriments, tree incorporation improves soil structures and thus can mitigate compaction issues that might otherwise occur from livestock use. Ammonia dust and other air pollutants are other big factors of interest in livestock production. Here again, tree incorporation has been shown to benefit as trees are effective scavengers of air pollutants and calculators exist to help farmers to model which trees work most effectively in different conditions and different distances from animal housing.

In general, incorporating some form of fodder production from trees within silvopastoral systems gives a diversified feed option on a farm that if performed optimally can reduce feed costs and can have increasing importance in seasons of difficult pasture growth or where imported feed costs increase.

 

So, what does it mean?

Agroforestry systems can provide many ecosystem service gains and ultimately over longer periods are frequently suggested to provide increased income over treeless equivalents. This means that farmers may need to be willing to play the long game and think carefully about what species of trees will grow best and provide the best sequestration vs productive economic harvest benefits. Mixed strategies show promise, for example, starting systems as silvoarable when younger trees block less light impacting the crop minimally. Following tree growth systems can convert to silvopastoral once crop yield reduction due to tree size and shade occur and loss in crop revenue can be at least partially mitigated by livestock production gains.

It has been suggested that the following strategies could flourish in the UK with the right legislation and support. Such options are also able to be combined into even more complex systems;

  1. Silvoarable with mixed grains (to mitigate pest and disease risks to mono-grains) along with nitrogen-fixing trees to reduce synthetic N requirements and other nutrition from tree species such as nuts (to capture non-meat-eating markets)
  2. Silvopastoral to improve animal welfare with trees producing fodder supplements to lower production costs
  3. Orchard intercropping (no spray) systems to produce food alongside nutraceuticals
  4. ‘Agrenforestry’ which would produce bio-renewable energy alongside food on land to prevent the conflict between these two resources – can also incorporate waste food into the renewable energy production alongside the woody biomass

Note – many studies suggest specific subsidies are needed to mitigate the economic losses associated with tree processing for energy in Agrenforestry systems.

There are, however, problematic considerations involved in transitioning to agroforestry. Firstly, the mindset for modern agriculture and forestry has very much been that of monocultures, as these are easier to predict growing trends rather than modelling complex mixed species systems, intercrops and systems combining trees and crops/livestock. Combining this with the fact that there is a lack of evidence surrounding the financial costs, risks and benefits associated with transitioning and the low levels of transition becomes apparent. Some studies have highlighted that agroforestry systems can reduce crop yields and increase weed impacts and many use these findings as a key negative focus in transitioning and management. This negativity was also raised in the recent UK ELM agroforestry assessment which suggested an underlying consideration from farmers that agroforestry is always more detrimental than beneficial to farm operations.

Long-term land tenure is another issue that can impact the uptake of agroforestry as often systems require time to develop. Considering more than 35% of UK farmland operates on a tenancy basis this could be a big factor to overcome and may push more farmers into the shorter turnaround systems. If farmers in such systems were to consider agroforestry at all they will be more likely to favour fast-growing biomass and timber options, which may not have as good prospects for long-term Csequestration.

 

Summary

In the UK it appears as though there is an understanding that agroforestry is an important future land management tool for land sharing purposes and environmental and biodiversity benefits amongst others. Whilst this understanding is clear it is also clear that newly proposed schemes such as the Sustainable Farming Scheme in Wales and ELM in England, will need to consider grants to incentivise correct integration of agroforestry and importantly provide resources to guide and train landowners and farmers in an area in which they may have little to no current experience. Whilst almost all agroforestry systems will benefit biodiversity and environmental factors, finding those which work well economically on a case-by-case basis will be key to the successful future of this strategy. Research suggests that this is possible but may require a re-examination of land system strategies and priorities.


If you would like a PDF version of the article, please contact 
heledd.george@menterabusnes.co.uk


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