Introduction

The "Green Revolution" was driven by public incentives and subsidies to achieve food self-sufficiency after World War II. It profoundly transformed the profile of farms, leading to the standardisation and compartmentalisation of both landscapes and know-how. In particular, this has led to the degradation or elimination of semi-natural habitats such as hedgerows and permanent grasslands, significantly contributing to a major biodiversity loss (Reidsma et al., 2006). Furthermore, farmers receive only a small proportion of the added value generated by their work, and many farms are weakened by high levels of debt and a strong dependence on fluctuating global agricultural commodity and input prices.

Organic agriculture has developed partly as a response to the negative impacts of this intensification and specialisation, in particular by underlining the importance of crop/livestock combinations. The "principles of organic agriculture" emphasise the notions of holistic health ("sustain and improve the health of soils, plants, animals, people and the planet, as one and indivisible") and ecology ("[base] oneself on living ecological cycles and systems, work with them, imitate them and help sustain them") (IFOAM, 2005).

Livestock farming systems have a large footprint on our planet: in 2006, they occupied 33% of the Earth's land surface, of which 30% of arable land was used to produce animal feed (Steinfeld et al., 2006). This sector accounts for 8% of global water consumption, and its effluents cause significant pollution of aquatic environments (Steinfeld et al., 2006; Schlink et al., 2010). According to the FAO, greenhouse gases emitted by livestock farming, mainly in the form of CH₄ and NO₂, account for 14.5% (in CO₂ equivalent) of anthropogenic emissions (Gerber et al., 2013).

In this context, agroforestry – the farming practice of combining trees with crops and/or livestock – is emerging as a possible response to current and future health, climate, economic and social crises. A modern reworking of ancestral practices, agroforestry is an attempt to maintain or even improve agricultural yields by benefiting from the ecosystem goods and services provided by the presence of trees. In an agroforestry system, the management of incoming flows (sunlight, water, nutrients, etc.) and outgoing flows (animal waste, crop residues, residual agricultural inputs, etc.) can be optimised compared to conventional systems with separate plots of cropland and woodland (Dupraz & Liagre, 2008). Habitats are created for wild micro and macro fauna, large quantities of atmospheric carbon are stored, and benefits can be expected in terms of crop health and/or animal welfare (Veldkamp et al., 2023). Agroforestry can therefore be seen as a form of agroecology capable of supporting the transition of farms to organic farming (Dupraz & Liagre, 2008), but also as a potential pathway for the future of organic farming (Rosati et al., 2021). Nevertheless, the presence of trees creates a number of constraints that can have an impact on working time, yield, and the farmer's perception of his work.

Agroforestry systems involving livestock (silvopastoral systems, Box 1) have been defined in various ways. These complex systems include not only animals (of different species and breeds) and trees (of different species and varieties), but also the soil, climate, plant cover, existing farming practices, as well as the farmers and the social, economic and political context in which they operate. The motivations for adopting silvopastoralism are linked to the synergies it can generate between animal feed, maintenance of the plant cover, animal welfare, etc. However, trade-offs between system components may also arise. These can be mitigated through careful system management.

Figure 1: Agroforestry methods in livestock farming (Silvopastoralism).

This article aims to document the diversity of silvopastoral systems and the potential synergies they produce, as well as their limitations and the new issues they raise, by considering different perspectives (tree, animal, farmer) and exploring the spectrum of tree-animal integration. This review is based on English-language scientific literature, grey literature (mainly in French), and preliminary results from ongoing projects. In the first part, it reviews the different challenges facing livestock farming, asking how silvopastoralism can contribute to coping with global change (mass extinction of species, climate crisis, etc.). Then, it tackles the management and performance of agroforestry livestock farming, before addressing the economic, social and regulatory issues about this practice.

The article focuses on silvopastoralism in temperate regions, with occasional references to the more abundant literature in tropical conditions. Some examples draw on data from silvo-arable systems, which are also important in livestock systems (production of concentrate), and particular attention is given to the association of livestock with specialised perennial crops (orchards, vineyards).

1. Agroforestry as a response to the environmental and climatic challenges facing livestock farming

1.1 Maintaining the productive capacity of farmlands

Although often perceived as a hindrance to agricultural productivity, trees are now recognised for their contribution to fodder self-sufficiency and the maintenance of soil quality and fertility.

a. More diversified forage production

Good fodder management is crucial in organic farming, since the standards require a large proportion of feed to be produced on the farm, for both ruminants and monogastric animals. The fodder purchased must be certified organic and come as much as possible from the farm area, which can involve considerable expenditure. Aiming for forage autonomy is therefore essential, not only for economic reasons, but also to cope with drought and summer heatwaves.

b. Changes in forage productivity and quality

The risk of reduced fodder yields can be a limiting factor to planting trees in agricultural environments. However, the effect of trees on crops varies according to planting density, soil and climate conditions and the species grown, making it difficult to draw any general conclusions about their impact on productivity (Torralba, 2016). Regarding grassland, the main effect of trees is the shade they produce, while competition for water and nutrients is minimal or even absent (DeBruyne et al., 2011).

In France, a canopy that is at least 60% open may not significantly affect grassland productivity, except for the grass at the foot of the tree (Béral & Moreau, 2020), but decreases have been observed in other temperate grasslands. Trees delay grass growth, allowing later grazing or mowing, thanks to the creation of microclimates (Karki & Goodman, 2015). Although the shade created by trees can reduce legume production (Béral & Moreau, 2020), grasses can be of better forage quality under the tree canopy (Kallenbach et al., 2006), which could lead to forages of equivalent quality between agroforestry and open grassland. Models for calculating the optimal tree density to maintain adequate productivity of the herbaceous resource have been produced (García de Jalón et al., 2018b), but they still need to be extensively tested against field data.

c. Better use of the herbaceous layer

The presence of trees also has an impact on the behaviour of animals, which are likely to make a better exploitation of the herbaceous layer.

An appropriate design of the grazing area is essential to encourage poultry to leave their rearing building (Béral et al., 2014). Chickens grazing on tree-planted areas consume more grass (Dal Bosco et al., 2014), promoting better weight gain (Germain, 2014). However, access to a run can also result in increased soil ingestion (Jurjanz et al., 2015). Therefore, maintaining the quality of the herbaceous cover is crucial to limit this effect and guarantee an adequate feed intake. This requires identifying palatable herbaceous species with interesting protein contents (Germain, 2014), while considering the light competition with trees. Trees protect pigs from the sun, making it easier for them to move around the grassland (Jakobsen, 2018). In addition, the consumption of on-the-hoof forage by pigs reduces the proportion of concentrate feed distributed and enhances the value of the carcass (Maupertuis & Desaint, 2023).

d. Trees as a source of fodder

Many tree species have been used since the Neolithic as livestock fodder (Rasmussen, 1989). Three factors need to be considered when evaluating their benefits for ruminants: yield, fodder quality and palatability.

Fodder yields remain poorly documented and vary depending on the species, age and management methods (pollard or high-stem¹, frequency of pollard pruning, etc.). Preliminary studies show that average leaf production per tree can exceed 800 g of dry matter per year for low pollards (Mesbahi & Novak, 2022). Measuring the yield from tall pollards, high-stem trees, and hedges is more difficult because these trees are larger, only partially accessible to animals and often densely interwoven. There is therefore very little data available to date, despite their greater presence in Europe.

Woody species show considerable variability in their nutritive values: they are often of higher quality than grasses in summer, but remain inferior to chicory (Novak et al., 2020a) (Table 1). Partially replacing alfalfa with white mulberry in goat rations increases milk fat production by 9 g per kg of milk (Boyer, 2022). Similarly to herbaceous species, the forage quality of woody species declines as the season progresses (Mesbahi et al., 2022a). Woody species can contain numerous secondary compounds with potential medicinal properties and/or that may reduce greenhouse gas emissions, but they may also have an anti-nutritional effect (see § 2.1.b). In addition to ruminants, preliminary studies have shown that finishing phase pigs consume available tree fodder (willow), but it does not fully offset the growth loss caused by a 30% reduction in the feed ration (Kongsted, 2022).

Cultivated (e.g. apples, cherries) and wild fruits (e.g. acorns, chestnuts) can also be consumed by livestock (Solagro, 2016). In the Lapoesie project, rabbits grazing in an orchard consumed 23% less feed than those raised indoors, thanks to their use of plant cover and fallen fruits (Savietto, 2023). In extensive Mediterranean pig production systems (dehesa, montado), the consumption of fruit during finishing improves the intramuscular fat and fatty acid profile of the meat, leading to a better evaluation by consumers (Lebret, 2008).

Assessing leaf palatability of woody species is essential, as a productive species with good forage quality is of no use if livestock does not consume it. Ash is traditionally used as fodder, yet various studies have shown that it is grazed very little by cattle if other resources are available (Vandermeulen et al., 2018; Mesbahi et al., 2022b), but that sheep readily consume it at the trough when it is presented alone (Bernard et al., 2020). Among the species regularly observed in Western Europe, hawthorn, dogwood, elm, hazel, black locust, and certain willows are particularly popular with ruminants (Vandermeulen et al., 2018) (Figure 2). The more Mediterranean white mulberry is also highly palatable to cattle, sheep and goats. These results on palatability need to be completed, however, as few studies have been carried out to date, and the determinants of food preferences have yet to be identified according to species, breeds, habits or even the balance of the animals' feed ration.

e. Healthy agroforestry soils

Agricultural soils generally benefit from the introduction of trees in fields: in arable farming, agroforestry improves soil structure, increases aggregate stability and water infiltration capacity, leading to greater resistance to erosion (Fahad et al., 2022). In traditional Mediterranean silvopastoral systems, a reduction in water erosion has also been observed, although this depends on grazing practices (Shakesby et al., 2002). Trampling by animals, but also the radial growth of tree roots, can compact the soil, although this effect is limited and reversible (Sharrow, 2007).

Part of the tree biomass ultimately becomes available to other components of the agroecosystem via the decomposition of fine roots and the annual fall of deciduous aerial organs. Trees facilitate the redistribution of water and minerals from the deep soil layers, normally inaccessible to the roots of herbaceous plants. In addition, certain species of trees and shrubs fix atmospheric nitrogen, enriching the soil through a symbiotic relationship with specific bacteria, which could promote the growth of neighbouring plants. These include species in the legume family (Fabaceae) such as honey locust (Gleditsia triacanthos), but also alders (Alnus spp) and sea buckthorn (Hippophae rhamnoides).

Agroforestry positively impacts soil biodiversity, more so in silvoarable systems (Marsden et al., 2020; Beule et al., 2022) than in silvopastoral systems (Cubillos et al., 2016; Poudel et al., 2022).

The presence of animals in treed areas (woodlands, perennial crops) results in the deposition of mineral-rich droppings (urine, faeces) beneficial to tree growth. For example, the biological activity of soils and the availability of nitrogen and phosphorus are improved in vineyards and olive groves grazed by sheep (Ferreira et al., 2013; Brewer et al., 2022). However, the effects can be reversed beyond a certain threshold: for example, while poultry can enrich wooded soils with nitrogen (Hilimire et al., 2013), prolonged presence at high stocking densities can result in nutrient overload, which is detrimental to fruit trees (Timmermans & Bestman, 2016).

1.2. Mitigating the negative effects of agriculture on the environment

a. Ecosystem services and biodiversity

Beyond improving soil fertility, the balanced integration of animals and trees can also help mitigate the environmental and climatic impacts of agricultural practices.

Deforestation to create pastureland results in the loss of the water regulation services provided by trees (Jose, 2009; Zhu et al., 2020; Smith et al., 2022). In silvopastoralism, the trees act as a “safety net” that limits the flow of water and nutrients not absorbed or recycled by the herbaceous layer, which could potentially lead to environmental pollution or reduced system efficiency (Udawatta et al., 2011; Zhu et al., 2020). This aspect is particularly important in pig farming, which generates high concentrations of nitrogen in localised hotspots ('latrines'), efficiently captured by tree roots (Jakobsen et al., 2019).

Agroforestry creates spatial heterogeneity, leading to diverse potential habitats, which generally has a positive effect on biodiversity. This effect is clear in silvoarable systems (Beillouin et al., 2021), but is more nuanced in silvopastoral systems: while some studies suggest that wooded pastures attract both forest species and those adapted to open habitats (Mcadam et al., 2007), others conclude that silvopastoralism does not support greater biodiversity than pastoral or forest environments (Mupepele et al., 2021). This result could be due to the observed decline in grassland-specific and threatened plant species when trees develop on permanent grasslands (Boch et al., 2019). The climatic zone also seems to be important: in Europe, the ecosystem services provided by silvopastoral systems are generally positive in Mediterranean environments, but neutral in temperate, continental and alpine environments (Torralba, 2016). The design of rangelands (choice of species, type of landscaping) and their connectivity to a diverse landscape mosaic are therefore crucial to maximising the effectiveness of silvopastoralism in terms of biodiversity (Béral et al., 2014).

Mechanical mowing in vineyards, orchards, and woodland is destructive towards poorly mobile arthropod species, which can therefore benefit from ground cover management by animal grazing. In addition, the presence of animal dung can attract coprophagous beetles specialised in decomposition, initially absent from cultivated areas, which in turn provide numerous ecosystem services (Nichols et al., 2008).

b. Carbon sequestration

The transition from conventional agricultural practices to agroforestry generally increases the carbon stock of the plot (De Stefano & Jacobson, 2018; Mayer et al., 2022), resulting from tree biomass production both above and below ground, and modification of carbon and nitrogen cycles due to tree-induced microclimates (Marsden et al., 2020). This effect is mostly observed in surface horizons, even though trees are also capable of storing carbon in deeper soil layers. Deciduous trees are associated with greater carbon sequestration, probably due to their deeper roots and the quantity and ease of degradation of their litter (Mayer et al., 2022).

However, the disturbance generated by the establishment of a silvopastoral system from a forest or grassland environment seems to potentially cause temporary or long-term soil carbon losses (De Stefano & Jacobson, 2018; Contosta et al., 2022; Mayer et al., 2022). Indeed, temperate permanent grasslands are nearing the saturation point of their organic carbon stock, and the establishment of trees can disrupt established herbaceous communities (Mayer et al., 2022). Nevertheless, the long-term carbon storage potential of silvopastoralism appears to be higher than that of open pastures (Contosta et al., 2022).

Conversely, converting temperate temporary crop plots to agroforestry can greatly increase carbon stocks, as these stocks are often initially low (De Stefano & Jacobson, 2018; Mayer et al., 2022).

In vineyards or wild cherry tree plantations, it has been shown that grazing leads to increased carbon storage, whatever depth is observed, provided that it is carried out correctly (Ferreiro-Domínguez et al., 2016; Brewer et al., 2022). Animal manure enriches the soil in carbon directly and indirectly via the activation of the microbial community (Brewer & Gaudin, 2020). However, it could also increase emissions of the powerful greenhouse gases N₂O and CH₄ (Lazcano et al., 2022).

In summary, agroforestry could contribute to limiting climate change, especially in alley-cropping systems. Indeed, the carbon stock has a greater potential for improvement in rotational plots than in permanent grasslands. Silvopastoral systems are also particularly effective at storing carbon in tropical environments, while in temperate climates their value lies mainly in decreasing water and wind erosion and improving microclimates (Mayer et al., 2022).

c. Methane emissions

In addition to their role in carbon sequestration, fodder trees can help reduce greenhouse gas emissions from ruminants. For example, tree browsing or the consumption of leaf plugs can reduce methane emissions, often to a small extent (Ramírez-Restrepo et al., 2010; Terranova et al., 2021). In addition, a reduction in urinary nitrogen emissions is sometimes observed, leading to a reduction in N₂O emissions, which is also a greenhouse gas (Terranova et al., 2021).

This reduction is often linked to the condensed tannin content of forages (Terranova et al., 2021), but other secondary compounds, such as saponins and phenols, could also play a role, although this is still poorly understood. The temperate-climate woody species with high tannin contents (> 50 g/kg DM) are, in ascending order: beech, kiwi, grapevine, hazelnut, willow and black locust (Novak et al., 2020a) – the last two species even having higher contents than sainfoin. However, these levels vary depending on the individual and the soil and climatic conditions. Hazelnut, willow and black locust are of particular interest, as numerous observations have shown that they are appreciated by ruminants.

2. Coexistence of trees and animals: potential synergies and technical challenges of agroforestry

2.1. Animal welfare and health

a. Microclimate and shade

Agroforestry creates microclimatic heterogeneities at plot level, allowing animals to choose the environment best suited for their well-being. Trees provide protection from the sun, buffer temperature variations, and limit wind speed (Karki & Goodman, 2015): temperatures can thus be reduced by 3 to 6°C during summer, compared with a grassland without trees (Béral et al., 2018). Under cold, rainy or windy conditions, trees also provide climatic protection: sheep actively seek out their cover, and cattle benefit from wintering in wooded areas if the soils allow it. A regularly wooded plot provides easy-to-reach shelter, which could limit the energy expenditure of livestock (Béral et al., 2018).

Overall, the protection and diversity of microclimates offered by trees improve the well-being of ruminants, pigs, poultry and rabbits by providing shelter from adverse conditions and allowing them to express their natural behaviours (Dal Bosco et al., 2014; Jakobsen, 2018; Savietto, 2023) (Figure 3).

b. Impact of the presence of trees on animal health

The introduction of trees into grazing areas or animal feed could be an opportunity to reduce the medicinal inputs used on farms, particularly by helping to control internal parasitism. However, the therapeutic or harmful effects of tree elements on farm animals, under operating conditions, are still uncertain and require more in-depth research to provide farmers with better support.

Wooded rangelands encourage animals to explore a larger environment (Germain, 2014), which may help to limit their concentration and hence their parasitic reinfestation. In addition, certain chemical compounds present in tree leaves could help limit intestinal parasite populations in grasslands before they are ingested by livestock. On the other hand, the cooler, wetter microclimate generated by the trees (and by irrigation in the case of orchards) may benefit these parasites; data is still lacking on this subject.

Animals are likely to spontaneously consume certain plants as a form of self-medication, although this behaviour is highly dependent on their breed and history, and is still debated by the scientific community (Villalba et al., 2014). For example, heavily parasitised goat kids start to eat Pistachio lentisca, which reduces infestation (Landau et al., 2010). Furthermore, lambs infested with parasites increase their consumption of tannin-rich forage (Lisonbee et al., 2009).

Indeed, the leaves, fruit, green wood and bark of trees can contain high levels of tannins (Novak et al., 2020a), which are recognised for their anti-parasitic effect. Ingestion of tannin-rich fresh willow reduces infestation in lambs (Musonda et al., 2009; Mupeyo et al., 2011). Consumption by pigs of fruits rich in tannins and sesquiterpene lactones (chestnuts, walnuts, hazelnuts, acorns, etc.) improves their tolerance to nematodes and pathogenic bacteria (Hassan et al., 2020).

However, the diversity of tannins, the molecules with which they associate and the environment in which the animals live make it impossible at this stage to draw any conclusion about the actual properties of the different tannins on health under rearing conditions. Some studies, for example, show the importance of combining tannins with low-protein rations, since tannins associated with proteins lose their effectiveness (Butter et al., 2000) and inhibit protein assimilation. Conversely, a protein-rich ration could enable the animal to expel more parasites, reduce weight loss and limit reinfestation (Butter et al., 2000).

Woody fodder can also play a role in the mineral nutrition of livestock. For example, leaves from fig, mulberry and lime trees contain 15 times more calcium than maize (Novak et al., 2020a). To optimise the utilisation of these minerals, it is therefore possible to plant "medicinal hedges" at the edges of pastured areas or along the paths used by herds.

However, some trees may contain toxic molecules. The risks are low for ruminants, but seem significant for monogastric animals. These risks are still very poorly understood, as they depend on the ingested quantities, proportion in the ration, phenological stage of the leaves or fruits consumed, animal species, herd's habits, interactions between molecules and possible 'cocktail' effects, etc.

c. Predation

Small farm animals (poultry, rabbits) are particularly prone to predation in the open, either from the air (birds of prey) or from the ground (foxes, martens, etc.) (Stahl et al., 2002).

Hens can protect themselves from ground predation by perching in trees, although this behaviour depends on the breed and individual, as well as on how they are reared (amputation of wing feathers). The protection provided by trees against aerial predation is also of interest, even though some raptor species, such as the goshawk, are capable of hunting in the undergrowth (Bestman & Bikker-Ouwejan, 2020).

The presence of trees provides a feeling of security for poultry and rabbits, but is not sufficient on its own to guarantee protection against predation and animal theft, which remain major challenges for agroforestry systems (García de Jalón et al., 2018a). Other protection systems, such as closed shelters for the night, electrified fencing and scaring systems, are therefore of essential importance (Knierim, 2006).

The layout of the tree cover could lead to changes in vigilance behaviour in relation to the risk of attack. Cattle would not lie down near a hedge, where wolves could hide, whereas they would allow themselves to rest in an isolated grove with good visibility (Kluever et al., 2008) - a behaviour that sheep do not seem to adopt (Monier S., personal communication).

2.2. Damage to trees

The association of animals with trees has a high potential for reciprocal benefits, but also comes with risks regarding the sustainability of the plantations. These depend on the animal species, stocking rate, duration of the animal presence, type of herd management, etc. The available literature on this topic mainly regards damage caused by wild ungulates or rodents, and cannot always be transposed to situations involving livestock.

a. Browsing

The first type of damage that animals can cause to trees concerns browsing, i.e., the consumption of leaves and twigs. This action is not necessarily harmful, and may even be desirable when grazing fodder trees, in moorland, scrubland or woodland (pruning of low branches), provided that the terminal bud is inaccessible (Gill, 1992b).

In intensive fruit-growing areas, however, this leads to a loss of yield in the area accessible to animals, which is also the most easily accessible for harvesting. In winter, the damage caused by sheep grazing is generally acceptable (a few buds consumed), but after bud break, vegetation can be consumed up to a height of 1.60 m (SSBA, 2017; Conrad et al., 2022), which is often considered prohibitive by farmers (AREFE, 2018). Some sheep breeds, such as Shropshire (Danish lineage) and Southdown, appear to be unable to stand on their hind legs, which could reduce the height at which damage occurs (Conrad et al., 2022). Trees can be protected from browsing by electric wire or barbed wire, while the application of repellents appears to be effective in the short term against browsing by sheep (Guittonneau et al., 2023a) but not so much against browsing by cattle (Novak et al., 2020b).

Obviously, larger animals (cattle, horses) are likely to cause browsing damage up to greater heights, not to mention the risk of branches being torn due to rubbing. In such situations, only relatively old woodland and high-stem orchards will be suitable.

In viticulture, although the same type of problem arises, it is interesting to note that some farmers (mainly in New Zealand and Australia) use sheep to carry out targeted leaf removal in the cluster area, with no damage to the grapes if the timing is controlled (Emms, 2010). Shoot thinning (removal of non-fruiting shoots) by sheep may also be possible (Conrad et al., 2022).

b. Debarking

Many animals are likely to consume or damage the bark of trees in their grazing paddocks, which can be prohibitive if the trees represent significant added value (orchards, valuable timber).

Sheep can debark apple trees massively and suddenly while grazing in orchards without previous incident (Figure 4). However, over several years of grazing, they appear to cause less cumulative damage than cattle or horses (López-Sánchez et al., 2020). Rabbits, on the other hand, are generalists, capable of debarking young trees on a massive scale, particularly in winter, but with a preference for fruit trees (Gill, 1992a). Adequate tree protection using sleeves is an effective way of mitigating this risk (Savietto, 2023). Preliminary observations by FiBL France indicate that fattening pigs can cause significant damage by eating bark and roots, with a greater predilection for certain species (apple, apricot, cherry, plum, whitebeam), while others seem to be spared (maple, narrow-leaf ash, wayfaring tree, common spindle). Even among trees of the same species, the genotype of certain individuals has an impact on the probability of debarking (Guerreiro et al., 2015).

For example, it has been observed that deer prefer to consume the bark of beech trees with the highest sugar content (Kurek et al., 2019). Ruminants seem to increase their propensity to debark trees when their diet is deficient in fibre, minerals or protein (SSBA, 2017; Nicodemo & Porfírio-da-Silva, 2019). Furthermore, Keenan (1986) observed that the debarking of eucalyptus by horses seemed to be linked to the presence of irrigation on their pasture. Farmers' testimonies also point in the same direction, linking abundant rainfall and low fibre content in herbaceous forage to sheep debarking behaviour. However, this factor was seen to be insufficient to provoke debarking behaviour among small groups of sheep grazing in apple orchards (Guittonneau et al., 2023b).

Many other factors seem to have an influence on the triggering of this behaviour: tree and stocking density, herd management, social learning and collective dynamics within the herd, self-medication (see § 2.1.b), stress, boredom, etc. (Figure 5). The debarking behaviour should therefore probably be considered as the result of a set of concordant factors – the relative importance of which remains to be understood. These uncertainties make grazing in agroforestry potentially insecure for herd and plot managers.

However, cases of mass mortality in plantations following the introduction of livestock are still relatively rare, especially when compared with the damage caused by wild animals such as voles, rabbits and deer. Herd monitoring and management are crucial factors in limiting debarking to light or moderate damage, but these can nevertheless lead to a loss of tree performance if frequently repeated (López-Sánchez et al., 2020). Studies are needed to document more precisely the impact of occasional debarking on tree physiology and productivity.

2.3 Specific features of grazing vines and orchards

a. Risks associated with plant protection products used in organic farming

When the tree component of the silvopastoral system is a high-added-value crop (e.g. vineyards, orchards), plant protection products are generally applied to the foliage. A wide variety of substances are used, and their effects on the health of humans, and a fortiori animals, are not always well known. Animals are thus potentially highly exposed to toxicity risks, particularly when they consume understory vegetation and/or soil components, or even tree leaves. Studies that assess pesticide toxicity are generally carried out on model animals (rats, dogs, etc.) and wild animals (fish, insect pollinators, etc.), but there is very little data on farm animals, and toxicity thresholds vary significantly from one species to another.

Organic farming makes extensive use of copper-based fungicides/bactericides, especially in viticulture and arboriculture (Andrivon et al., 2019; Lamichhane et al., 2018). Copper is well tolerated by many animals, even promoting growth in pigs, poultry and rabbits. However, even a relatively small amount of copper ingested over several months can be lethal to cattle and sheep (National Research Council, 2005; Suttle, 2010). In sheep, copper is mainly accumulated in the liver, from where it can be released suddenly into the bloodstream following a stressful event (change of diet or plot, parturition, etc.), causing the animal to die within a few days.

Winter grazing of vines and orchards by sheep can nevertheless be carried out without risk of intoxication if it occurs sufficiently late after the last copper spraying. Copper is then diluted by plant growth and washed away by rain, reducing its concentration to non-toxic levels within a few weeks (Trouillard et al., 2021; Dufils et al., 2022). The situations requiring particular monitoring are therefore those where grazing takes place quickly after the application of copper-based products: spring/summer for apple or walnut trees (Trouillard et al., 2023), winter/spring for peach trees, winter in apple orchards that have undergone early defoliation with copper chelates - or when the consumption of vine leaves by sheep is deliberate (Emms, 2010). If necessary, the risk of intoxication can be estimated based on copper, molybdenum and sulphur levels (antagonists of copper absorption) in the plant cover (Trouillard et al., 2021).

Phytopharmaceutical applications could occasionally produce beneficial 'collateral effects' in veterinary medicine: for example, azadirachtin, used to treat infestations of various insects in organic orchards, might have an anti-parasitic effect on sheep gastrointestinal strongyles (Iqbal et al., 2010) and on swine mange (Pasipanodya et al., 2021). It remains to be determined whether the doses ingested by the animals offer them any real health benefits.

The issue of pesticide residues in animal products intended for human consumption is complex, due to the diversity of substances used (Dasenaki et al., 2023). Copper, on the other hand, accumulates very little in animal muscles, and humans are generally not affected by it at the encountered doses (Anses, 2012).

b. Orchard prophylaxis linked to grazing

While the main goal of integrating animals into arboriculture is weed management, it can also be a means of controlling pests and diseases (Paut et al., 2021), particularly in the case of poultry. However, the integration of animals should be used as a preventive measure, not as a curative solution (Laget et al., 2015). Animals can provide direct prophylaxis by predating pests, and indirect prophylaxis by making the environment unfavourable to their presence and development.

Direct prophylaxis mainly involves the ingestion of pests by animals: hens can predate on certain fruit crop pests such as the Japanese beetle (Popillia japonica) and the tarnished plant bug (Lygus lineolaris) (Clark & Gage, 1996). Animals can also consume pests or pathogens in or on fruits that have fallen to the ground (Lavigne et al., 2012). Following the introduction of pigs into apple or pear orchards, almost all of the fallen fruits were consumed, thereby helping to control apple maggot (Rhagoletis pomonella) or reduce the inoculum of codling moth (Cydia pomonella) and oriental fruit moth (Grapholita molesta) (Nunn et al., 2007; Buehrer & Grieshop, 2014).

In terms of indirect control, the presence of chickens in an orchard reduces the populations of aphid mutualist ants, which could limit the impact of aphids (Hilaire & Mathieu, 2000). The consumption of the herbaceous layer by herbivores exposes rodents (Wilson & Hardestry, 2006) and insects (Witt et al., 1995; Clark & Gage, 1996) to their natural predators and to an unfavourable climate. Sheep trampling also appears to destroy the galleries and mounds of voles that consume fruit tree roots (Pype & Venineau-Delvalle, 2016), and could reduce the inoculum of apple scab (Venturia inaequalis) by degrading the litter, although this remains difficult to confirm (Dufils, 2017).

To characterise the prophylactic potential of the animal species to be introduced, it is essential to consider its feeding preferences, foraging behaviour (scratching the ground, browsing, grazing, etc.) and morphology. A balance must be found between the desired prophylactic effect and the risk of damaging the orchard, such as animals passing under low branches or causing soil compaction. Adaptations of the silvopastoralism management may be necessary (relocation of shelters, rotating pens) to synchronise the animal presence with the vulnerability stage of the targeted pest, in order to optimise prophylactic effectiveness.

3. Agroforestry as a way of facing the economic and organisational challenges of livestock farming.

3.1 Income diversification, but additional costs

In addition to providing ecosystem and agronomic services, the integration of a new production system on a farm offers new opportunities for income and savings. Tree cultivation can provide timber, wood fuel, fruit and other commodities (Figure 6), and livestock farming generates marketable animal products: eggs, meat and other co-products such as wool (Moreno et al., 2018). Animal waste provides nutrients for crops (see § 1.1. e), which can make a significant contribution in a context of high volatility in the price of fertilisers. In addition, animals in vineyards and orchards are often used for branding purposes, potentially enhancing the economic value of products (Mohamed, 2015).

The diversification generated by the association of trees and animals becomes a source of economic resilience, since separate production types are impacted in different ways by climatic (late frost, drought) or economic hazards (variation in buying and selling prices) (Cubbage et al., 2012). This effect is reinforced by the timing differences between productions (Smith et al., 2022). Short-term valuation of livestock products can provide income while awaiting longer-term valuation of fruit or timber trees (Dupraz & Liagre, 2008; Smith et al., 2022). Similarly, the seasonal income from the sale of fruit can, for example, complement the more regular income generated by egg production.

Complementarities between productions can express themselves in time but also in space: savings can be made thanks to the coexistence and synergies between simultaneous productions in a limited space, particularly in terms of cycle completion (Rocchi et al., 2019). A third area of complementarity lies in the methods of economic valorisation in long or short distribution channels and/or self-consumption, etc. – potentially contributing to stabilising farm economics by enhancing their potential for subsistence production (Moreno et al., 2018; Guittonneau & Pellissier, 2023).

However, setting up an additional production system is also costly (Paut et al., 2021). Investments are needed to develop a livestock farm: fencing, buildings, treatment facilities, and possibly processing and packaging facilities for animal products. The tasks specific to livestock farming (monitoring, care, watering, feeding, etc.) and their cost are added to the expenses already incurred for vineyards or orchards, which can be difficult to manage for some farmers (Moreno et al., 2018; Guittonneau & Pellissier, 2023).

Introducing trees in a livestock farm also involves some investment and regular costs to ensure their proper development: plant material, protective devices, fertilisation, irrigation, mulching, etc. (Béral et al., 2014). The costs associated with harvesting operations (felling, cutting, transport) must also be factored into the planned economic model (Solagro, 2016).

In all cases, new knowledge and skills need to be acquired, and mistakes made through inexperience can affect the success of the farm and/or the economic value of the trees. A silvopastoral system, therefore, requires careful management, which may be resource-consuming, but has the potential to generate additional income if managed properly (Jose et al., 2017; Pent, 2020).

3.2 Organisation and workload, regulations

Developing a silvopastoral system from a livestock farm does not fundamentally alter how the farm is organised. The presence of trees provides a certain amount of flexibility: mobilisation of tree fodder resources in the event of a shortage, and/or rapid cash flow from harvesting mature trees (Dupraz & Liagre, 2008). On the other hand, mechanisation (hay production, shredding of feed refusals) can become more tedious. Young trees need regular care (irrigation, weeding, etc.), which can be burdensome. From a regulatory point of view, the planting of vines can require administrative procedures for monitoring, and trees of the Prunus genus may be subject to control visits to check for the absence of plum pox virus. Obviously, the trees must be managed organically in order to enable the organic certification of the animals that graze therein.

When a livestock unit is developed on a fruit, wine or forestry farm, some aspects of work organisation can be facilitated: the timing of the introduction of animals into the plots is flexible, which is important for controlling bio-aggressors (see § 2.3.b), carry out a cultivation operation at a lower cost (e.g. de-budding and leaf removal by sheep, see § 2.2.a), or limit the height of the grass in anticipation of a spring frost. Animal waste provides manure and compost, which improves organic matter self-sufficiency (see § 1.1.e).

Farmers who practise this combination nevertheless mention additional organisational constraints: increased workload and complexity, and administrative management (García de Jalón et al., 2018a). Peak workloads for fruit crops are difficult to reconcile with the time constraints inherent in livestock farming, and the presence of animals can be incompatible with work on perennial crops (phytosanitary treatments, presence of employees, farm machinery, etc.), leading to livestock movements and/or the use of fallback plots (Moraine et al., 2012). These may also be justified in terms of forage resource management (Dufils, 2017) or preservation of the herbaceous layer from soil-disturbing animals such as hens (Bosshardt et al., 2022) or pigs.

The establishment of local partnerships between livestock farmers and tree growers (see § 3.3) makes it possible to retain some of the services provided by the animals in the plots (Moraine et al., 2012), without requiring a radical reorganisation of the way the farm operates. The success of such an association depends on the willingness of the parties involved to work together and to make the necessary mutual adjustments, especially in terms of timing.

Silvopastoralism has several specific regulatory aspects, which are often not addressed in current legislation: common agricultural policy (CAP) declaration in the event of mixed land use, required waiting period after the application of a plant protection products, animal exclusion time before harvesting, biosafety rules requiring the creation of a specific fence in a grazed vineyard or orchard, etc. (Riffard & Liagre, 2023). Regulatory loopholes can sometimes give farmers flexibility, but they can also represent bottlenecks or areas of concern if a problem arises. The French example shows that a text such as the “Guidelines for good hygiene practice [...] in fresh fruit productions”, mentioning that “as far as possible, [animals] should be kept away from crops”, may potentially be legally enforceable since it has been published in the Journal officiel (official journal) (CTIFL, 2012).

According to the EU regulations, organic farming allows non-organic animals to graze on labelled land for up to four months a year. The reciprocal (grazing of organically labelled animals on non-organic plots) is not possible, even if the grazed plot has trees. Forest and agroforestry trees are exempt from organic certification as long as they “do not produce marketable agricultural products” (INAO, 2023), which raises questions if their aim is to be primarily used as fodder.

3.3. Integration into the local environment and heritage

At the beginning of the 20^th century, farming was still based on a close, synergistic relationship between plants and animals (Mazoyer & Roudart, 2017), but the movement towards agricultural “modernisation” (mechanisation and “chemicalisation”) has led to a dissociation between these two worlds. This division between animal and plant productions now structures both space, which is divided into specialised production 'basins', and sectors.

The territorial scale seems both relevant and promising for reconnecting these two worlds (Moraine et al., 2016; Napoleone et al., 2022). Several recent or ongoing research-action projects, in France and elsewhere, bear witness to this renewed interest in practices that are often traditional but threatened with decline: for example, the complementarity between olive trees, vines and sheep in Mediterranean areas (Mohamed, 2015) or grazing in walnut groves bordering mountainous summer pasture areas.

In these cases, a form of temporary association between a livestock farmer and one or more tree growers is developed to generate territorial synergies. Although this type of association requires a great deal of coordination between the parties involved, it is generally formalised by a simple oral agreement. A number of recent projects have proposed workshops and/or documents for reaching an agreement, promoting understanding between livestock farmers and growers and anticipating possible conflicts, while maintaining the spontaneous nature of their collaboration (Lyazid et al., 2021). From an operational point of view, online tools have recently been developed in France to connect local stakeholders.²

Livestock farming creates and maintains social and agronomic links on a regional scale. Herd movements generate organic matter in areas where livestock farming is scarce, reducing their dependence on external fertilisers and their ecological footprint. Exchanges between lowland and upland areas can help to make livestock farms more self-sufficient in forage (Napoleone et al., 2022), and (re-)use land that has been abandoned. In some cases, the grazing of wooded areas has enabled 'landless' or 'herbage-only' sheep farmers to set up in business, thereby contributing to the establishment of farmers with no family farming history. Fruit, wine and cereal producers in a given region may agree to offer a grazing area large enough to justify the relocation of a livestock farmer.

Reintroducing animals into an area of a specialised crop changes the visual, acoustic and olfactory environment of farming. The presence of farm machinery is reduced, but new potential nuisances may arise: noise from animals, smell of droppings, risk of animals escaping, etc. The introduction of trees into landscapes that are more or less devoid of them (intensive agricultural plains, but also mountain pastures and extensive pastures without hedgerows) has a generally positive impact on the quality of life of residents (García de Jalón et al., 2018a; Elbakidze et al., 2021). Tree planting patterns can break up the monotony of landscapes by adopting circular layouts or following contour lines (Dupraz & Liagre, 2008; Giambastiani et al., 2023), especially if the main agricultural activity is livestock rearing with little or no mechanisation.

Areas with an agroforestry tradition (bocage, meadow orchard, dehesa, montado, streuobst, etc.) tend to claim this aspect of their landscape as a valuable cultural feature, and to promote it as part of their heritage and tourism (Moreno et al., 2018), as well as a brand image for agricultural products (see § 3.1). The multifunctionality of agroforestry (Veldkamp et al., 2023) is thus fully revealed at this scale of the territory, producing positive economic spin-offs for farmers, but also an ecological and landscape network which can lead to the definition of a local cultural identity (Jeanneret et al., 2021).

Conclusion and challenges for research

Organic livestock farming still faces many challenges, and agroforestry has the potential to meet a number of them. Silvopastoral systems can make farms more resilient by providing fodder resources that are less affected by climatic hazards, and by diversifying the sources of income. Herbivores can consume the plant cover in specialised orchards and fertilise their soil. Trees can provide shelter and a medicinal resource for the animals, which in turn can help to control pests and diseases of perennial crops. All these potential synergies between animals and trees mean that agroforestry strongly aligns with the principles of organic farming: health, ecology, equity and precaution.

There are, however, some limitations to these benefits, which pose new challenges: for instance, animals can damage high-value trees or be poisoned by plant protection products; the carbon storage potential of agroforestry applied to permanent grassland is low; agroforestry livestock farming can be difficult to manage, as well as time- and money-consuming.

These different aspects of agroforestry in livestock farming, discussed throughout this article, are summarised in Figure 7.

Like organic farming, agroforestry is about balancing agronomic performance with biodiversity preservation or enhancement, farmers' income, social connections, etc. It requires fine-tuning of a complex system to position the agroecosystem on a series of productive and environmental optimums. This involves intensive management of the interactions between the elements of the system, a potential mental workload increase, as well as a time and financial investment in the design of the system and in the acquisition of equipment and skills. Specific constraints linked to legislation and social acceptance also need to be taken into account.

Scientific research has a role to play in generating new knowledge in this field, contributing to the development of tools that support the design of silvopastoral systems, helping to inform the decision-making, and facilitating the training of farmers and agroforestry advisers. We believe that the priority tracks for research are:

i) understanding mechanisms and identifying threshold effects, beyond which synergies are effective or, on the contrary, disservices appear;

ii) economic profitability of agroforestry, a major driving force behind its development;

iii) relationships between agroforestry stakeholders, in the territories and within society as a whole.

Organic agriculture is guided by strong principles and backed by a very positive brand image, but it has so far only partially succeeded in abolishing the separation between animal and plant production that has prevailed since the second half of the 20^th century. Agroforestry can be a part of this reconnection, since it is highly relevant from an agronomic point of view but also bears an emotional, 'philosophical' or even spiritual dimension for farmers (Foyer et al., 2020). Until recently, livestock farming was always envisioned and practised in landscapes that included trees, and organic farming cannot afford to ignore such precious allies.

Authors' contributions

Co-ordination, M.T.; Conceptualisation, M.T., F.D.; Fund acquisition, M.T., A.D., B.D., F.D., G.M. and S.B.; Writing - original version, M.T., G.M., M.T., G.M., A.D., B.D., F.D. and S.B.; Writing - revision and editing, M.T., S.B., A.D., F.D. and G.M.; Design, layout and adaptation of figures: M.T. Revision and editing of the English version: M.T., G.M. All authors have read and approved the published version of the manuscript.

Acknowledgments

This article was first translated with www.DeepL.com/Translator, and the authors warmly thank Elsa Kanner (FiBL) for English language editing.