Raphaël Paut
email: raphael.paut@inrae.fr

Arnaud Dufils
email: arnaud.dufils@inrae.fr

Floriane Derbez
email: floriane.derbez@gmail.com

Anne-Laure Dossin
email: annelaure.dossin@bio-provence.org

Servane Penvern
email: servane.penvern@inrae.fr

Orchard Grazing in France: Multiple Forms of Fruit Tree-Livestock Integration in Line with Farmers' Objectives and Constraints

Keywords: grazed orchards, sylvopastoralism, agroforestry, agroecology

Although the grazing of extensive standard orchards has long been a common practice in Europe and continues to take place on a considerable portion of existing traditional orchards, it is more unusual for current specialized and intensive orchards (with bush trees) to be grazed. The way in which animals are integrated into these modern forms of orchards differs according to the animal and tree species as well as to the place relegated to livestock as well as the expected and provided ecological services of that place. However, little literature is available on these modern forms of sylvopastoralism. The objective of this paper is therefore to provide the first overview of the advantages and limitations of these systems as perceived by the actors involved. Based on several research programs, we first tracked on-farm innovations to describe a diversity of systems. We then conducted a multifactorial analysis to characterize these systems according to: (i) structural farm variables; (ii) farmer motivations to integrate livestock; (iii) technical adaptations generated by sylvopastoralism; and finally, (iv) observed services and disservices provided by livestock in orchards. A total of 34 farms and 21 variables were used to differentiate three types of systems that differed according to animal species, grazing patterns, the degree of system redesign, and compliance between initial farmer motivations and the observed services. The results showed that while the practice of livestock grazing in orchards can be agronomically effective and economically viable, its success depends on the ability of growers to integrate all of the dimensions of livestock farming into their orchard system for a win-win association. There are a large number of variables that are involved in successful orchard grazing that result in both challenges and opportunities, but success is closely linked to the grower's ability to adapt the production system to suit the intended role of livestock and to acquire new skills. This typology paves the way for numerous combinations between orchards and livestock. The analysis of the determinants, obstacles, and benefits provided by orchard grazing provides some preliminary elements that are necessary to adapt agricultural support to a diversity of integration patterns in integrated tree and livestock systems.

Introduction

Combining livestock and trees in different forms of silvopastures has long been a traditional practice in Europe, but one that is now outdated given the current highly specialized production systems that exist at the present time [START_REF] Herzog | Streuobst: A traditional agroforestry system as a model for agroforestry development in temperate Europe[END_REF][START_REF] Hilimire | Integrated crop/livestock agriculture in the United States: A review[END_REF]. Today, in the context of a shift towards agroecological and more sustainable agricultural systems, some fruit growers are diversifying and have chosen to reintroduce livestock into their specialized "bush" (or low-stem) orchards [START_REF] Burgess | Lessons Learnt-Grazed Orchards in England and Wales[END_REF][START_REF] Macadam | System Report: Grazed Orchards in Northern Ireland[END_REF]. In general, integrated crop-livestock systems are considered to be an efficient design for sustainable, ecologically based farming systems since they rely on the complementarities between crops and livestock and the connectedness of livestock to the land [START_REF] Bonaudo | Agroecological principles for the redesign of integrated crop-livestock systems[END_REF]. The complementarities first correspond to the ability of animals to enhance natural and cultivated vegetal resources, especially non-food biomass.

Abundant literature on crop-livestock combinations exist, with many studies suggesting typologies and discriminating factors to describe the diversity of crop-livestock systems according to the level of coupling [START_REF] Bell | Integrated crop-livestock systems in Australian agriculture: Trends, drivers and implications[END_REF][START_REF] Mischler | Savoir caractériser les complémentarités entre cultures et élevage pour accompagner la reconception des systèmes de polyculture-élevage dans leurs transitions agroécologiques[END_REF], achieved autonomy [START_REF] Schiere | The role of livestock for sustainability in mixed farming: Criteria and scenario studies under varying resource allocation[END_REF], or ecological functioning [START_REF] Bonaudo | Agroecological principles for the redesign of integrated crop-livestock systems[END_REF], either at the farm [START_REF] Stark | Crop-livestock integration determines the agroecological performance of mixed farming systems in Latino-Caribbean farms[END_REF] or territory [START_REF] Martin | Crop-livestock integration beyond the farm level: A review[END_REF][START_REF] Moraine | Co-design and assessment of cropping systems for developing crop-livestock integration at the territory level[END_REF] scale. There is also abundant literature concerning forms of silvopastures that may, in particular, provide insight into animal feeding and foraging behavior [START_REF] Fraser | Sheep grazing for vegetation management in the northern forests of British Columbia and Alberta[END_REF]. However, these forms of crop-livestock integration are relatively different from those considered in the present study, where animals are fully combined (in reference to Hilimire's typology [START_REF] Hilimire | Integrated crop/livestock agriculture in the United States: A review[END_REF]) with high-value trees (in reference to Den Herder's stratification; [START_REF] Herder | Current Extent and Trends of Agroforestry in the EU27[END_REF]), i.e., intensive, perennial and productive orchards with low-stem trees.

The integration of livestock into specialized and intensive fruit orchards may not be particularly well-suited to sylvopastoralism, but additional benefits for grass, pest, and disease management may be expected and may motivate this practice. These innovative systems may have specific characteristics that consider the benefits that they can offer and the drawbacks that they need to overcome. As for traditional meadow orchards with scattered trees, grazing orchards could facilitate grass management [START_REF] Lavigne | Poultry for biological control of weeds in orchards[END_REF][START_REF] Geddes | Shropshire Sheep Control Weeds in Orchards[END_REF] and thus decrease herbicide use and tillage, which have drawbacks in terms of labor, machinery, and the environment. Many sheep breeders graze their animals in vineyards during the winter to avoid forage expenses-an ancestral practice that is regaining interest [START_REF] Niles | Ecological and economic benefits of integrating sheep into viticulture production[END_REF]. Another additional benefit that may motivate fruit growers is the regulation of aerial pests (plum curculio-Conotrachelus nenuphar; Japanese beetle-Popillia japonica), or pests that live all year long (vole-Microtus duodecimcostatus) or part of the year (codling moth-Cydia pomonella) in the soil, contributing to the sanitation of the orchard through the consumption of fallen leaves and leftover fruits [START_REF] Burgess | Lessons Learnt-Grazed Orchards in England and Wales[END_REF][START_REF] Clark | Effects of free-range chickens and geese on insect pests and weeds in an agroecosystem[END_REF][START_REF] Buehrer | Postharvest grazing of hogs in organic fruit orchards for weed, fruit, and insect pest management[END_REF]. With regard to animals, grazing may offer benefits in terms of animal health and welfare [START_REF] Ginane | Sheep grazing on wooded pastures: Which effects of trees on animal behaviour, welfare and performance[END_REF]. Depending on how it is implemented, it may even represent some form of additional income and a reduced environmental footprint for the agriculture sector [START_REF] Paolotti | Combining livestock and tree crops to improve sustainability in agriculture: A case study using the Life Cycle Assessment (LCA) approach[END_REF]. Nevertheless, the reintegration of animals into bush orchards may also provide disservices such as soil compaction (the overgrazing of ground vegetation can lead to soil and feeder root problems), phytosanitary issues (the impacts of the pesticides used in orchards on animals are still unknown), and socio-economic considerations (labor availability and investment costs are critical to profitability) [START_REF] Wilson | Targeted grazing with sheep and goats in orchard settings[END_REF][START_REF] Hardesty | Silvopastoral orchard management options[END_REF].

Overall, the benefits may counterbalance the technical and structural constraints and may explain farmer motivations to explore such an innovative practice. This practice has not yet been the subject of much experimentation, and very few technical references are available regarding the services or disservices that are provided by this practice. Moreover, the little literature that does exist encompasses various combinations according to the plantation density of the trees in different types of orchards (from orchard meadows to low-stem fruit trees) and the species that should be considered: tree species (various species or fruit trees) and animal species (chickens, geese, pigs, sheep). Likewise, a diversity of grazing forms in orchards and modes animal integration into those orchards can coexist. Some growers may own the flock while others will cooperate with breeders; grazing may be permanent or seasonal; and livestock integration may result in no or a significant adaptation of a system, resulting in a diversity of forms that will undeniably condition the benefits and challenges of animal reintegration [START_REF] Moore | Crop-livestock farming systems in Australia: What levels of integration result in different benefits[END_REF].

Meanwhile, over the past 10 years, there has been a growing interest among farmers and even neo-farmers in the grazing of commercial orchards. This interest has given rise to a dynamic among various research and development actors to characterize and evaluate these emerging forms of livestock integration. Through our involvement in these research and development projects, we have initiated several studies to characterize the systems and level of animal integration as well as farmer motivations and trajectories. We compiled data from surveys, and in this article, we propose the first typology of this practice according to the type of systems, motivations, and services expected by farmers. Such an overview aims to provide insight to help adapt systems to the diversity of contexts that farmers work in and proposes criteria that are essential for producers that could be considered for the further evaluation of these systems.

Materials and Methods

Identifying and Characterizing a Diversity of Orchard and Livestock Integration

We focused our analysis on the integration of livestock in commercial low-stem orchards. Our aim was thus to explore and describe why and how livestock could be integrated into the system. A sample was built to encompass a large diversity of systems and integration patterns. Individuals were therefore chosen for their diversity rather than for their representativeness. Considering the marginal nature of low-stem grazed orchards, farms were identified following a snowball process [START_REF] Lewis-Beck | Snowball Sampling[END_REF] starting with research and development projects that aimed at characterizing innovative orchard systems:

•

The focus group "Vergers + Durables" (V + D), 2008-2013 [START_REF] Capitaine | The "sustainable orchards" group produces and capitalizes knowledge to design and lead orchards otherwise[END_REF][START_REF] Penvern | Farmers' management of functional biodiversity goes beyond pest management in organic European apple orchards[END_REF], which brought together researchers, extensionists, and fruit growers from France, Belgium, Switzerland and Spain; Objective: to analyze different forms of sustainable orchard prototypes and to share experiences; Method: semi-directive interviews; System: sheep grazing in orchards (n = 2 farms).

•

The research project "ARDU", 2014-2016 [START_REF] Cazaux | Determining Factors and Trajectories Analysis to Support System Diversification: Mixed Orchard Animals and Mixed Orchard Vegetables Systems[END_REF][START_REF] Lamine | ARboriculture Durable: Approches interdisciplinaires[END_REF]; Objective: to study motivations, determinants and trajectories of producers who implement diversified orchard systems, including grazed orchards [START_REF] Cazaux | Determining Factors and Trajectories Analysis to Support System Diversification: Mixed Orchard Animals and Mixed Orchard Vegetables Systems[END_REF]; Method: semi-directive interviews and workshops with practitioners and advisors; System: sheep or poultry grazing in orchards (n = 5 farms).

•

The agricultural development project, "IRAEE", 2016 [START_REF] De | Le Pâturage en Vergers; Livret de références de l'inter-réseau Agriculture Energie Environnement[END_REF][START_REF] Colleu | Reconnexion Verger-Élevage[END_REF], which was composed of development actors engaged in farm energy savings; Objective: to study grazed orchard farming system models by substituting some mechanized work with livestock; Method: semi-directive interviews; System: sheep or poultry grazing in orchards (n = 9 farms).

•

The action research project "PEI", 2018-2022 [START_REF]Pei Depasse Intégrer des Animaux aux Cultures Pérennes. Une Démarche Agro-écologique à Accompagner en Provence-Alpes-Côte d'Azur[END_REF], which was composed of agricultural research and development actors; Objective: regional development of pastoralism in perennial crops; Method: semi-directive interviews; System: sheep or poultry grazing in orchards and olive groves (n = 14 farms).

•

Diversified orchards surveyed independently of any research and development program (HD); Objective: exploration of different forms of grazed orchard systems; Method: semi-directive interviews; System: sheep or poultry grazing in orchards and olive groves (n = 4 farms).

The data were collected through semi-directive interviews that had been conducted within the framework of the different projects, resulting in a heterogeneous dataset. The 34 farms that were finally selected (Figure 1) are those for which we had sufficient, homogeneous, and quality data for all of the selected variables and are presented in Table 1. To combine information for a great diversity of farms, the variables were coded and divided into four groups: (i) structural farm descriptors; (ii) farmer motivations to diversify into grazed orchards; (iii) technical adaptations resulting from the integration of trees and livestock; and (iv) observed benefits and disadvantages of the practice (Table 1). Ben-

Data Processing and Statistical Analysis

As highlighted by several authors [START_REF] Bell | Integrated crop-livestock systems in Australian agriculture: Trends, drivers and implications[END_REF][START_REF] Mischler | Savoir caractériser les complémentarités entre cultures et élevage pour accompagner la reconception des systèmes de polyculture-élevage dans leurs transitions agroécologiques[END_REF][START_REF] Hendrickson | Principles of integrated agricultural systems: Introduction to processes and definition[END_REF], farm typologies and performance evaluations differ considerably based on the criteria that is used. These criteria might be defined according to the objectives of the analysis. Since the aim of the present study was to analyze the redesign of systems that integrate livestock and orchards, in the absence of stabilized references on this type of system, we based our analysis on farmer perceptions of the advantages and limitations, constraints, and levers for implementing this practice.

To combine information for a great diversity of farms, the variables were coded and divided into four groups: (i) structural farm descriptors; (ii) farmer motivations to diversify into grazed orchards; (iii) technical adaptations resulting from the integration of trees and livestock; and (iv) observed benefits and disadvantages of the practice (Table 1). Benefit and disadvantage assessments were constructed based on the farmers' own assessments and were based on their own expertise or on previous experiences. We then conducted a multiple correspondence analysis (MCA) followed by a hierarchical cluster analysis on principal components (HCPC) on the sample of 34 farms. MCA is a multivariate exploratory analysis that makes it possible to analyze the pattern of relationships between several categorical dependent variables [START_REF] Husson | Multiple Correspondence Analysus[END_REF]. MCA is an extension of simple correspondence analysis in that it is applicable to a large set of categorical variables [START_REF] Husson | Exploratory Multivariate Analysis by Example Using R[END_REF]. Its graphical output provides a structural visualization for variables in a multi-dimensional space, which is useful for identifying patterns and associations between the investigated variables in a dataset. In the present study, the analysis was conducted on the categorical variables presented in Table 1. To identify the variables that explain each cluster, we tested each group of farmers with regard to the whole sample. A significant difference implies that the tested variable discriminated the cluster of individuals (Vtest with p value ≤ 0.05) and contributed to characterizing the cluster. All statistical analyses were conducted using R software [START_REF] Core | R: A Language and Environment for Statistical Computing[END_REF] with the FactoMineR package [START_REF] Lê | An R Package for Multivariate Analysis[END_REF].

Farm Sample Description

Among the 34 farms that were surveyed, nearly half raised sheep (n = 18), and the other half raised poultry (n = 14). Tree species found on these farms were fruit trees (genus Malus, Prunus and Pyrus; n = 28) and olive trees (Olea europaea L. n = 6). The integration of animals into the orchard can occur at different stages of the farm's trajectory (Figure 2). In our sample, it not only occurred at the set-up and at orchard replanting, but it also occurred during the growth or production phases of an orchard as well. The farms that integrated sheep are relatively older (average set-up date: 1995) than the farms that integrated poultry (average set-up date: 2002). However, the integration of animals is a rather recent dynamic in both farm types since the average diversification dates are 2010 and 2013 for sheep and poultry, respectively.

Results

The multiple correspondence analysis followed by hierarchical clustering resulted in three clusters (Figure 3). The first two MCA dimensions explained 52.6% of the variability among the farms. Dimension 1 contributed to 29.3% of the inertia and is mainly explained by the animal species (animal_species), the utilized agricultural area (UAA), the temporality of tree-livestock integration (Div_duration), and grass management variables (mo-tiv_grass; positive_grass). Dimension 2 contributed to 23.3% of the inertia and is mainly explained by the adaptation of tree management practices (Adapt_trees), time constraints (Limit_time), and livestock ownership (Div_property). The maximization of inertia gains occurred when the clustering resulted in three groups of farmers (see Appendix A Figures A1 andA2 for statistical analysis details).

Results

The multiple correspondence analysis followed by hierarchical clustering resulted in three clusters (Figure 3). The first two MCA dimensions explained 52.6% of the variability among the farms. Dimension 1 contributed to 29.3% of the inertia and is mainly explained by the animal species (animal_species), the utilized agricultural area (UAA), the temporality of tree-livestock integration (Div_duration), and grass management variables (motiv_grass; positive_grass). Dimension 2 contributed to 23.3% of the inertia and is mainly explained by the adaptation of tree management practices (Adapt_trees), time constraints (Limit_time), and livestock ownership (Div_property). The maximization of inertia gains occurred when the clustering resulted in three groups of farmers (see Appendix A, Figures A1 andA2 for statistical analysis details).

Results

The multiple correspondence analysis followed by hierarchical clustering resulted in three clusters (Figure 3). The first two MCA dimensions explained 52.6% of the variability among the farms. Dimension 1 contributed to 29.3% of the inertia and is mainly explained by the animal species (animal_species), the utilized agricultural area (UAA), the temporality of tree-livestock integration (Div_duration), and grass management variables (mo-tiv_grass; positive_grass). Dimension 2 contributed to 23.3% of the inertia and is mainly explained by the adaptation of tree management practices (Adapt_trees), time constraints (Limit_time), and livestock ownership (Div_property). The maximization of inertia gains occurred when the clustering resulted in three groups of farmers (see Appendix A Figures A1 andA2 for statistical analysis details). In addition to the statistical interpretation, the typology was analyzed using the expertise gained from the numerous farmer surveys and from about ten seminars, workshops, and field visits organized with farmers, extension services, and researchers to share experiences and to discuss the benefits, conditions, challenges, and levers for the wider integration of livestock into orchards.

Poultry in Orchards (Group 1)

Group 1 is composed of 14 farms, all of which are involved in poultry farming. Most of the farms in this group (n = 13) integrate livestock and fruit trees over small areas (<5 ha; p < 0.001) and on a permanent basis (p < 0.01). The area allocated for these mixed systems is generally a part of the farm (n = 12; p < 0.05), and the fruit growers own the animals (n = 13; p < 0.05). The farmers in Group 1 mainly choose to integrate poultry within their fruit trees to improve the biocontrol (n = 10; p < 0.05) of pests with a diapause or pupation phase in the soil during their biological cycle (e.g., Cydia pomonella, Bactrocera oleae, Ceratitis capitata, Grapholita molesta) or pests that attack the root system (e.g., Microtus duodecimcostatus). The positive effects observed on these farms were related to grass management (n = 9; p < 0.05).

As revealed by the statistical analysis for the first group, pasture management is organized through the rotation of plots throughout the year, which are structured in fixed or mobile pens, and an arduous work of moving mobile henhouses. Farmers mainly chose to integrate poultry to improve the biocontrol of pests (different species of lepidoptera), but this service is highly dependent on the ability of the poultry to explore all of the plots. In practice, it has been found that in windy, hot, or bare ground conditions, this exploration is limited. However, pasture rotation is also designed to preserve the herbaceous layer, a layer that has been reported to be particularly useful in orchards to preserve structure, biological functioning, and soil-bearing capacity. Farmers may also have chosen to diversify their income with an extensive vision of breeding; in particular, this may have been to meet consumer demands for eggs in short food supply chains. Moreover, the orchard tree later represents an environment that offers more comfort to the animals but that can also evolve into a grazing area in a situation where there is limited access to land. The integration of animals has sometimes led to a reversal of the activity hierarchy, putting eggs and even the sale of animals before fruit production.

However, interviews report that this type of system is subject to constraints. For fruit production, the presence of hens implies access to fallback plots to facilitate the main interventions in the orchard. The constraint of moving the animals contributes to reducing technical interventions such as phytosanitary treatments and soil preparation (weeding, decompaction). However, the farmers reported that they do not try to save on feed rations. They provide a standardized ration to secure egg laying regularity. However, both feeding and watering can be done manually, which contributes to additional workloads. Recurrent problems encountered in these systems are predation (foxes, dogs, raptors, etc.), which leads farmers to investing in protective devices. Finally, another constraint that was mentioned is the implementation and compliance with biosecurity, a set of very demanding preventive and regulatory measures aimed at reducing the risks of the propagation and transmission of infectious diseases such as avian influenza.

Sheep Grazing in Orchard Managed by Fruit Growers (Group 2)

Group 2 is composed of ten farms that all integrate sheep (p < 0.001) with fruit trees (n = 10; p < 0.001), mainly over large areas (n = 8; p < 0.05). Most of the farmers in Group 2 own their animals (n = 9; p < 0.05). The initial farmer motivations were mainly focused on grass management (n = 7; p < 0.05), and positive impacts were indeed observed on grass management (n = 7; p < 0.05) and biocontrol (n = 8; p < 0.05). Adaptations occurred in terms of tree training (n = 9; p < 0.001), buildings (n = 8; p < 0.001), and phytosanitary treatments (n = 7; p < 0.05). The disadvantages associated with this form of integration were related to time n = 8; p < 0.001) and money (n = 6; p < 0.01).

The fruit growers in Group 2 chose to acquire a flock, often of limited size (a flock with less than 50 sheep on average), in order to benefit from two main agronomic services: (i) to increase their autonomy in terms of weed management through sheep grazing and (ii) to optimize the prophylactic action of sheep on orchard diseases and pests (e.g., Venturia inequalis). For example, some farmers graze their flock as early as possible after fruit harvest in order to consume and degrade as much wormy fruit that has fallen on the ground as possible so as to reduce the inoculum. In addition to these agronomic advantages, despite the expenses associated with breeding (purchase of animals, fencing and accessories, veterinary care, shearing, administrative and regulatory costs, etc.), there are also some economic advantages to this diversified system with the sale of lambs, which are sometimes highly sought after for certain breeds (e.g., Shropshire breed).

However, depending on the organization of the grazing under the trees and, in particular, the periodicity (temporary or quasi-permanent over the year), the technical adaptations of the orchard can be null or consequent, such as the optimization of the trellising or the irrigation system for the movement of the animals or the enhancement of fruiting by means of the removal of low branches and the formation of higher trees in order to minimize the risk of loss due to animals and to maintain a stable yield.

Finally, in terms of workload, the presence of the herd becomes an additional factor to consider depending on the nature of the interventions to be conducted in the orchard. Much of the extra work involved in breeding consists of managing the grass resources, organizing rotational grazing under the trees using mobile pens, and observing the animals in order to detect situations where there are insufficient grass resources and to guard against the risk of the animals attacking the tree bark. Breeding also involves more work (e.g., lambing period) and new regulatory and sanitary constraints. It was also necessary for the farmers to open up to new technical networks far from the tree farming sector (veterinarian, shearer, slaughterhouse, etc.).

Sheep Grazing in Orchards in Cooperation with a Shepherd (Group 3)

Group 3 is composed of ten farms. The grazing animals are sheep (n = 10; p < 0.001), but most of the farmers in Group 3 do not own the herd (n = 8; p < 0.001). Grazing is mainly implemented over medium (5-20 ha; n = 6; p < 0.01) and large areas (>20 ha; n = 4; p < 0.05) and on a temporary basis (n = 10; p < 0.001).The entire agricultural area is allocated to grazing (p < 0.001). The main motivation for farmers is related to grass management (n = 10; p < 0.001), and positive impacts can be observed on grass management (n = 8; p < 0.05) and soil fertility (n = 7; p < 0.05). Nevertheless, adaptations are required for phytosanitary treatments (n = 6; p < 0.05).

For the fruit growers in Group 3, in the majority of cases, the decision to make orchards grazeable for sheep very often stemmed from a request from a shepherd looking for grazing areas. The fruit growers discovered the impact of the presence of the animals on their plots while benefiting from the support of a livestock professional at the same time. Depending on local opportunities, shepherds graze their large herds in the orchards from fruit harvest until the bud burst of the fruit trees the following spring in specialized arboricultural areas with the agreement of the landowners. This arrangement benefits both parties: on the one hand, the flock has access to a grass resource, and on the other hand, the flock contributes to the maintenance of the grass cover, eliminating at least on wedding operation.

The main disadvantages of these types of arrangements are organizational. First, after consultation between the shepherd and the fruit grower on the modalities of the presence of the sheep, a set of plots is selected to be grazed. These must offer simplified access to the flock and should include a block of plots that is large enough to constitute a continuous grass resource for several weeks. Second, these plots must have mature trees so as not to compromise the development of young trees. Third, the technical management of the orchard must not require interventions that are incompatible with the presence of the sheep (e.g., phytosanitary treatments, machinery).

Over the years, a trusting relationship was established, and the temporary grazing was renewed annually, resulting in the fruit growers not being required to significantly modify their tree practices or to invest in any specific equipment. In most cases, the initial partnership extended to neighboring tree farms, thus facilitating the breeder's long-term involvement.

Synthesis on Grazed Orchard Management

The farm typology presented supra (Sections 3.1-3.3) was built on data concerning the structural dimensions of the studied farms, a set of initial farmer motivations, and on the modalities of orchard management practices as well as on the empirical evaluations of the services and disservices resulting from this practice according to the farmers. Concerning the practical aspects, it appears that a certain number of characteristics must be taken into account to optimize the management of grazed orchards, regardless of the groups considered. These are mainly: (i) breeding management by fruit growers and (ii) the pasturing temporality of the animals under the trees (Table 2). Indeed, the time dimension is a factor that appears to be decisive in the management of these systems. Time is not only an indicator of the workload (peaks of activity, on-call duty, etc.), it is also a management lever throughout the seasons. Group 3 breeders assess the duration of grazing according to the abundance of the grass resources. The farmers in Group 2 coordinated the grazing periods with the annual evolutionary cycles of the fruit species on their farms and the biological cycles of pests and diseases. The farmers in Group 1 define the annual frequency at which the animals return to the plots to the plots to preserve the herbaceous layer and the health of the herd.

Discussion

An Exploratory Approach to Record Farmer's Experiences with Fruit Trees and Livestock Integration

Considering the marginal nature of the system studied here, our analysis is based on a limited number of individuals. Indeed, farms were chosen for their diversity rather than for their representativeness. We focused on low-stem orchards, whereas other fruit treelivestock combinations exist, namely cattle grazing in high-stem orchards [START_REF] Pantera | Agroforestry for high value tree systems in Europe[END_REF] or with other perennial crop species such as vineyards [START_REF] Conrad | Suitability of different breeds and possible breeding objectives[END_REF]. These other forms of livestock integration might provide knowledge and technical references of the benefits and implementation conditions for these systems. For example, the literature describes the advantages of geese or hogs for weeding and fertilizing trees but warns of the damage that they can cause to the trees [START_REF] Lavigne | Poultry for biological control of weeds in orchards[END_REF][START_REF] Clark | Effects of free-range chickens and geese on insect pests and weeds in an agroecosystem[END_REF][START_REF] Buehrer | Postharvest grazing of hogs in organic fruit orchards for weed, fruit, and insect pest management[END_REF][START_REF] Bijja | Les services rendus par les systèmes de production de foie gras agroforestiers: L'exemple de l'association entre oies et noyers en Périgord[END_REF].

The exploratory approach presented in this study is then based on a limited number of variables that were gathered by analyzing surveys, workshops, and visits with various stakeholders. The presented multivariate analysis could explain a large part of the variability, but more situated information was collected and assessed through interviews. For instance, the regulatory aspects (sanitary and pesticide regulations) that could not be taken into account in the statistical analysis were often mentioned by farmers and advisors in workshops. Moreover, the methodology presented here is based on perceptions and narratives of the farmers regarding the advantages and constraints of grazed orchards. These data could be supplemented with quantitative data to more accurately measure the level of integration and support system design according to its expected performance [START_REF] Martel | Effectively combining crop and livestock systems on conventional and organic farms: A means for increasing system sustainability[END_REF][START_REF] Ryschawy | Mixed crop-livestock systems: An economic and environmentalfriendly way of farming[END_REF]. On-farm experimentation on the interaction between animals and the orchard are necessary to provide such data. Concerning the economic aspects in particular, it remains unclear whether the integration of livestock into a highly specialized conventional system is profitable [START_REF] Pissonnier | A methodology for redesigning agroecological radical production systems at the farm level[END_REF]. Research is still needed to prove the concept with a model that reveals some of its limitations.

Another important aspect in the analysis of grazed orchards is their temporality. As shown in Figure 2, these systems are at an early stage of development. The perception and the criteria to assess their benefits and limitations may change depending on how long the practice is used. According to Penvern et al. [START_REF] Penvern | Farmers' management of functional biodiversity goes beyond pest management in organic European apple orchards[END_REF], the longer a practice is implemented, the more benefits mentioned by the farmers. With the experience and knowledge gained after using these systems over longer periods of time, systems can improve, adjust, and thus create new benefits or opportunities. The analysis of the temporal dynamics in the changes in farmer practices may be useful to identify levers of change, constraints to adoption, and adaptations of practices that suit the farmer's own priorities [START_REF] Chantre | Transitional pathways towards input reduction on French field crop farms[END_REF][START_REF] Mawois | Transition to legume-based farming systems requires stable outlets, learning, and peer-networking[END_REF]. This might also highlight how successive changes in farmer practices lead to a transition of the farm [START_REF] Lamine | Transition pathways towards a robust ecologization of agriculture and the need for system redesign. Cases from organic farming and IPM[END_REF][START_REF] Catalogna | Diversity of experimentation by farmers engaged in agroecology[END_REF]. For all of these reasons, regular monitoring of the farms involved in the present study would make it possible to capitalize on their technical trajectories and their impacts on the system properties.

The Balance between Contraints and Benefits as an Optimal Degree of Integration

The three types of farms described here display different levels of integration according to the livestock management and the grazing temporality of the animals under trees (Table 2), each with different benefits and limits. The systems in Groups 1 and 2 might provide more benefits in terms of pest and disease management (from the grower's perspective) than those in Group 3. Group 2 induces more tree training practice adaptations, whereas the system described in Group 1 results in an increase in the complexity of the management of the herbaceous stratum. Group 3 requires the presence of livestock farms in close proximity and arrangements that can be made to provide fallback plots for the flock at key periods: spring for grass growth and autumn for scab. This overview suggests that multiple solutions and trade-offs can be found in response to a diversity of situations.

Beyond this exploratory survey, technical and economic references are needed to help farmers design their own system that optimizes benefits and risks and to find trade-offs among integration options. While our approach makes it possible to better characterize the coupling between orchards and livestock in mixed systems, it remains unclear whether high or low levels of integration are desirable. This characterization raises a methodological difficulty that lies in the diversity of the definitions of "integrated systems", in which the coupling of several activities can be based on agronomic, economic, structural, capitalist, or labor approaches [START_REF] Mischler | Savoir caractériser les complémentarités entre cultures et élevage pour accompagner la reconception des systèmes de polyculture-élevage dans leurs transitions agroécologiques[END_REF]. The integration of trees and livestock is not an objective in itself but is instead a means to achieve other objectives. Thus, different forms of grazed orchards would not aim at the same objectives, nor have the same environmental or economic performance [START_REF] Martel | Effectively combining crop and livestock systems on conventional and organic farms: A means for increasing system sustainability[END_REF].

In terms of economic and environmental impact, the assessment of mixed farms by their level of integration between crops and livestock showed that an increasing level of integration led to a win-win strategy comprising good economic and environmental performance [START_REF] Bonaudo | Agroecological principles for the redesign of integrated crop-livestock systems[END_REF][START_REF] Kronberg | Integration of Crop and Livestock Production in Temperate Regions to Improve Agroecosystem Functioning, Ecosystem Services, and Human Nutrition and Health[END_REF]. Moraine et al. [START_REF] Moraine | Un cadre conceptuel pour l'intégration agroécologique de systèmes combinant culture et élevage[END_REF] refer to the process of substituting inputs with ecosystem services. In our case study, all of these hypotheses still remain to be tested and deserve further investigation. Dynamic models [START_REF] Paut | How to reconcile short-term and long-term objectives in mixed farms? A dynamic model application to mixed fruit tree-Vegetable systems[END_REF][START_REF] Do | Decision analysis of agroforestry options reveals adoption risks for resource-poor farmers[END_REF] seem to offer a useful framework to support the design and dynamic evaluation of mixed system properties [START_REF] Sabatier | Action versus result-oriented schemes in a Grassland agroecosystem: A dynamic modelling approach[END_REF]. In addition, farm-level simulation tools such as those developed by Pissonnier et al. [START_REF] Pissonnier | A methodology for redesigning agroecological radical production systems at the farm level[END_REF] can be useful for evaluating the properties and conditions for the success of alternative "radical" systems such as orchard-sheep systems.

At the territorial scale, in correspondence with the systems in Group 3, researchers have [START_REF] Martin | Crop-livestock integration beyond the farm level: A review[END_REF][START_REF] Moraine | A participatory method for the design and integrated assessment of crop-livestock systems in farmers ' groups[END_REF] proposed participatory methods based on simulation models to design croplivestock integration at a regional scale. Indeed, organization at the territory-scale makes it possible to overcome constraints and to bring out similarities between specialized farms in a mixed territory or between closely linked territories, as is the case in the context of transhumance. Upstream of these approaches, the aim is to identify and bring together interested fruit growers and breeders. While shepherds are more often in search of grass for their herds, fruit growers are less familiar with opportunities to build this type of partnership. Spatially explicit matchmaking tools are being developed in some areas where these two types of farming can coexist. However, cooperation between farmers is not only determined by their geographical proximity but also by their social proximity [START_REF] Asai | Critical factors for crop-livestock integration beyond the farm level: A cross-analysis of worldwide case studies[END_REF]. Collective initiatives are thus useful tools to bring the involved stakeholders together, share objectives, find arrangements, and to build trust and commitment between the interested parties.

A Gradient of Farm Redesign

Finally, our results show that expected services depend on the extent of the change in the system and on the initial investment, regardless of the group concerned. A more detailed comparative analysis of the case studies within the groups reveals a certain diversity in the trade-off between fruit production and breeding activities, which is highly dependent on the initial objectives of integrating breeding into orchards. Thus, for Group 2 fruit growers, having objectives that are limited to weed management result in breeding management that is similar to the use of a weeding tool. The grazing period and rotation between plots are thus only organized by considerations related to the fruit trees. The economic value of breeding may even be null as long as the agronomic value is present. On the other hand, more global objectives, both economic and environmental, lead to the consideration of breeding as an activity on the farm in its own right. It may even be optimized for the purposes of supplementing fruit production, which remains the main activity.

In reference to Hill and MacRae's framework [START_REF] Hill | Conceptual Framework for the Transition from Conventional to Sustainable Agriculture[END_REF], some farmers may thus prefer and subsequently adopt substitution rather than redesign strategies-strategies that can take place at different points in their trajectories. Animal integration can also be temporary, a technique that can be attempted without jeopardizing the farms, or progressive. This technique can therefore be adapted to all types of more or less specialized systems. Nevertheless, minimum practice adjustments are necessary in all cases to facilitate grazing and to ensure animal health. First, this particularly concerns the use of copper, which is widely used by fruit growers as a fungicide and can be toxic depending on the doses and application conditions. In the face of the legitimate concerns related to copper, no instances of intoxication due to copper applications were reported in our sample. A recent study by Trouillard et al. [START_REF] Trouillard | Sheep Grazing Organic Vineyards And Orchards: What About Copper Poisoning[END_REF] highlights the protective role of molybdenum against copper toxicity in sheep grazing perennial crops. Despite the presence of risk indicators for intoxication, no clinical symptoms were observed in this study, even after 60 days of grazing. This appears to be favorable for the wider development of this practice in orchards. However, it calls for repetitions in a broader range of situations and for the consideration of other pesticides commonly used in arboriculture in order to fully recommend this practice. In the absence of more stabilized references, farmers use alternatives to this active ingredient (e.g., calcium hydroxide, calcium sulfide) or choose to exclude the animals from the plot for 20 to 30 days after copper application. For more intensive farms, these measures could also be considered by limiting grazing periods to the post-harvest only in order to exclude the spring season, which is prone to numerous phytosanitary treatments. Second, growers seeking to reintegrate animals face challenges stemming from their lack of knowledge about animal husbandry and regulations. As demonstrated by Hilimire [START_REF] Hilimire | Integrated crop/livestock agriculture in the United States: A review[END_REF], these challenges result from a long history of specialization. New regulatory and sanitary constraints must be respected, and new professional networks must be built (veterinarians, shearers, slaughterhouses, etc.). As emphasized by our results, reintegrating animals into orchards may also imply the mastery of a diversity of components, processes, and dimensions. Third, as advocated by many authors [START_REF] Birner | From Best Practice to Best Fit: A Framework for Designing and Analyzing Pluralistic Agricultural Advisory Services Worldwide[END_REF][START_REF] Bakker | Assessing the effects of Farmer Field Schools on farmers' trajectories of change in practices[END_REF], the diversity of farmer practices and trajectories of change that were revealed in this study reinforces the assertion that advisory services should be adapted to the farmers' local conditions and should be focused on reinforcing the innovation capacities of farmers.

Conclusions

Orchard grazing systems are currently experiencing renewed interest among fruit growers and breeders. Traditionally present in agricultural areas with high-stem fruit production, new types of sylvopastoralism are emerging within more intensive fruit-growing systems. These innovative forms of tree and livestock integration raise new questions and present many challenges for farmers. Through the analysis and characterization of 34 farms, we proposed a typology of grazed low-stem orchards in the present study. By describing different tree-livestock integration systems, we highlighted original practices that have been developed by farmers. Our results indicate that the motivations underlying the choices of their practices differ according to the type of integration, and the resulting systems generate different technical adaptations and provide various services and disservices. This exploratory analysis provides a basis for the identification of systems that should be explored to a greater extent through the implementation of measurements in farmer fields or through experimentation. The relationships between practices, motivations, and services provided described in this paper makes it possible to highlight innovative solutions that can be adapted by other farmers or that can be studied in greater depth through agronomic research.

Figure 1 .

 1 Figure 1. (a) Map of the 34 surveyed farms, France; (b) sheep (Shropshire breed) and commercial apple, Farm no. 24 (Malus Domestica); (c) laying hens and commercial apricot (Prunus Armeniaca), Farm no. 6 (photo credits: M. Compagnone).

Figure 1 .

 1 Figure 1. (a) Map of the 34 surveyed farms, France; (b) sheep (Shropshire breed) and commercial apple, Farm no. 24 (Malus Domestica); (c) laying hens and commercial apricot (Prunus Armeniaca), Farm no. 6 (photo credits: M. Compagnone).

Forests 2021 ,

 2021 12, x FOR PEER REVIEW 6 of 16

Figure 2 .

 2 Figure 2. Diversification date as a function of set-up date, livestock species, and orchard type. Points on the bisector represent grazed orchards designed at the set-up date; the points far from the bisector represent diversification trajectories that took place after the set-up date.

Figure 3 .

 3 Figure 3. Distribution and grouping of the 34 farms surveyed; (a) cluster dendrogram; (b) result of the hierarchical clustering (HCPC). Ellipses are centered on the barycenter of the groups (non-labeled large points). The first two dimensions of the MCA explain 52.6% of the variance (Dimension 1, 29.3%; Dimension 2, 23.3%).

Figure 2 .

 2 Figure 2. Diversification date as a function of set-up date, livestock species, and orchard type. Points on the bisector represent grazed orchards designed at the set-up date; the points far from the bisector represent diversification trajectories that took place after the set-up date.

Figure 2 .

 2 Figure 2. Diversification date as a function of set-up date, livestock species, and orchard type. Points on the bisector represent grazed orchards designed at the set-up date; the points far from the bisector represent diversification trajectories that took place after the set-up date.

Figure 3 .

 3 Figure 3. Distribution and grouping of the 34 farms surveyed; (a) cluster dendrogram; (b) result of the hierarchical clustering (HCPC). Ellipses are centered on the barycenter of the groups (non-labeled large points). The first two dimensions of the MCA explain 52.6% of the variance (Dimension 1, 29.3%; Dimension 2, 23.3%).

Figure 3 .

 3 Figure 3. Distribution and grouping of the 34 farms surveyed; (a) cluster dendrogram; (b) result of the hierarchical clustering (HCPC). Ellipses are centered on the barycenter of the groups (non-labeled large points). The first two dimensions of the MCA explain 52.6% of the variance (Dimension 1, 29.3%; Dimension 2, 23.3%).

Table 1 .

 1 Summary of variables and respective categories organized into groups.

	Group Description	Variables	Code	Modalities
		Tree component	initial_prod	Orchard, Olive grove
		Livestock component	animal_species	Poultry, Sheep
	Farm structural descriptors	Utilized Agricultural Area (UAA) used for tree-livestock integration (ha) Land structure	UAA Land_structure	Small (<5 ha); medium (5-20 ha), large (>20 ha) Merged, Fragmented
		Time since tree-livestock integration (years)	Div_date	<2005, 2005-2015, >2015
		Ownership of livestock	Div_property	Yes, No, Both
		Economic motivation	Motiv_eco	Yes, No
	Farmers' motivations	Grass management motivation Biological control motivation	Motiv_grass Motiv_biocontrol	Yes, No Yes, No
		Improving nutrient recycling motivation	Motiv_nutrients	Yes, No

Table 1 .

 1 Cont.

	Group Description		Variables	Code	Modalities
		Adaptation of tree management practices (training, pruning, shaping)	Adapt_trees	Yes, No
		Adaptation of fertilization practices (modification or reduction of doses)	Adapt_ferti	Yes, No
	Technical adaptations	Adaptation of phytosanitary practices
		(postponement of treatment, reduction, or	Adapt_phyto	Yes, No
			substitution of products)
		Adaptation of farm buildings (irrigation, trellising)	Adapt_buildings	Yes, No
		Temporality of tree-livestock integration	Div_duration	Permanent, Temporary
		Proportion of land diversified within the farm	Land_proportion	All, Part
			Biological control	Positiv_biocontrol	Yes, No
	Benefits and disadvantages Forests 2021, 12, x FOR PEER REVIEW	Grass management Soil fertility improvement Time constraint	Positiv_grass Positiv_soil Limit_time	Yes, No Yes, No Yes, No	4 of 16
			Financial investment constraint	Limit_money	Yes, No

Table 2 .

 2 Synthesis of the main characteristics of a grazed orchard management system.

	Characteristics	Benefits	Limits
	Livestock management by	Owner of both activities	Flexible management Economic valuation of livestock	Investments Plot structuring Sanitary regulations Workload
	fruit growers	Partnership between fruit grower and herder	Possibility to delegate the entire livestock farming management (no skills required)	Appropriate grazing area Communication and coordination between farmers Adapting orchard operations to the herder's organization
				Grass resource
	Grazing temporality	Permanent grazing	Better efficiency of grazing and prophylaxis	Tree training Animal parasitism
	under trees			Reduction in floral diversity
		Temporary grazing	Simplification of orchard management	Need for fallback plots

Acknowledgments:

The authors would like to thank all of the farmers and herders who gave their time and experience to help develop the grazed orchards. The authors would also like to thank the trainees who contributed to the data acquisition: Laura Cazaux, Mathie Compagnone and Lucie Maurel.

Data Availability Statement: Data available from the authors on request and with the permission of the respondents.

Author Contributions: Conceptualization, R.P., A.D. and S.P.; data curation, R.P., F.D., A.-L.D. and S.P.; formal analysis, R.P.; funding acquisition, A.D., F.D. and S.P.; investigation, A.D. and S.P.; methodology, R.P.; software, R.P.; validation, A.D., F.D., A.-L.D. and S.P.; Writing-original draft, R.P. and S.P.; writing-review and editing, R.P., A.D., F.D., A.-L.D. and S.P. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.

Conflicts of Interest:

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Appendix A. Results of the Multiple Correspondence Analysis

Forests 2021, 12, x FOR PEER REVIEW 13 of 16

trainees who contributed to the data acquisition: Laura Cazaux, Mathie Compagnone, and Lucie Maurel.

Conflicts of Interest:

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Appendix A. Results of the Multiple Correspondence Analysis


Bassel Othman

Giovanni De Nunzio

Domenico Di Domenico

Carlos Canudas-De-Wit

Urban Road Traffic Fuel Consumption Optimization via Variable Speed Limits or Signalized Access Control: A Comparative Study

This work focuses on comparing the ecological potential of variable speed limits (VSLs) and signalized access control. A synthetic two-region network composed of an urban and a peri-urban area is considered. This study aims at improving the energy efficiency in both areas. A microscopic traffic simulator (SUMO) is used to model the dynamics of the system. It is controlled by a nonlinear model predictive control (NMPC) framework based on a macroscopic traffic model, which is an adapted version of the cell transmission model (CTM). The controller is coupled with an artificial neural network (ANN) to predict the fuel consumption. Finally, microscopic physical energy and NOX models are used to evaluate the performance of both control actuators. The results reveal that VSLs are more promising due to the smoother variation of the densities.

I. INTRODUCTION

Epidemiological research has highlighted that mortality and several cardiovascular and respiratory diseases are highly correlated with air pollution. A study conducted in 30 countries revealed that PM 2.5 concentrations above minimum exposure levels were responsible for 22% of infant deaths [START_REF] Heft-Neal | Robust relationship between air quality and infant mortality in africa[END_REF]. Although traffic related pollutants (NO 2 , NO X , PM 10 , and PM 2.5 ) have been on a downward trend in major cities since 2010 due to more efficient vehicles, pollution levels remain well above the recommended European limit value [START_REF] Font | A tale of two cities: is air pollution improving in paris and london?[END_REF]. To improve air quality in urban areas, the development of new engine technologies can be combined with connectivitybased solutions [START_REF] Othman | Connectivity and Automation as Enablers for Energy-Efficient Driving and Road Traffic Management[END_REF] and a change of individual habits induced by public authorities [START_REF] Quarmby | Air quality strategies and technologies: A rapid review of the international evidence[END_REF].

Although most variable speed limit (VSL) approaches in urban areas are not energy-efficiency oriented [START_REF] Tumash | Boundary and VSL Control for Large-Scale Urban Traffic Networks[END_REF], [START_REF] Tajali | Dynamic speed harmonization in urban street networks[END_REF], [START_REF] Van Den Berg | Integrated traffic control for mixed urban and freeway networks: A model predictive control approach[END_REF], speed limits have a strong impact on energy consumption and pollutant emission because of their impact on traffic dynamics [START_REF] Madireddy | Assessment of the impact of speed limit reduction and traffic signal coordination on vehicle emissions using an integrated approach[END_REF]. In this sense, ecological VSL strategies are promising in urban environments as they can adapt to the traffic demand [START_REF] Khondaker | Variable speed limit: an overview[END_REF], [START_REF] Othman | Variable speed limits control in an urban road network to reduce environmental impact of traffic[END_REF]. However, studies that focus on the control of urban networks usually overlook the performance in the peri-urban areas.

The use of signalized access control actuators to optimize the flow of vehicles between different areas of a network is a well-known subject in the literature [START_REF] Haddad | On the stability of traffic perimeter control in two-region urban cities[END_REF], [START_REF] Sirmatel | Stabilization of city-scale road traffic networks via macroscopic fundamental diagram-based model predictive perimeter control[END_REF]. These approaches usually focus on developing control systems Bassel Othman, Giovanni De Nunzio, and Domenico Di Domenico are with IFP Energies nouvelles, Rondpoint de l'échangeur de Solaize, BP 3, 69360 Solaize, France {bassel.othman, giovanni.de-nunzio, domenico.didomenico}@ifpen.fr Bassel Othman and Carlos Canudas-de-Wit are with Grenoble Alpes University, CNRS, Inria, Grenoble INP, GIPSA-lab, 38000 Grenoble, France carlos.canudas-de-wit@gipsa-lab.fr for efficient congestion management in large-scale urban networks.

In this study, we propose to consider a synthetic road network of intermediate size consisting of a peri-urban and an urban area. The objective is to analyze the impact of VSLs and signalized access control on energy efficiency, pollutant emissions, and traffic performance. To achieve this, a nonlinear model predictive control (NMPC) framework is implemented with either the speed limits or the signalized accesses as actuators. The controller considers an adapted version of the cell transmission model (CTM) to evaluate the traffic dynamics, and an artificial neural network (ANN) is used to predict the global energy consumption in the network. To evaluate the performance of both controllers, simulations are run with a microscopic traffic simulator (SUMO), and the energy consumption and pollutant emissions are calculated with a physical model.

The main contributions of this paper are:

• A comparison of the effect on energy efficiency of two control actuators, namely the speed limits and the signalized access actuators, is realized. Both actuators are controlled in an NMPC framework that has the same parameterization.

• The CTM is adapted in order to be consistent with traffic light signals (TLS) and to consider additional endogenous demands inside the network. The body of this paper is organized as follows. Section II presents the problem setting. Section III describes the optimal control strategies. The results are presented in Section IV. Finally, Section V contains concluding remarks.

II. PROBLEM SETTING

This study considers a road network composed of a periurban (B) and an urban (A) area, as represented in Fig. 1. It aims to compare the potential energy efficiency impact of

• VSL control in the urban area A,

• signalized access control from the peri-urban B to the urban A area. To be more representative of the traffic dynamics, vehicles are generated upstream from the peri-urban area (constant exogenous demand d in ) and within the urban area through parking lots for example (intermittent endogenous demand d i (k), where i denotes the cell and k is the time step). As represented in Fig. 1, the vehicles all have destinations downstream from the urban area (vehicle sinks of supply s out ). Note that in this study, the parking lots described above are considered as sources only. In a future work, parking lots and households could also behave as vehicle sinks. In this work, the objective is to improve the energy efficiency in both areas. To achieve this, an optimal regulation of the flow of vehicles through the network is necessary, either through VSLs or access control from the peri-urban to the urban area.

Peri-urban area B

A. Traffic model

In order to model traffic dynamics, we use the cell transmission model (CTM) [START_REF] Daganzo | The cell transmission model: A dynamic representation of highway traffic consistent with the hydrodynamic theory[END_REF], which is a temporal and spatial discretization of the Lighthill-Whitham-Richards (LWR) model [START_REF] Lighthill | On kinematic waves ii. a theory of traffic flow on long crowded roads[END_REF], [START_REF] Richards | Shock waves on the highway[END_REF]. The road network under consideration is discretized into 60-meter-long cells. Based on the conservation of the number of vehicles, the density in cell i at time step k + 1 is

ρ i (k + 1) = ρ i (k) + δ t δ x ϕ in i (k) -ϕ out i (k) (1)
where ϕ in i and ϕ out i denote respectively the inflow and the outflow of cell i. The parameters δ t and δ x are respectively the time step duration and the cell length. In this framework, the densities ρ characterize the state of the system.

The sets of cells A and B consist respectively of the cells of the urban and the peri-urban areas. To characterize the road connections, sets P i and N i are associated with each cell i. They correspond respectively to the upstream and the downstream cells connected to i. The function |.| returns the number of cells in each set. A few illustrating examples are given in Table I. Moreover, we define C ⊂ B as the set of peri-urban cells that are at the interface with the urban area. In this work, access control actuators are located at the downstream end of cells C. Their behavior is characterized by the green function g defined as follows

g(k) = 1, if k ≤ k T cycle + κT cycle 0, otherwise (2)
where the duty cycle κ takes on continuous values between 0 (closed access) and 1 (open access), and is defined as

κ = T green T cycle (3)

TABLE I: Examples of cells connections.

Cells structure

|P i | |N i | i 1 1 i 1 2 i 2 1
The expression of the cells inflows and outflows depends on the number of upstream and downstream connections. For example, to calculate the flows at an intersection in the urban area, it is essential to take into account the split ratios β i , as well as the behavior of TLS, which are fixed and characterized by binary green functions α i . Based on the trapezoidal fundamental diagram introduced in [START_REF] Daganzo | The cell transmission model: A dynamic representation of highway traffic consistent with the hydrodynamic theory[END_REF], the possible expressions are summarized in Table II, where the demand and supply functions are respectively defined as

D i (k) = V i ρ i (k) + d i (k) (4a) S i (k) = w(ρ M -ρ i (k)) (4b)
The fundamental diagram parameters V i , ϕ M i , w, and ρ M denote respectively the speed limit, the maximum flow, the backward wave speed, and the maximum density. Based on the trapezoidal fundamental diagram, the average speed in each cell can be calculated as follows

v i (k) = min V i , ϕ M i ρ i (k) , w ρ M -ρ i (k) ρ i (k) (5)
Finally, the total travel distance (TTD) is defined in each area over a simulation period T as

T T D 1→T A = δ t δ x i∈A T k=1 ρ i (k)v i (k) (6a) T T D 1→T B = δ t δ x i∈B T k=1 ρ i (k)v i (k) (6b)
TABLE II: Expression of cells outflows and inflows, depending on the number of upstream and downstream connections.

|N i | Expression of ϕ out i (k) 0 min D i (k), ϕ M i , s out (k) 1 If i ∈ C: g(k) × min D i (k), ϕ M i , S N i (k) Else: min D i (k), ϕ M i , S N i (k) ≥ 2 α i (k) min D i (k), ϕ M i , S j (k) β j j∈N i |P i | Expression of ϕ in i (k) 0 min d in (k), ϕ M i , S i (k) 1
If P i ∈ C: g(k) × min D P i (k), ϕ M P i , S i (k) Else: min D P i (k), ϕ M P i , S i (k) ≥ 2 β i j∈P i α j (k) min D j (k), ϕ M j , S l (k) β l l∈N j

B. Energy model

To predict fuel consumption, the calibration of an ANN is preferred to the use of existing macroscopic energy models. First, it captures the impact of density and average speed variations without the constraint of introducing a macroscopic acceleration (defined as the discrete derivative of the average speed), which would be strongly linked to the simulation step [START_REF] Zegeye | Integrated macroscopic traffic flow, emission, and fuel consumption model for control purposes[END_REF]. Then, the ANN has a greater sensitivity to fuel overconsumption at low average speeds. Finally, this ensures that the energy model is adapted to the type of vehicle and the network topology under consideration. However, it is essential to keep in mind that this ANN-based model may not be adapted to all types of networks, vehicles, and driving styles.

The proposed ANN predicts the average fuel consumption rate per vehicle in cell i at time k, expressed in liters per second per vehicle and denoted y k i . It is composed of three hidden layers, whose nodes use the rectifier activation function that returns the positive part of its argument. The training data are obtained by microscopic simulation performed in the road network presented in Fig. 1. It is then processed by a microscopic physical energy model (cf. Section III-A), considering Euro 4 diesel passenger cars. Two initial density levels (0% and 70% of the maximum density ρ M ), and four different constant speed limits (20 km/h, 30 km/h, 40 km/h, 50 km/h) are considered in the simulations. The data are measured at a sampling frequency of 1 Hz, generating about 3.5 million data points.

The ANN inputs can be listed as follows

y k i = f (ρ i (k), ρ i (k -1), v i (k), v i (k -1), V i , α i (k)) (7
) They include the densities and speeds at current and previous time steps, the speed limit, and the TLS state. In case cell i is not regulated by a TLS, we consider ∀k, α i (k) = 1. The densities and speeds at previous time steps are included in the model inputs as they add information on the dynamics of the system. This partially compensates for the fact that the macroscopic CTM variables do not contain any information on vehicles accelerations.

Finally, the total fuel consumption in areas A and B over a simulation period T are calculated as follows

E 1→T A = δ t δ x i∈A T k=1 ρ i (k)y k i (8a) E 1→T B = δ t δ x i∈B T k=1 ρ i (k)y k i (8b)

III. OPTIMAL CONTROL

In this section, we present the energy-oriented optimization problem and we detail the two proposed approaches to solve it.

A. Model predictive control

To control the system, we propose to implement an NMPC framework because the traffic dynamics is corrected at each control iteration. The associated block diagram is given in Fig. 2.

The dynamics of the controlled system is modeled using a microscopic traffic simulator, namely SUMO [START_REF] Lopez | Microscopic traffic simulation using sumo[END_REF], with the car-following intelligent driver model (IDM) [START_REF] Treiber | Congested traffic states in empirical observations and microscopic simulations[END_REF]. To calibrate the maximum acceleration a and comfortable deceleration b of vehicles, real-world driving data collected in French cities are used. This is particularly useful as very different values can be found in the literature for these parameters. In practice, the speed profiles of 648 journeys have been recorded at a sampling frequency of 1 Hz using the drivers' smartphones. In order to reduce the influence of outliers (unusual driving behaviors, other means of transportation such as motorbikes, measurement errors), the 1% highest accelerations and the 1% highest decelerations are filtered out, resulting in a = 2.4 m.s -2 and b = 2.8 m.s -2 . Note that the calibration of the comfortable deceleration b is not of primary importance in the sense that vehicles can always perform emergency brakings (up to 9 m.s -2 ) in order to avoid crashes, to stop at a red TLS, etc. Also, the impact of b on the vehicles' energy consumption and pollutant emission is smaller than the one of a, as vehicles' energy efficiency mostly depends on acceleration phases. The other IDM parameters are set to standard values, namely s 0 = 2 m, δ = 4, τ = 1 s, v 0 = 50 km/h [START_REF] Treiber | Congested traffic states in empirical observations and microscopic simulations[END_REF].

Additionally, microscopic physical energy and NO X models are introduced to evaluate the performance of the controller. These models calculate the wheel force and torque by using Newton's second law of motion and expressing the resistive forces. Based on the vehicle's speed and gear ratio, it is then possible to calculate the engine torque and speed. Finally, a two-dimensional map is used to obtain the fuel consumption rate as a function of the engine torque and speed. The NO X emission rate is calculated using a linear regression of the burned gas rate (obtained from another twodimensional map of the engine torque and speed) and the in-cylinder fuel mass per stroke and displaced volume [START_REF] Thibault | A new gps-based method to estimate real driving emissions[END_REF]. In this study, the average fuel consumption per vehicle and per distance, expressed in liters per 100 kilometers, is a metric of particular interest because minimizing it ensures a global improvement of the energy efficiency. It can be defined as the ratio of the energy consumption and the TTD. The objective function to minimize is the weighted sum of the average energy in each area, as follows

J 1→T = λ E 1→T A θ A T T D 1→T A + (1 -λ) E 1→T B θ B T T D 1→T B ( 9
)
where θ A and θ B are normalization coefficients, and λ is a weighting coefficient. The two terms are normalized because the fuel consumption rates can be very different in both areas due to the presence of TLS and to different speed limits. In this sense, the actuators seek to obtain a compromise of the energy efficiency between both areas.

To reduce the computational complexity due to the large size and the non-linearity of the problem, the control is updated every T cycle , and the optimizer finds a piecewise constant control over the prediction horizon T p . The parameterization of the controller is summarized in Fig. 3. For illustration purposes, the diagram is realized with T p = 3T cycle . In this study, we actually consider T p = 5T cycle .

B. Variable speed limits

In this approach, the control actuators correspond to the speed limits V i inside the urban area. They appear explicitly in equations 4a, 5, and 7 of the model. In order to reduce the number of control variables, and because of the high symmetry of the network under consideration, we propose to define two clusters of roads that have common speed limits. As indicated in Fig. 1, the first cluster is composed of the roads entering the urban area (speed limit V 1 ). The second cluster is composed of the inner urban roads (speed limit V 2 ). Hence, the controller solves the following problem at each control iteration k

Time k State k + T cycle k + 2T cycle k + Tp k + 1 Past Future • • • • • • • • • • • Time k Control k + T cycle k + 2T cycle k + Tp k + 1 u k δt T cycle

Tp

Fig. 3: Representation of the NMPC framework, for

T p = 3 T cycle . min V1,V2 J k→k+Tp (ρ, V 1 , V 2 ) s.t. ρ i (e + 1) = ρ i (e) + δt δx ϕ in i (e) -ϕ out i (e) , ∀e ∈ [k, . . . , k + T p -1] V 1, V 2 ∈ [20, 50] km/h (10)

C. Access control

In this approach, the control actuators are the access points between the peri-urban and the urban area, whose behavior is characterized by the duty cycle κ. This control actuator appears explicitly in equation 2 and in the expressions of flows in Table II. Due to the network symmetry and the fact that the exogenous demands are all equal, we propose to control all the access points with the same control variable. Therefore, the controller solves the following problem at each control iteration k

min κ J k→k+Tp (ρ, κ) s.t. ρ i (e + 1) = ρ i (e) + δt δx ϕ in i (e) -ϕ out i (e) , ∀e ∈ [k..k + T p -1] κ ∈ [0, 1] (11)
The fact that the controller optimizes the duty cycle κ, which takes continuous values between 0 and 1, rather than the binary function g leads to a nonlinear programming (NLP) problem and avoids a mixed-integer nonlinear programming (MINLP) formulation, whose complexity would be much higher.

IV. RESULTS

In this section, the simulation scenarios and results are presented.

A. Simulation scenarios

Three traffic scenarios are defined 1) Uncontrolled baseline: the accesses from the periurban to the urban area are permanently open. The speed limits are constantly equal to 70 km/h in the peri-urban area and 50 km/h in the urban area. 2) Controlled VSL: the accesses from the peri-urban to the urban area are permanently open. The speed limits are constantly equal to 70 km/h in the peri-urban, but they are dynamically optimized in the urban area, taking continuous values between 20 km/h and 50 km/h. 3) Controlled access: the accesses from the peri-urban to the urban area are controlled, but the speed limits are constant over the simulation duration (70 km/h in the peri-urban area and 50 km/h in the urban area). To satisfy the Courant-Friedrichs-Lewy (CFL) [START_REF] Courant | On the partial difference equations of mathematical physics[END_REF] condition, the time step duration δ t is set to 1 s. The control and prediction horizons of the NMPC strategy are respectively T cycle = 1 min and T p = 5 min. The weighting coefficient λ is set to 0.7 in order to privilege the environmental sustainability in the urban area as it is supposed to be denser with more traffic.

In the urban area, all roads are one-way, of equal length and capacity. At each intersection, both downstream roads have the same split ratio β i = 0.5.

We consider a simulation of duration T = 1 h during which the exogenous demand is set at d in = 0.2 veh/s and the intermittent endogenous demand is

d i (k) =      0.1 veh/s, if 6 min ≤ k ≤ 9 min 0.1 veh/s, if 31 min ≤ k ≤ 34 min 0 veh/s, otherwise (12)

B. Simulation results

The results of the three scenarios defined in Section IV-A are summarized in Fig. 4. The fuel consumption, the NO X emissions, and the TTD are calculated using the data of the microscopic traffic simulator SUMO and the microscopic physical energy and NO X models.

The average fuel consumption and NO X emissions presented in Fig. 4a & 4b indicate that the VSL controller is able to improve the energy efficiency both in the urban (18%) and in the peri-urban (14%) areas. Similar results are observed with NO X emissions, which are reduced by 18% in the urban area and by 13% in the peri-urban area. However, the access control approach only allows to improve the environmental sustainability in the urban area (fuel consumption reduced by 7% and NO X emissions reduced by 6%) in exchange for a deterioration of the performances in the peri-urban area (fuel consumption increased by 21% and NO X emissions increased by 19%). Fig. 4: Comparison of the energy efficiency, pollutant emission level, and traffic performance in the baseline, the VSL, and the controlled access scenarios.

Fig. 4c reveals that the impact of both controllers on the TTD is lower, which is conceivable because it is not explicitly the optimization criterion. The VSL controller improves it both in the urban (5%) and in the peri-urban (1%) areas. The access control approach deteriorates it in the urban area (2%) and does not affect it in the peri-urban area.

As a result, the VSL controller is more appropriate in this study because it gives better results in both areas in terms of energy efficiency, pollutant emissions, and traffic performance. In practice, both controllers aim at reducing the inflow in the urban area during peak times because the congestion deteriorates the energy efficiency. The difference is that the access control is much more discriminating towards vehicles in the peri-urban area as it forces them to stop and start, which is very energy consuming. On the other hand, the VSL controller leads to smoother density variations.

V. CONCLUSION

In this work, an NMPC approach has been employed to compare the effect on energy efficiency of two control actuators, namely the speed limits and the signalized access actuators. For the chosen parameterization and synthetic tworegion network of intermediate size, it appears that the VSL approach is more efficient. In fact, it reduces the global fuel consumption up to 18% in the urban area, against 7% for the access control which also decreases the performance in the peri-urban area. The reason is that VSLs leads to smoother density variations, which are more energy-efficient.

To go further, a scalability analysis with more complex and asymmetrical road networks could be of particular interest, especially in scenarios that combine the benefits of both actuators.

2 Fig. 1 :

 21 Fig. 1: Representation of the road network and the access control actuators. The urban area corresponds to the solid line roads and the peri-urban area is represented by the dashed line roads.

Fig. 2 :

 2 Fig. 2: Block diagram of the NMPC approach.

  (a) Average fuel consumption.