PAP 55: The multiple roles of forest edges in rural landscapes

assets for agroecology

Marc Deconchat, février 2022

Le Collectif Paysages de l’Après-Pétrole (PAP)

Under the effects of climate change as well as voluntary decisions, rural landscapes will change significantly in the future. The choice of varieties and crop associations, rotations, but also new crops or non-agricultural uses of land, new land uses will be established. These changes will redefine the size of plots and their distribution in the landscape, modifying the delimitation of spaces and the way in which humans use their contact zones. Within these landscapes to be invented, the discontinuities between the different land uses will present new characteristics.

Together, they will form zones with certain ecological and socio-technical characteristics that will have an impact on the agriculture of tomorrow. As a space of interaction between forest environments and other environments, notably agricultural, the forest edges will have a particularly important place in these new landscapes. Sheltering a particular biodiversity but also threatening certain species, forest edges are at the center of many issues in rural landscapes. Edges are interfaces between the forest and agricultural worlds. They are the points of passage for exchanges between these environments, the object of negotiation between the actors concerned by their management, and also a reservoir of resources for the future. The study of forest edges shows the importance and complexity of the ecological and social processes they induce. However, they remain poorly understood.

Forest edges and their particularities

Natural edges are induced by soil discontinuity: rocky outcrops or a water table prevent the expansion of a forest and thus create an edge. More transient edges form after a fire or landslide. In rural temperate landscapes, most forest edges have been defined by human activities: in Europe, it is essentially the ancestral confrontation between forests and agricultural land, either cultivated or grazed, that has drawn these edges. Land appropriation determining the fixed limits of a parcel system has also contributed to establish the discontinuities that shape the mosaic landscapes found in many parts of Europe.

In other parts of the world, these processes of differentiation and structural fixation have been less prominent. The resulting landscapes have more transition zones and less marked discontinuities. For example, the transition from savannah to dense forest in West Africa is very gradual without it being easy to distinguish a clear discontinuity. In the same way, the boundaries tend to become blurred in landscapes undergoing agricultural abandonment due to the natural extension of wooded areas. The wastelands and natural thickets form transitory and intermediate occupations on the ground between purely agricultural and forest environments, making it difficult to identify a discontinuity and an edge zone.

Place of edges in landscapes

The diversity of terrestrial landscapes is due to the morphology of the landforms, with their different types of vegetation, and to the way in which the human economy has settled on these configurations. There are primary forests, others exploited by gathering, and others entirely managed by man. On the portions of the land where man has installed agriculture, the interface zones with these forests and the multiplicity of cultivated land use modes generate as many edges. These confrontation zones occupy significant areas on a global scale. The most recent estimates are based on satellite data: nearly 20% of the forest area is within 100 m of an edge, 70% within one kilometer, with these values increasing due to forest fragmentation and deforestation (Haddad et al. 2015). In Europe, the Joint Research Centre (JRC) estimates that 40% of forest area is within 100 m of other land uses, and 60% of edges are in contact with intensive land uses (Estreguil et al. 2012).

In France, the National Forest Inventory estimates that 50 m of linear edge can be counted for each hectare of forest, which gives 805,000 km of edge for metropolitan France, slightly increasing in recent decades, including 25,600 km for state forests (Drège 2006). The CAP defines edges with cultivated plots as areas of ecological interest that are included in the environmental accounting of farms, but there are no data available on them.

Effects of edges on biodiversity

An edge alters the ecological characteristics of adjacent environments and the distribution of plant and animal species that define local biodiversity (Ries et al. 2004). Reference is often made to an « edge effect », the impacts of which are assessed differently in the scientific literature. In connection with the fragmentation of spaces, some contributions present the edge as a threat to biodiversity. Others emphasize its richness in terms of flora and fauna. This divergence corresponds to two levels of analysis that do not focus on the same processes. Large areas of habitat are suitable for species that prefer to remain under the cover of trees. Fragmentation of these habitats into smaller, more dispersed areas creates as many edges but reduces their living space, which is a threat to the most fragile biodiversity. Conversely, when we analyze the local influence of the edge portions and when the edge is old enough, we find a juxtaposition of species specific to the two adjacent environments, as long as these species tolerate this proximity, as well as species inflicted on the intermediate environments. The result is a biodiversity that can be different and richer in the edges than in the adjacent environments. It is often made up of species that are not very demanding and more ubiquitous than those that belong to the core habitats of the environments. The edges are therefore locally rich in species, but rather common.

The edges can shelter a biodiversity whose various species are brought to circulate because of the complementarity of the environments that it articulates. These biological flows between very different environments, such as forests and crops, contribute to complex ecological functions that remain poorly understood because of the methodological difficulties in analyzing them. Fields bordering woods are often exposed to game damage: various recent studies clearly show that ungulates have particular vigilance behaviors when they cross an edge, the boundary between a fairly poor but protected environment, the forest, and a rich but risky environment, the crops. Moreover, pollinating insects regularly cross edges and find refuge there at key moments in their life cycles, as do carabids, predators of the main plant pests (Roume et al. 2011). Proximity to edges thus provides some positive effects for agriculture due to the uses of these species. These favorable influences of edges are similar to those of hedgerows, and more broadly of agroforestry.

The effects of edges for ecologically intensive agriculture

The development of rural areas that aim to preserve biodiversity and to develop an agro-ecological agriculture that mobilizes the benefits of natural processes benefits from taking into account the effects of edges. Several recent results show that the total length of edges in a landscape is a reliable indicator of a good level of ecosystem services regulating the functioning of cultivated plots. Landscape heterogeneity is thus an asset for promoting natural processes beneficial to agriculture. Heterogeneity of configuration, involving the most interfaces and edges, appears to be more important than heterogeneity of composition, defining the types of land use. Smaller area of agricultural plots and more contact with non-agricultural environments are key factors for productive agroecological landscapes using the services of preserved biodiversity (Sirami et al. 2019).

A wide range of conditions can influence the ecological processes present in edge diversity. It would be useful to know more about this variability and its effects to be able to identify edge types that are favorable to certain desired functions and to be able to propose recommendations, developments, and management practices capable of limiting undesired effects. This variability of edges has been little studied and remains poorly known quantitatively. The factor most frequently used to characterize edges is the structure of the vegetation. Edges are described schematically as strips of vegetation arranged on either side of the boundary between forest and non-forest environment. One thus speaks about grassy hem, shrubby then raised, their composition and their structure evolving jointly. Depending on the presence, absence or size of these different strips, we distinguish between abrupt edge facies, where the transition from open to wooded areas is almost seamless, and gradual edges, where different vegetation strips follow one another from one area to another (Crémer et al. 2010). Steep edges are considered less favorable for biodiversity than gradual edges that offer a greater diversity of ecological conditions. Moreover, the transversal variability of the vegetation is composed with a longitudinal variability: straight edges would shelter less biodiversity than sinuous edges. It should be noted, however, that because of the tendency of tree vegetation to establish itself on a continuous front in open spaces, gradual and sinuous edges tend to become spontaneously steeper and straighter if the vegetation is left to develop without regular intervention. Maintaining gradual, meandering edges requires voluntary interventions to limit tree expansion.

The age of the edge, i.e., the length of time since its creation, is an important differentiating factor, linked to the dynamics of the vegetation as it adapts to soil conditions and the availability of light and water. In the long term, the structure of the vegetation can affect the permeability of the edge. In general, gradual edges are assumed to be more permeable to a large number of species than steep edges where the vegetation may form a wall that is difficult to cross, shaped by the development of vegetation that takes advantage of the high light availability. This characteristic depends on the species, their capacities and modalities of movement. The permeability of the edge may or may not be a desired characteristic, depending on the ecological processes targeted. The nature of the environment adjacent to the forest introduces a variability that can significantly affect the ecological functioning and management of an edge. Edges with cultivated plots often present a steep structure leaving little room for a grassy hem, with no shrub transition, and with trees whose crowns are pruned to prevent them from shading the crops or hindering the movement of machinery. In contrast, edges with grazed grasslands are often more progressive, with the crowns left in place so that they overhang the grassland, providing shelter and forage resources for the animals.

In the northern hemisphere, south-facing edges receive more light and thus energy than north-facing edges, which influences both the vegetation and the shelter capacity of the edges. We have shown that in the hillsides of Gascony, hoverflies, pollinating and predatory insects, find refuge in the southern edges during the winter and in the northern edges in the heat of the summer, thus benefiting from conditions favorable to their survival (Arrignon et al. 2008). When we look at the edges, we see that other differentiating factors are involved, rarely considered in the literature. These are micro-topographical elements such as erosion slopes, paths, ditches or low walls that materialize and arrange the divisions into parcels. Far from being anecdotal, the ecological roles of these frequent elements in European landscapes can be major, notably by creating a perennial micro-variability from which certain species can benefit. For example, the slopes that are gradually formed by the difference in erosion rates, which are faster in the open part than in the forest, are known to provide suitable roosts for solitary bees that nest in burrows (Carrié et al. 2017). Traditional landscapes are made up of small patches that follow the differentiations in soil level and configuration. They offer a diversity of environments conducive to that of the living. By redeveloping an agriculture that knows how to bring these interactions into play, the cultivated environment will benefit from the contributions of its close environment.

The scientific study of agroecological experiments on edges

The effects of edges vary greatly from one situation to another. Their study in the field requires the consideration of many factors. Theoretical work has proposed general typologies of edges based on the structure and morphology of the environments they articulate. We can mention the very complete work of Strayer et al (2003) which proposes a classification of ecological boundaries based on these criteria. In addition, other works aim to characterize edges from satellite or airborne spatial data (Grybas and Congalton 2021).

An important characteristic of forest edges is the depth of their effect. Much of the scientific literature focuses on its study. The presence of the discontinuity, arbitrarily materialized by an edge line, induces for some variables different effects related to the distance to this line. The edge is defined as the whole area where a given characteristic evolves in relation to the distance to the edge. The areas where this characteristic varies independently of this distance correspond to the hearts of the adjacent environments. It is on this theoretical basis that many studies have sought to measure the size of this edge zone, i.e. the depth to which variables of interest are statistically related to distance from the edge. The magnitude of the edge effect measures the amount of variation induced by the edge. This variable is important because it potentially defines which parts of the environments are under mutual influence and which are isolated. It is also essential for mapping and quantifying edge extent. The effects of forest fragmentation are dependent on this depth: the greater the depth, the deeper the influence of the external environment penetrates into the fragments and the less untouched space remains where sensitive species can shelter. As a result, depending on the depth, some fragments that are too small or with loose shapes may be composed only of edge zone and have no interior environment.

The depth of edge effects varies with many variables. Wind, for example, has a deeper effect than light entry. It is therefore difficult to define a single edge effect distance. Also, gradual edges generally induce greater depth than steep edges. The orientation or slope of the land can also have very significant impacts. Regarding vegetation composition, one would expect it to vary with distance, depending in particular on the light gradient. While this has been observed in some situations, it does not appear to be systematic. It is also likely that « negative » results showing no vegetation response were more frequently discarded from the scientific literature than those that supported this hypothesis.

In order to establish comparable metrics, it was sometimes necessary to set a depth of edge effect on both the forested and open sides. For convenience, this depth is generally close to the height of the dominant trees in the edge, which in France is between 20 and 30 m. The JRC has thus fixed the depth of the edge at 25 m for its estimates at the European scale, which also corresponds to 1 pixel of the spatial data used.

Edge management and the future of rural areas

The edges are places of social and technical interactions between different actors. Important issues are concentrated there. Forest edges are generally the result of a human intention. A structure built more or less voluntarily has been maintained over time for different reasons. Forest edges have many points in common with hedges, linear tree structures created and maintained by humans for multiple uses.

Maintaining forest edges involves regular maintenance and therefore constraints. Without maintenance, trees and other woody plants of the forest environment naturally tend to colonize the available open space on the surface, in the ground and in the air. Tillage in crops and grazing by domesticated herbivores contribute to this maintenance, but are often not sufficient. Several types of edge maintenance are possible, depending on the equipment available and the objectives, but they are poorly known because they have not been studied. The forms of this maintenance induce very visible differentiations according to whether the edges are with crops or with meadows. Qualitative socio-technical surveys of farmers have highlighted the perceived importance of the maintenance of edges, and more broadly of wooded areas, in the tasks imposed by current agricultural techniques (Blanco et al. 2020). However, an evaluation remains to be done to quantify what this workload represents and how it is actually carried out. Edges, such as hedgerows, play a special role in fuelwood harvesting. They must be maintained regularly to contain their development. They are more easily accessible than intra-forest patches, and trees are thought to grow faster there than in the forest interior (Li et al. 2018; Albiero-Júnior et al. 2021). Therefore, edges are maintained by longitudinal and shallow cuts. Due to the small area involved, these cuts are poorly quantified by aerial inventory methods. As they do not require prior authorization and are often part of a family or neighborhood economy, they remain invisible in traditional statistics even though they constitute a significant contribution to the domestic economy. Today, these practices are relatively anecdotal, but they will undoubtedly develop more widely in the future, when the need for biomass to replace fossil fuels will increase. The use of shredded wood will become widespread for heating, for livestock bedding and for soil enrichment. It will be included in the farm’s accounts.

Today, the edges are neither part of the agricultural nor the forestry world. In the current disciplinary, political and technical division, edges are intermediate, marginal objects, which rarely receive attention. Territory managers consider them as a result of the heterogeneity of landscapes linked to the juxtaposition of different land uses. In fact, the edges are the concrete place where the competition between forestry and agricultural issues for land use is materialized. The retreat of forests in some places, and their expansion in others, displace the historical edges. These evolutions are directly linked to the social and demographic dynamics of the territories. In order to think about the new balances to be expected between agricultural and forestry production in a human economy impacted by ongoing climate change, and to anticipate the adaptations to be expected in terms of reducing CO2 emissions or capturing carbon from the various agricultural systems, an overall vision weighing the contribution of the various land uses to a revisited economy will become necessary.

Edges are common in many landscapes around the world. Their origins are diverse, their characteristics complex and their ecological consequences, particularly on biodiversity, far from being unambiguous. Although they occupy a relatively small area in landscapes, they have important roles that it would seem useful to know more about in view of the considerable changes that climate change will impose on our production systems. In order to integrate the theme of edges in the reflections on future development of rural territories, we currently lack knowledge about them. Another obstacle is the difficulty of identifying their managers. In order to design the landscapes of the future, it will be necessary to know how to trace these lines of force that structure the space and mark its different compartments as well as their interactions. In the near future, post-oil landscapes will be able to give them new roles and new meanings.

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