Le "grain bocager" :a tool for better managing agricultural hedges
Hedges, far more than simple field boundaries, are fundamental elements of agricultural landscapes. They provide many services: microclimate regulation, shelter for wildlife, erosion control, water quality improvement…
Yet, in France, the network of hedges continues to decline rapidly: an estimated loss of 23,500 km of hedges per year. The causes are well known: parcel enlargement, inappropriate maintenance, aging woody plants not renewed.
This decline contributes to impoverishing landscapes and fragmenting habitats, which strongly affects biodiversity. Over the last 25 years, there has been a 33% decrease in birds in agricultural areas, and in Europe, 76% of flying insects have disappeared in 27 years.
To better preserve and manage these hedges, it is essential to be able to measure their role in the landscape. This is precisely what a new tool allows: the bocage grain.
Definition of bocage grain
The bocage grain is an index that describes the structure of hedges in an agricultural landscape. It is not just about counting hedges, but about analyzing their arrangement, their height, their type (hedges, groves, trees isolated), as well as the distance separating them.
The more numerous, close, and tall the hedges are, the finer the grain is said to be. Conversely, when they are rare and distant, the grain is called "coarse." This notion of "grain" is essential to understand the cumulative effect of hedges on their environment: microclimate, biodiversity, ecological connectivity, etc. For example, close and parallel hedges can create a buffering effect over an entire field (shade, windbreak, humidity), while scattered hedges will have a more limited effect.
A precise calculation method
The 3D bocage grain is calculated from data provided by the National Geographic Institute (IGN), notably the Canopy Height Model (CHM), which indicates tree height.
The method includes three steps:
- Identify wooded elements: hedges, isolated trees, groves.
- Assess their zone of influence, taking into account their height and type. A formula calculates an effect distance around each element.
- Average the results within a radius of 250 to 300 meters (sliding window), to obtain a global index at the scale of a landscape or a parcel.
Thus, the territory can be mapped into zones of finer or coarser grain, according to the density and cumulative effect of hedges.
Interpretation and value classes
Bocage grain maps are generally presented in four classes, based on biodiversity data (e.g., carabid beetles in Brittany):
- Fine grain (blue) : areas rich in close and tall hedges, favorable to forest biodiversity. Note: excessive closure can also limit some uses.
- Functional grain (green) : balanced, well-connected network. Hedges should be maintained in good condition.
- Potential grain (yellow) : degraded but improvable network. Replanting or restoring hedges would restore their ecological function.
- Coarse grain (white) : highly fragmented network. Reconnecting hedges is a priority to restore ecological continuities.
A threshold effect has been observed: the presence of forest species becomes significant when the bocage grain is below 0.33, but this is effective only if hedge quality is also good.
Concrete applications in the field
The bocage grain is already used in several regions, and it can serve at many scales:
- At the scale of a territory (municipality, watershed, etc.) : it enables ecological diagnostics, targeting restoration areas, and guiding bocage recovery projects (e.g., SAGE of Bas-Léon).
- In urban planning and land management : it is used in PLUi to identify areas to preserve, strengthen, or reconnect.
- On farms : it feeds Sustainable Hedge Management Plans (PGDH), helping to choose the best areas to plant or rebuild an effective bocage network.
- In local outreach : bocage grain maps serve as educational tools in workshops with elected officials, farmers, citizens, or technicians, to jointly define coherent actions.
A tool to better manage hedges and the landscape
The bocage grain allows very concrete visualization of the effects of hedges on microclimate and biodiversity. It can guide technical choices on farms, facilitate access to planting subsidies, and strengthen agroecological transition initiatives.
It is an ecological treasure map: each color indicates the current quality of the landscape and the strategies to adopt to improve it. Thanks to this tool, it becomes possible to think of the hedge no longer as a constraint, but as a lever for production, resilience, and biodiversity.
Sources
Boussard, H., Meurice, P., Rolland, D., Baudry, J., & Commagnac, L. (2023). The bocage grain: an index to characterize bocage and its microclimatic effects for forest biodiversity management – Concept, method and applications.
