Understanding the vulnerability of tree species to gain a better understanding of fire risk

Climate change is radically transforming the risk of wildfires in France. The models are clear: the area burnt is expected to increase by a quarter by 2030 compared with the period 2001–2020, whilst the fire season is gradually lengthening, spreading beyond traditionally vulnerable areas into new territories — as far as the Centre-Val de Loire region and to higher altitudes in the Alps and Pyrenees.

In this context, gaining a better understanding of what makes a forest stand more or less vulnerable to fire is becoming essential for managers. This is the aim of the VULNEFEU project, led by researchers from INRAE (the URFM unit in Avignon and the LESSEM unit in Grenoble), in collaboration with the ONF, the Department of Forest Health and the INRAE ETTIS unit in Bordeaux, as part of the AFORCE programme.

A three-part definition

The project is based on a definition of forest vulnerability comprising three components:

• the flammability of a stand, that is to say, the ease with which it catches fire and the amount of energy released,
• its resilience, that is, the ability of trees to survive a fire,
• and its resilience, or the ability of forests to regenerate after being damaged.

This breakdown enables a more detailed analysis of risk depending on the scale under consideration — that of the individual tree or that of the stand as a whole.

From understanding fire to preventative management

The team drew on scientific publications, technical reports and expert opinions to cover the entire spectrum of risk understanding: recent trends in wildfires in France, the mechanisms underlying fire behaviour, the response of different tree species to flames, and preventive management strategies.

On this last point, the project distinguishes between two main approaches to preventive forestry: strengthening forest fire defence systems (DFCI), and investigating the inherent resilience of stands in the absence of external intervention. This work has, in particular, resulted in species-specific fact sheets. Designed as a practical tool that can be used directly by forest managers, they assign each forest species a rating for flammability, resistance and resilience to fire.

Our understanding is still incomplete

To document the vulnerability of tree species, the team conducted a survey of twenty-eight experts in fire ecology. This work also highlighted its own limitations: current knowledge remains largely focused on Mediterranean tree species, which have historically been the most exposed. However, the risk of wildfire is now spreading northwards, exposing temperate tree species for which there is far less research on their behaviour in the event of a fire. Filling these gaps is one of the team’s future priorities, alongside developing research into the combustibility of stands and optimising preventive management strategies at the landscape scale.

Partners: ONF, Department of Forest Health

INRAE units: UR URFM, UR LESSEM, UR ETTIS