Alpine forests increasingly vulnerable to fire

As a result of climate change, the risk of forest fires in the French Alps has increased tenfold. Which regions and species are the most vulnerable? An Irstea study is addressing the question. 

Alpine forests provide a wide range of ecosystem services that are beneficial to all, including wood production, protection against rockfalls, leisure activities, biodiversity, etc. Ordinarily subject to small, low-intensity fires, these forests have been at risk of more frequent and more intense fires since the 1980s due to ongoing climate changes and a longer fire season (2 to 4 days more per decade). However, in contrast to Mediterranean species, those found in the Alps are often poorly adapted to resisting during or regenerating after a fire. How will these forest species be impacted by the intensification of fires over the coming decades?

A recent study by Irstea Aix-en-Provence analyzed 2,600 forest plots in the French Alps to answer this question. It provides forest managers with an assessment of the vulnerability of forests to fire and provides welcome support to help them in their choice of forest species.

Simulating future fires in the Alps

Irstea scientists simulated fire behaviors in 2,600 Alpine forest plots, based on severe weather conditions that are similar to those expected during the coming decades. Each plot was characterized by ground cover plants and forest composition and structure, all of which define fire characteristics. The simulations were used to estimate the intensity of the fire on the ground as well as the probability of propagation through the treetops, the main cause of tree death.

Fire intensity varies according to forest area

The simulations revealed that fires with the highest intensity will be located in the southern Alps, as well as in subalpine and high-altitude dry areas. The results also highlight the effect of dry autumns and wet springs which, by stimulating the growth of herbaceous plants and shrubs, increase fire intensity at ground level.

In the dense, humid centers of the alpine forests, fire intensity should increase while remaining moderate, particularly in beech, pine and spruce forests. In these large, dense forests, the lack of light will reduce ground plants and maintain humidity levels. As a result, fires are rarely intense enough to spread to the treetops, except in years with exceptional weather, such as 2003. Despite the fact that these species are poorly adapted to fire due to their thin bark, most of the trees would survive a low intensity fire.

In contrast, the hotter/lower altitude and drier/higher altitude margins of French alpine forests will fare less well. It is in these areas that fires can become very intense. The study suggests that climate change will produce changes to the fire regime.1 Low intensity surface fires will evolve into a mixed fire regime with surface fires and treetop fires.

The most exposed areas will be in the Southern Alp forests, single species or a mix of oak (particularly downy oak) and pine (Scots and black pines). These forests, found at low or medium altitudes, have an open structure with high levels of combustible plant matter close to the ground (forest litter, grasses, shrubs) and are regularly subject to long periods of weather favorable to fires. Treetop fires are frequent and can destroy the forest.

The driest forests of the internal Alps, populated by larch and Swiss stone and dwarf pines, are also particularly exposed at altitudes of over 1,500 meters. Located in areas that are dry and rich in herbaceous plans and shrubs, they are conducive to treetop fires. Larch should resist fire better than pines due to its thick bark and high treetops. However, the fragmented populations of dwarf pines and small Swiss stone pines would be more significantly threatened and affected by the fires.

The effects of climate change on fires and alpine forests will therefore primarily be felt in areas with a drier, hotter climate. Both low and high altitude oak and coniferous forests will be more vulnerable due to treetop fires. The study led by Irstea provides recommendations to help forest managers chose species best adapted to fire according to forest zones.

 

This study was carried out as part of a doctoral thesis by T. Fréjaville, and as part of the European FP7 FUME (no 243888) project and the CR PACA Vulnerability of Provençal forests to forest fires.

Fréjaville T., Curt T., Carcaillet C., 2018. Higher potential fire intensity at the dry range margins of European mountain trees. Journal of Biogeography, 1-13. DOI: 10.1111/jbi.13386

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1- A fire regime is described by the frequency of fires, their size, season, intensity and type (surface fire vs. treetop fire)