Repeated Fire And Drought: A Menace For Mediterranean Forests

Fires represent one of the most serious disturbances to Mediterranean forest ecosystems, where 600,000 hectares burn every year. However, it is not so much the size of the burnt zones that worries the Cemagref researchers as the impact of fires on plant communities and the capacity of the ecosystem to regenerate itself.

Thanks to the IRISE program, coordinated by Cemagref, we now know that forests are not destroyed by a single fire, but when the frequency of fires is too high. This three-year, multi-disciplinary project brought together scientists from three research institutes (Cemagref, CNRS, INRA) and three universities from the Aix-Marseille and Lyon regions. It was possible to determine the critical frequencies above which the ecosystem is no longer balanced and its regenerative capacity is reduced. Key mechanisms involved in the regeneration or collapse of fragile ecosystems were identified. Taking advantage of the reduced rainfall from 2003 to 2008, the scientists also studied the interaction between repeated fires and droughts.

50 years to repair the effects of a fire

In areas regularly subjected to fire, biological activity in the soil is concentrated in the first centimetres where most of the organic matter is found and is exposed to combustion and erosion. Following a fire, most physico-chemical parameters of forest soils return to their initial levels after 15 to 25 years.

But a full 50 years are required before the ecosystem regains is overall and qualitative resilience. Before the 50 years have elapsed, the bacterial communities and soil fauna, essential elements in the regenerative process, are less diversified and active. What is more, after a fire, the organic matter contains high levels of poorly degradable or toxic substances that are likely to partially inhibit the biological activity of the soil. It is only after 150 to 200 years without a fire that the carbon stored in the soil increases significantly and the structure and composition of the vegetation improve.

The first and fourth fires are critical

A single fire is sufficient to interrupt the restoration process, however it cannot compromise the long-term regenerative capacity. The latter is also not affected by one or two additional fires over a 50-year period. However a fourth fire over the same period, or two fires within a very short time span (less than ten years) can be fatal. If a fourth fire occurs, species and communities that are essential for ecosystem operation become more rare and the stock of organic matter is reduced in both quantity and quality. The fire releases a large quantity of CO₂ and the forest can no longer play its role as a carbon sink, which, in the end, contributes to the greenhouse effect. The plant community changes and the forest can gradually give way to shrubs and bushes.

When drought chimes in

An increase in the frequency of dry periods, as in 2003 to 2008, combined with a high frequency of fires, leads to a collapse of the biological operation of the ecosystem. A long dry period after a fire slows or even stops regeneration of the forest. Similarly, the impact of a fire is greater on an environment that has recently suffered a long dry period. Four successive dry years would appear to constitute a critical threshold in the resistance of forests to fire. Climate change, by intensifying the combination of fire and dryness, can only increase the fragility of ecosystems, which makes it difficult to foresee their condition over the mid and long term.

Taken together, the above work provides the means to formulate priorities for the management of Mediterranean forests. Zones that have suffered a number of recent fires and that could be irreparably damaged by another fire must receive priority attention, before other forests that have not burned for decades and are more resilient. The rare old forests (over 150 years) must also be protected at all costs. Due to the importance of the stock of organic matter in soil for forest resilience, the adjunction of compost to enhance the fertility of forest soil and environmental dynamics could be a solution in the most fragile zones. Experiments have been launched in order to assess the effectiveness of such a policy.

(1) The IRISE program (2005-2008) took place thanks to the European Forest-Focus regulation, via the French Agriculture ministry and with the support of the "Mediterranean centre for environmental sciences" federative research institute.