Preparing the future forests of the Mediterranean

Mediterranean forest © Irstea
Mediterranean forest © Irstea

With increasing temperatures and decreasing rainfall in spring and summer, the Mediterranean region has been particularly affected by climate change and is even more vulnerable to fire. Between 1998 and 2017, the region suffered from several exceptionally dry and hot years. Irstea scientists are analyzing the response of forest ecosystems to climate change and fire to prepare the forests of the future.

Predictions based on 25 years’ study of the Mediterranean ecosystem

Between 1995 and 1998, Irstea researchers set up a plant observation system in Provence. Comprising 325 sample plots (specifically located and defined areas used to perform an inventory of plant species), the system was used to create a database to calibrate a model simulating plant evolution in relation to changes in climate and soil. This predictive tool is used to monitor the way in which Mediterranean flora has adapted to climate change, thereby helping managers and policymakers.

In 2008, 10 years after the first measurements were taken and following a period of severe drought and heat that took place between 2003 and 2007, researchers launched a new field campaign across the various sites. This made it possible to confirm the accuracy of Irstea’s model of plant behavior in relation to water availability, revealing variations of 5 to 25% in the diversity of flora, with the most water-intensive species partially replaced by species more resistant to heat and drought. “This field observation reinforced the hypothesis that these differences, which are more pronounced in cool and humid environments, are due to climate change. A new inventory in 2019 will refine these initial results, and long-term monitoring has been scheduled. Across other similar systems, the major drought of 2015-2017 caused a new type of massive dieback in the driest and hottest areas where conditions became unbearable, even for the most resilient species that had been spared until then,” specifies Michel Vennetier, forest specialist at the Irstea Aix-en-Provence center.

To deal with climate change, trees have two options: migrate by distributing seeds beyond the extremities of their usual territory towards milder environmental conditions, or adapt to the new environment. This adaptation involves advancing the start of their growing period to early spring along with their flowering and fruit maturity while increasing the depth of their roots and reducing their height, the size of their leaves and the diameter of sap vessels, for example. If a species is unable to migrate or adapt, it will become either partially or totally extinct in the local area. “Scots pine are particularly suffering. Approximately 50% of this species’ stands are in decline. Cork oak, white oak and fir trees are also experiencing a significant downturn in the Mediterranean region. However, the holly oak and Aleppo pine are doing well,” clarifies Vennetier. Tree mortality nevertheless remains a difficult process to understand within a forest ecosystem as it is generally affected by a combination of several causes: heat, cold, thirst, hunger, attacks (insects, fungi, illness), and sometimes even inappropriate management.”

Mortalité massive Chênes Kermès
Massive mortality of kermes oak and rosemary in 2016 © M. Vennetier - Irstea

 

Pins d'Alep gelés au printemps 2012
Frozen Aleppo pines in spring 2012 © M. Vennetier - Irstea

 

Dépérissement pin sylvestre Alpes du Sud
Scots pine dieback in the Southern Alps © M. Vennetier - Irstea

 

Pins sylvestre et canicule
Impact of a heat wave on Scots pines © M. Vennetier - Irstea

 

 

L’impact du changement global sur la végétation

Le changement global a un effet bénéfique et néfaste à la fois : la hausse du CO2 atmosphérique peut favoriser temporairement la croissance des arbres en facilitant la photosynthèse et en limitant les pertes d'eau, mais contribue au réchauffement climatique. Une température plus chaude allonge la saison de croissance mais peut accroître aussi la durée et l’intensité de la sécheresse en été et le risque de dégâts de gel en automne et au printemps, avec des effets défavorables. L’accessibilité de l’eau conditionne la composition de la flore méditerranéenne et sa répartition. La flore est inévitablement confrontée à des périodes plus longues de stress hydrique

 

The impact of global climate change on vegetation

Global climate change has both a beneficial and detrimental effect: increases in atmospheric CO2 can temporarily encourage tree growth by facilitating photosynthesis and limiting water losses but also contribute to global warming. Higher temperatures lengthen the growing season but can also increase the length and intensity of any summer droughts and the risk of frost damage in autumn and spring, with adverse results. Access to water determines the diversity of Mediterranean flora as well as its distribution. Plants are inevitably confronted with longer periods of water stress.

Repeated fires and droughts threaten Mediterranean forests

Fire is one of the most important disturbances that can affect Mediterranean forest ecosystems, with 600,000 hectares burnt each year (10,000 ha in France). In addition to the extent of burnt plots, scientists are focusing on the impact of fires on the behavior of plant populations and the ability of the ecosystem to regenerate. Through the IRISE1 project, coordinated by Irstea, we now know that forests are not destroyed by single isolated forests but rather by too many fires.

The increase in frequency and length of drought episodes, such as those suffered between 2003 and 2007 and then between 2015 and 2017, combined with a significantly high number of fires, is leading to a collapse in the biological operation of the ecosystem. Persistent drought after a fire will slow down, or even stop, forest regeneration. Similarly, the impact of a fire on an environment that has recently suffered a prolonged drought is exacerbated. Four successive years of drought constitutes a critical threshold in a forest’s resistance to fire. By intensifying this combination of fire and drought, climate change is weakening these ecosystems.

50 years to erase the effects of a fire

Fire
Forest fire in the Massif des Maures mountains in 2007 © R. Valentin - Irstea

In areas that regularly suffer from fires, biological activity in the soil is concentrated in the first few centimeters of soil, which is exposed to combustion and erosion and where most of the organic material is found. After a fire, most of the physico-chemical parameters of forest soils quantitatively return to their initial levels within 15 to 25 years. However, it takes 50 years to see any overall resilience in the ecosystem. Below this threshold, essential elements of the regeneration process such as bacteria communities and soil fauna are less diverse and less active. It is only after 150 or 200 years without fire that a significant increase in carbon stores in the soil can be observed, along with an improvement in the structure and diversity of the vegetation.


Although it only takes one fire to interrupt this recovery process, in the long run, this does not compromise the forest’s ability to regenerate. This ability is also not affected by one or two additional fires over a 50-year period. However, a fourth fire during this period, or two fires occurring within a short period of each other (i.e., less than a 10-year interval), can be fatal. At the “4th fire” threshold, species and communities essential to ecosystem operation become increasingly rare and there is a drop in the amount and quality of stored organic matter. Thus, although this fire will have caused high emissions of CO2, the forest will no longer be able to fulfill its role as a carbon sink, which in turn contributes to the greenhouse effect.

This knowledge makes it possible to identify priorities in the management of Mediterranean forests: areas that have suffered several recent fires, which yet another fire could degrade irreversibly, are considered a priority, in contrast to forests that have not burned in several decades and are therefore more resilient. The all too rare ancient forests (older than 150 years) must also be protected at all costs. Given the importance that levels of organic matter play in forest resilience, the addition of compost to restore the fertility of forest soils and the dynamics of the environment should be considered in areas most weakened by fire or drought.

In order to prepare all forests, and Mediterranean forests that have already suffered significant dieback in particular, for the effects of global climate change, scientists recommend clearing, rejuvenation and diversification of tree populations. “Clearing to encourage young trees helps them avoid competition and removes old trees that are more sensitive and less capable of adapting to changes. Diversification makes it possible to plant or encourage species that will ensure the future,” concludes Vennetier.

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1 - For 3 years, this multidisciplinary project brought together scientists from 3 research institutions (Irstea, CNRS, INRA) and 3 universities in the Aix-Marseille and Lyon region. The IRISE program (2005-2008) was undertaken due to the European Forest-Focus Regulation, overseen by the French Ministry of Agriculture with support from the federal research institute Pole Mediterraneen des Sciences de l’Environnement [Mediterranean Center of Environmental Science].