In the Pyrenees, a significant revegetation process can be observed, characterised by the progression of layers of bushy vegetation and forest.

During the past few decades, changes in vegetation have in some cases led to a certain homogeneity in the vegetation cover, and yet in other cases have given rise to increased heterogeneity.

Homogeneity mainly occurs in forests which are already in place and which become denser if not harvested, while heterogeneous processes tend to occur on uncultivated land on which a wide variety of vegetation thrives, depending on the period during which the land stopped being cultivated, the way it was managed and natural conditions.

The uppermost treeline and the alpine area in Europe are in the process of shifting towards higher altitudes. In the case of trees, shifts to higher altitudes must be analysed with caution, because they may be related to pasture abandonment in the uppermost regions; the lack of any cultivation in these pastures paves the way for the return of forest vegetation to these areas now left free. This in turn leads to a return to the natural upper treeline, such as it existed before human activity intervened (up to 300 m higher than the current level).

However, at medium altitude, the changes to the forest treeline are not as rapid and have been marked by a gradual “densification” throughout the previous century, caused by pasture abandonment and accelerated by climate change.

By far the most restrictive factor, the lack of water could reverse the trend and cause a decline in the productivity of French forests by around 2050. Peak heat levels combined with more frequent, severe periods of drought could lead to a reduction in the productivity of forests by amplifying the water stress to which trees are subjected. The effects can already be felt in the southernmost zones of the areas in which certain species are found – reduced growth and decline have already been observed in these zones, due partly to insufficient water and high temperatures in the summer months (Jump et al., 2006). These major changes in ecosystems can have a negative impact on the wide range of ecosystemic services provided to society by its forests (protection against natural risks, repository for plant and animal biodiversity, regulation of the quantity and quality of water resources and harnessing and storage of CO₂ from the atmosphere).

Management is the primary lever for helping forests to adapt to climate change: by acting on the composition, structure, density, etc. of forest populations, forest managers will be able to help forests adapt to these future mutations, for example by limiting competition between trees (for water resources or soil nutrients, etc.) or by enhancing a community’s degree of resilience.

Forests have the ability to limit the triggering of such phenomena (by stabilising the land through the root system, reinforcing the blanket of snow in areas where avalanches develop, limiting surface runoff, etc.) and/or to reduce their impact (slowing down, channelling or stopping rock falls on slopes of average gradient, etc.). The impact of climate change on the protective role of forests therefore needs to integrate two approaches: the natural dynamic of forest environments and the biotic and abiotic risks that can affect them.

Indeed, all the plant formations and tree varieties found in forests do not offer the same degree of “protection” against natural risks. Climate change can therefore have an influence on the way in which vegetation cover will evolve but can also lead to a whole series of factors which do not depend on the natural dynamic of the vegetation, thereby altering the protective role played by forests.

As a result, the way in which these mountain slopes are managed, particularly as regards the vegetation that grows there, can affect forests’ ability to control an unforeseen event. The time scale involved in forest management means that forest managers have to be particularly vigilant and must have access to state-of-the-art diagnosis, management and communication tools, in a context marked by an increasingly high level of social demand regarding the protective role of forests. 

The most telling parameter used as an indicator for climate change is defoliation, i.e. an estimation of the rate of leaf loss for the trees studied (regardless of other contingencies that may affect them). In addition, we have access to a fairly large amount of historical data for this parameter (1997).

Trends indicate that several signs of deterioration are starting to become apparent: defoliation and branch mortality in the upper part of tree crowns.

We can therefore conclude that defoliation is on the increase, especially in the Mediterranean area but also in the Mediterranean part of the Pyrenees.

The main determining factors identified are variations in water supply throughout the year in question as well as during the two preceding years: precipitation and difference between precipitation and evapotranspiration. This confirms the prominent role played by climate change and its presumed impact on forest ecosystems in the long term, if the pattern of rainfall and temperature variations alters.

In addition to the deteriorating health of trees, an amplification of the trends identified might lead to a worryingly high mortality rate among certain species that are less well adapted to change in the medium and long term, thereby modifying the range of flora found in the ecosystems, with a shift in habitat further northwards or to higher altitudes.

However, we must not overlook the considerable genetic variability of trees, which is an important asset in the context of climate change. Finally, anthropogenic actions may help us prepare for the expected changes by adapting forestry practices even now, and modifying our choice of tree varieties for replanting in the years to come. 

At present, the vast majority of forest fires in the Pyrenees occur in the winter months (due to dry vegetation resulting from very low temperatures). The risk of summer wildfire mainly concerns the eastern edge of the mountain range. The summer period is also synonymous with pasture management operations, particularly stubble burning.

In this respect, then, a shift towards a more continental climate in the Pyrenees (or even a more Mediterranean climate in certain sectors), combined with a drop in the amount of snow cover and an increase in the availability and continuity of combustible material (mainly due to a decline in farming), is likely to increase the risk that major wildfires will break out and spread, both in the winter and summer months, depending on the sector.

Effective management of the environment (foresters and farmers), monitoring, risk prevention actions adapted to the specific context of the Pyrenees and a reinforced risk culture among all Pyrenean stakeholders are essential means of preventing the outbreak and spread of major forest fires in the future, whilst limiting their impacts on the environment and the social and economic stakes inherent to our regions.

Forests and forest management must be considered from a long-term perspective. The decisions made today will determine the state of our forests in the future. In order to boost the resilience of Pyrenean forests and counter the negative effects of climate change, we must therefore:

• Improve our understanding – in the specific context of the Pyrenees -  of the effects and impacts of climate change on forests (particularly the most vulnerable in the short and medium term) and adaptation processes (both natural and anthropogenic),
• Maintain and develop agricultural, farming and forestry activities in mountain areas, since these activities guarantee sustainable management of the forest environment with numerous positive effects: lower volume and reduced continuity of combustibles, patchwork of more resilient housing, risk control and increased resilience of forests in the event of health or climatic hazards, etc.,
• Promote enhanced awareness of technical and scientific progress among non-specialist populations.

Pyrenean stakeholders, including first and foremost owners, scientists, forest managers and public authorities, must therefore take action right now to ensure that our forests are able to develop in the best possible conditions and are well prepared to adapt to these changes.