The Mediterranean forest is an ecosystem that has long been accustomed to hot dry summers, which its plants have to cope with during their optimal growth period. Now climate change scenarios indicate an increase in temperatures and a decrease in rainfall during summer, i.e. more severe drought conditions, which forests will have to deal with.
Summer drought and climate change affect the water and carbon cycles and the dynamics of plants in the Mediterranean, especially when they are at the limits of their distribution ranges. For the past several years, such prospects have set the pace for the work of Guillaume Simioni, an ecophysiologist, and Bruno Fady, a geneticist, both of whom are researchers at the Mediterranean Forest Ecology Unit in Avignon, France.
Signs for concern, a shared observation
In Mediterranean forests, canopy levels are clearly defined, and many species grow side-by-side and one after the other. At the Font-Blanche site (13), the Aleppo pine (Pinus halepensis) stands from after the Second World War are still young. “You would expect such stands to continue growing,” explained Guillaume Simioni.
But that would not take into account climate change, whose effects on the growth of all tree species have already been noted by researchers, e.g. increased water stress, defoliation, and mortality. Given the Aleppo pine's high tolerance for extreme drought, it may survive better than other species that are less equipped to deal with more arid conditions.
For trees and forests that are less susceptible to drought
To reduce their vulnerability to drought and climate change, trees may migrate towards more favourable environmental conditions by dispersing their seeds beyond their initial distribution area. However, that raises other questions. Take the example of the Aleppo pine: Will it be able to adapt to the cold winters it is likely to find in regions farther north or at higher altitudes? Will it be able to quickly adapt to the cold?
The topographic, edaphic or even climate gradients that characterise Mediterranean forests are an advantage for plant species that migrate via their seeds. “With climate change, it will be easier to stay in a preferred habitat in areas with sharp slopes, because distances are shorter than in the lowlands,” pointed out Bruno Fady.
Through natural selection, sometimes facilitated by forest management, trees can also adapt to their new environment.
Font-Blanche, a valuable observatory for improving knowledge on how Mediterranean forests function
The Font-Blanche station allows researchers to study many different aspects of forest ecology, including tree plasticity, i.e. a specimen's ability to modify its functioning, growth, or morphology, in response to environmental factors. The site has a plot that is partly sheltered from the rain to artificially increase its water deficit. “It would appear that there is no real physiological adaptation to drought, but rather adjustments to the foliage surface area to adapt to declining water resources,” noted Guillaume Simioni.
For forests, especially managed ones, replacing vulnerable species with better adapted forest material or controlling cover and regeneration through forestry techniques are alternatives for coping with drought and climate change.
At the heart of experimental schemes known by the delightful name of “common gardens”, scientists and forest managers can compare the genetic values of the trees, whose seeds they plant, which come from a variety of geographic locations. The idea: select the geographic origins and then those specimens that should have the desired traits, such as resistance to drought or cold, growth rate, trunk volume, etc. “At the present time, for the Aleppo pine, we are focussing on comparing geographic origins since there is no strong economic pressure on the species and an important issue is learning how to manage existing forest resources,” pointed out Bruno Fady.
Designing ecosystem mosaics to respond to climate change
At a wider level, characterising the genetic diversity of trees and their ability to adapt to stress, particularly water stress, and managing that diversity are major challenges for geneticists (GenTree Project, EU 2016-2020) and other stakeholders in this field.
This issue is also the focus of the French national forest genetic resources committee (CRGF) chaired by INRAE and of its European counterpart, the Euforgen (European Forest Genetic Resources) Programme, for which INRAE serves as France's representative. More so than individual variability, population and higher orders of variability may make it possible, for example, to devise ecosystem mosaics that could temper and/or mitigate climate change.
Thinning, a common forestry management practice, promotes production of more commercially profitable boles. Scientists have also demonstrated that this practice helps promote drought resistance in the Atlas cedars of the Valliguières (30) community forest. Eliminating some of the foliage in a stand reduces its water requirements and improves the water balance of the remaining trees.
While forest managers can take action to make their stands more resistant to drought and, more broadly, climate change, a number of questions remain. What is the limit of such changes in practices? Are they economically viable? How should they be implemented, etc.? Such questions underlie major issues that need to be considered such as land management issues given local variability in the environment.
Specimens with adapted traits can pass them down to their descendants unlike specimens that do not have such traits and will not survive. Genetic adaptation does exist and can develop rapidly, over the span of one generation. But will it be enough to offset climate change? That is far from certain! The Aleppo pine reproduces after only a few years, while the fir tree takes about 40 years. The average age of forest stands at harvest ranges from 50 to 100 years - a time span that means the climate such stands experience at the end of their cycles will be very different from the one that existed when they were first planted.
A complex question
After the 2003 heat wave, the fir trees of Mont Ventoux were hit by an upsurge in mortality that continued for several years. Today wide-scale regeneration of those stands has begun spontaneously: new specimens are appearing, offering a ray of hope to a situation that seemed irreversible.
However, the speed and scale of climate change now and in the future pose major unprecedented challenges to forests in France: on the one hand, coping with the effects of climate change; and, on the other, as one of many ecosystem services, ensuring carbon sequestration to mitigate climate change. Will forests be able to provide this service over the long-term? An ongoing study on the 2019 heatwave as part of the European ICOS - Integrated Carbon Observation System, which INRAE is a member of, has revealed that heat and drought negatively impact a forest's carbon storage capacity.
In addition to those aspects, there are others such as an increased fire risk and damage caused by pests, for example. All those factors combine in such a way that even today, the reaction of trees and forests to drought and, more broadly, to climate change remains a process that is difficult to fully grasp.
Vinceti B. et al. 2020. Managing forest genetic resources as a strategy to adapt forests to climate change: perceptions of European forest owners and managers. Eur. J. Forest. Res. (2020). https://doi.org/10.1007/s10342-020-01311-6
Courbet F. et al. 2019. Projet REDSURF : Peut-on rendre les forêts moins sensibles à la sécheresse par la sylviculture ? Effet de la réduction de la surface foliaire et de sa répartition dans le peuplement. Approches empirique et fonctionnelle. Rapport final d’activité. https://hal.archives-ouvertes.fr/hal-02791401