The acceleration of the effects of climate change and its share of extreme hazards (heat waves, storms, fires, hailstorms, etc.), combined with the arrival of new diseases and insect pests, represent real sources of uncertainty and therefore concern about the future of our forests .
In a recent health report for French forests , the IGN reported a 30% increase in the stock of trees less than 5 years old that were dead on their feet, that is to say standing trees that no longer show any signs of life. These declines are observed on a European and even global scale.
A major question then emerges: will tree species be able to adapt to these new threats?
Before studying this point in more detail, it is appropriate to recall that previous declines, documented in particular in oaks , have rarely led to local extinctions of stands, which implicitly testifies to the adaptive capacities of oaks.
The holy grail of fitness
Most of today's forests are replenished by natural regeneration , that is, renewal is carried out by seeds produced by trees before they die naturally or are felled.
Before becoming a majestic specimen, a tree was a simple seed, among hundreds of thousands of others. Why did this seed become a mature tree when others did not survive?
A tangible answer to this question is that this individual has taken over the others, by better growth towards the light, or by better resistance to fungal diseases and insect pests, and probably also because it was very lucky. Still, natural selection has done its work: it has eliminated the least suitable individuals, among a very large number of candidates.
A less noticeable answer is that trees have great genetic diversity . However, the ability of a population to adapt is highly dependent on this level of genetic diversity (which varies according to two main components, the rate of mutation and the number of individuals in the population). Each DNA variation comes from a mutation that appeared at one time in a tree, then was transmitted to one or more descendants generation after generation, and then disseminated by pollen flow. Genetic diversity is therefore shaped over time, over hundreds of thousands, even millions of years of evolution.
It is on the basis of this diversity that the most suitable individuals are selected. The propagation of this diversity to the next generation therefore remains a crucial point. From a forest management point of view, this consists of leaving a sufficiently large number of trees (called “seed trees”) to produce pollen and ovules for the next generation and thus guarantee this transmission of diversity.
[ More than 85,000 readers trust The Conversation newsletters to better understand the world's major issues . Subscribe today ]
To illustrate how great this diversity is in trees, note that two oak acorns from the same plot differ by approximately 7 million simple genetic differences in their DNA sequences! On the scale of a regenerating stand, and its hundreds of thousands of acorns, this constitutes so many unique genetic combinations, giving the new seedlings a more or less good ability to adapt to their environment.
A measurement (difficult to apprehend in the forest) allowing us to know if certain genetic combinations are more favorable than others, consists in studying, for an individual of generation n, the number of his descendants alive at the following generation n+1: this is called the adaptive value.
In a recent study we conducted on oaks , we showed that the genetic variation in fitness of oaks was among the highest in the plant kingdom. Some genetic combinations have thus varied in frequency more than others between two successive generations. Concretely, this indicates that certain trees have in their genome genetic combinations which have been more favorable to the survival of the seeds which carried them, hence an increase in the frequency of these favorable alleles between two generations, according to the principle of natural selection .
High genetic diversity promotes rapid adaptation
The context of current climate change represents a new test in the life of these young trees. Young plants in particular will face more frequent and more pronounced periods of drought.
In another study , we analyzed the evolution of oak trees in three French forests over the last three centuries, from the cold period of the Little Ice Age to that of warming due to human activities. We showed that they had evolved in a concordant way in the three forests, to adapt to this climatic transition which took place over a few decades.
Surprisingly, adaptation to this climatic transition (cold -> hot) was almost “immediate”. This counter-intuitive result is explained by the very large number of unique genetic combinations formed in each generation during natural regeneration, which allows natural selection to be very efficient in sorting out the fittest to survive and reproduce.
Analysis of the DNA of these oaks has also shown that these genetic changes have affected a very large number of regions of the genome, and not just a few genes.
In summary, this study has clearly shown that the current adaptation of populations is linked to a large number of genetic variations, each of low effect, and having an ancient evolutionary origin.
Somatic mutations as a driver of adaptation: a real red herring
Several recent popular articles such as this one published by the Botanical Society of France and this one published by Epsiloon magazine have echoed the adaptive role that new mutations accumulated during the growth of a tree could play (we speak of somatic mutations, unlike the pre-existing genetic diversity described above).
Why is this a red herring for adaptation?
Because if these mutations do exist, they are very infrequent: 17 for a 230-year-old Swiss oak , 46 for a 100-year-old French oak . Even if the number of mutations has been underestimated because of the complexity of their detection, this diversity remains ridiculously low compared to the diversity present from the formation of the glans: the 7 million differences indicated above. We are talking about a few dozen new variants on the one hand, millions of pre-existing ones on the other!
Moreover, it is not a single mutation that provides the adaptation of the oak to the environment, but a whole genetic combination.
Moreover, even if these mutations had an effect on the survival of the individual carrying them, they would still have to be able to be transmitted to other trees for them to have an effect on the level of the entire population. Not to mention that the diffusion of mutations (possibly favorable) by pollen flow requires several generations before they can fuel the adaptation of the population...
As authors of the French oak study, we ourselves had explicit reservations about the adaptive interpretations of these mutations. This discourse has unfortunately been widely misused to feed scenarios of rapid adaptation to change via these somatic mutations, although this hypothesis seems extremely unlikely.
What concrete applications for foresters?
These reminders of the nature and origin of the genetic diversity contributing to the adaptation of trees inevitably lead to mention of the human interventions making it possible to enhance the benefits.
First of all, it seems futile to propose an aging of stands, on the sole pretext that they would produce more somatic mutations. The recommendation would rather be to do the exact opposite. Since natural selection takes place mainly at the young stage, faster cycles of regeneration would probably be more favorable to rapid adaptation.
Moreover, promoting abundant natural regeneration, by accumulating several years of flowering and fruiting of the seed trees, makes it possible to maintain a good level of genetic diversity within the seedlings and consequently to reinforce the action of natural selection.
Ensuring the production of a very large number of seedlings during natural regeneration will also make it possible to benefit from a higher selection intensity at the juvenile stage. Maintaining stands will be all the easier if the “sowing” is dense. Indeed, natural selection will be more effective in selecting – from among a very large number of individuals – those who are best adapted to the change in the environment.
Genetic diversity and intensity of natural selection are therefore the two main factors in the adaptation and resilience of our forests.