In the current context of climate change, and given that the natural migration of trees is only a few kilometres per century, the concept of assisted migration is spreading widely with the aim of speeding up the process and thus maintaining the productivity of forests.
But French researchers are warning : planting trees from hot, dry regions in colder, wetter regions can have negative consequences for the overall functioning of the ecosystem. Translocated tree species (those moved across natural barriers) are certainly more resistant to drought, but they also have reduced height and smaller, thicker, evergreen leaves. Less dense foliage from smaller trees is less effective in fulfilling its essential role of attenuating the effects of heat waves in the forest. The microclimate of the undergrowth is therefore drier and less cool than that of the natural undergrowth of forests in temperate regions, dominated by broad-leaved trees. Impacted by this warmer microclimate, some undergrowth species are at serious risk of disappearing.
In addition, the risk of forest fires could be increased. This is because southern species have thicker, drier leaves that are richer in volatile molecules, generating a more flammable carpet of dead leaves. Without knowing the potential negative repercussions that assisted migration can have on the functioning of forests, it should therefore be used with great caution. Solutions do exist, however. These include encouraging intraspecific diversity in the sspecies already present, or selecting genotypes from populations originating in warmer, drier areas within the natural range of a given species. In this way, as well as favouring the trees with the greatest genetic aptitude for heat resistance, we can maintain their range of species!