Recently, Pedlar and colleagues (2012) stated that assisted migration in forestry (forestry AM) differs from species rescue assisted migration (species rescue AM) because the risks of invasiveness, hybridization with local species, and the spread of diseases are minimized in managed forests. The rationale behind this assertion for forestry AM is that it involves the translocation of populations within the existing geographic range of the species, whereas species rescue AM involves the introduction of exotic species.
However, although we agree that forestry AM is less risky than species rescue AM for the recipient ecosystem, not only can forestry AM fail, but it can also incur enormous financial costs. The failure of efforts that involved planting maritime pine (Pinus pinaster Aiton) trees in southwestern France (Aquitaine) with seeds from more southerly populations from Portugal for production purposes is a textbook case.
The climate variability in Aquitaine includes periods of intense frost that are sufficiently rare (every 10–20 years) to be overlooked when establishing tree populations. The frost of the winter of 1985 was the most intense frost event since records began, with temperatures dropping as low as -22 degrees Celsius (°C; Boisseaux 1986), affecting about 350 square kilometers of tree plantations in the region (Doré and Varoquaux 2006). The highest mortality related to frost was observed in populations harvested from Leiria, in Portugal, for which nearby records show that the absolute minimum temperature was only -7.8°C in the last 60 years. Climate averages over the last 30 years differ only slightly between Leiria and Aquitaine, which would erroneously suggest that samples from Portugal would have survived in the Aquitaine region.
Newly emerging climates (Williams et al. 2007) and the uncertainty related to extreme climate events (Easterling et al. 2000) will make the search for southern locations with climatic conditions similar to those of northern populations of trees extremely difficult. Policies of forest adaptation to climate change should account for extreme cold events in the target populations, even if climate change will likely decrease the number of extreme cold events (Easterling et al. 2000), which remain, in our opinion, the hidden element behind the maladaptation of southern populations to northern locations.
