Altitudinal assisted migration of Mexican pines as an adaptation to climate change

Since shifts in altitudinal range are expected in response to climate change, we explored the effect on survivorship and growth of moving populations of three Mexican pine species (Pinus devoniana, P. leiophylla and P. pseudostrobus) to higher altitude, aiming to realign the populations to projected future climates in an experimental assisted migration. Twelve populations were collected across an altitudinal gradient (1650-2520 m above sea level [asl]) in a mountainous zone in the central-west region of the Mexican Trans-Volcanic Belt, and were planted in common garden tests at three forest sites of different altitudes (low: 2110, medium: 2422 and upper: 2746 m asl). Climate was estimated using a spline climatic model at the seed source and test sites and also measured using in situ data loggers. Survivorship and seedling height were evaluated in the field during the second and third growing seasons. Results were analyzed using mixed models to include the effect of climatic transfer distances (difference in climate between seed source and test site). Significant differences were found in seedling growth among Pinus devoniana, P. pseudostrobus and P. leiophylla, and among populations within the former two species. These were associated primarily with climatic transfer distances of extreme temperatures (minimum temperature in the coldest month and mean temperature in the warmest month). There was a significant decrease in growth in P. devoniana when the transfer exceeded 650 m of upward altitudinal shift or a reduction of 1.5 degrees C with transfer to colder sites. There was also a decrease of growth in P. pseudostrobus when transfer exceeded 400 m of upward altitudinal shift or 1.5 degrees C, with a significant decrease in survivorship. Pinus leiophylla, however, exhibited similar growth at all altitudes tested, probably due to phenotypic plasticity. Although further research is required with field tests using commercial spacing and trees of older ages, the results suggest that an assisted upwards migration of 300 m in altitude, in order to approach a realignment of the populations to the climate projected for the decade centered around the year 2030, appears to be a viable strategy with which to accommodate the effects of climate change