In order to determine the genetic variation among populations of Pinus hartwegii along altitudinal gradients, and the possible adaptive decoupling due to climatic change, a common garden test was set up with 13 provenances collected along an altitudinal transect (3150 to 3750 masl; collection sites were every 50 m of altitudinal difference) from Pico de Tancítaro, Michoacán, west-central Mexico. The test evaluated trees aged 4.5 to 5.5 years, and contemporary and future (year 2030; six model-emission scenarios) climate variables were estimated. We found significant (P<0.005) differences among populations in total elongation, final seedling height, basal diameter, rate of elongation, dry weight of needles, branches and stem, total dry weight and first principal component. We found a significant association between population performance, expressed as first principal component values, and their climate, where the best climatic predictor was the frost free period (r2=0.90, P<0.0001). This result indicates that the patterning of genetic differentiation among populations is strongly associated with a climatic and altitudinal gradient. We propose a re-delineation of fixed climatic and altitudinal seed zoning for seed and seedling movement, based on intervals of 26 days of difference of frost free period: Zone 1, from 240 to 214 days of frost free period or 12 to 11°C or 3150 to 3350 masl; Zone 2, 214 to 188 days or 11 to 10°C or 3350 to 3550 masl; and Zone 3, 188 to 162 days or 10 to 9°C or 3550 a 3750 masl. Projecting at what altitude will occur the same frost free period values by year 2030, we suggested that populations should be shifted 400 m attitudinally upward, through a program of assisted migration, aiming to realign the populations to the climate for which they are adapted. Seedling deployment should use future (year 2030) fixed seed zones, with the same projected climatic interval values than contemporary climate, although at the following altitudinal limits: Zone 12030, from 3550 to 3750 masl; Zone 22030, 3750 to 3950 masl; and Zone 32030, 3950 to 4150 masl.
