Altitudinal genetic variation among Pinus oocarpa populations in Michoacán, Mexico

Journal Article

Transfer Guideline: Recommendation

Pinus oocarpa has a large natural distribution in the sub-tropical forests of Mexico. Populations, however, are rapidly disappearing particularly in the Michoaca´n State as native forests are converted to avocado (Persea sp.) orchards.We investigated the patterning of genetic variation among P. oocarpa populations for quantitative traits along an altitudinal gradient by establishing a provenance/progeny test from wind-pollinated seeds collected along an altitudinal transect (1100–1500 m) near Uruapan, Michoaca´n, Mexico. Genetic variation was analyzed in relation to normalized climate records (temperature, precipitation, degree days >58 C and annual moisture index) for the provenances and the test site for the contemporary climate and for climates projected for the decades beginning in 2030, 2060, and 2090. Estimates of future climates used output from the Canadian and Hadley General Circulation Models. Results of a field test suggested an altitudinal pattern of genetic differentiation in juvenile height among P. oocarpa populations. Seedlings from populations originating from lower altitudes tended to grow more than seedlings originating from populations at the higher altitudes. However, this trend abates at the lowest altitudinal limit of the species distribution, a probable conservative growth strategy for avoiding drought stress. Thus, the cline appeared to arise from selection along a climatic gradient reflecting in a moisture index (ratio of degree days to precipitation) and is dependent, therefore, on a balance between temperature and moisture. For guiding seed and seedling transfer in ecological restoration, conservation of genetic resources, tree breeding and mitigating the effects of global warming, we suggest guidelines based on delimitation of three altitudinal seed zones of about 200 m in breadth. Alternatively, one can limit transfer to three climatic zones of about 0.75 units of annual moisture index. Predictions of future climates indicate an average annual temperature increase of 3.8 C by year 2090, and, judging from an increase of an annual moisture index of 26%, an increase in aridity by the end of the century. However, the more difficult period for adaptation of P. oocarpa populations to the new climate should be between 2030 and 2060, when the increase in aridity is expected to be most pronounced. Changes of this magnitude should alter the natural distribution of the species and would create an adaptational lag, as the adaptedness of extant populations deteriorates. Mitigating these effects will require seeds to be transferred upwards in altitude, perhaps as much as 150 m initially.