Genetic Differentiation among Populations of Pinus ponderosa from the Upper Colorado River Basin

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Abstract
Genetic variation among 62 populations of ponderosa pine was studied by comparing seedlings from all populations according to (1) growth and development of 4-yr-old seedlings in three disparate common gardens and (2) patterns of shoot elongation of 2-yr-old seedlings in a greenhouse. Genetic variation was detected among populations for 19 of the variables, most of which were intercorrelated. Two principal components accounted for 60% of the total interpopulation variance. Multiple regression analyses were used to relate genetic variation in 19 variables and two principal components to the elevation and geographic origin of the seed. The regression models produced values of R2 as large as .78 and accounted for more than 40% of the variance among populations for 14 of the variables, including both the first and second principal components. These models described genetic variation as occurring along a relatively steep elevational cline and along both primary and secondary geographic clines of relatively gentle slope. All clines paralleled patterns of environmental variation, particularly the length of the frost-free season and patterns of precipitation. Because genetic variation occurs along three clines simultaneously, genetic differentiation can be described as rampant. Nevertheless, similar genotypes tend to recur in similar environments.