GENETIC VARIATION IN THE PONDEROSAE OF THE SOUTHWEST

Ninety‐five seedling populations of southwestern ponderosa pine (Pinus ponderosa var. scopulorum) along with single populations of Pinus engelmannii and Pinus arizonica were compared in four environmentally disparate common gardens. Differentiation among ponderosa pine populations was detected for a diverse assortment of variables that included patterns of shoot elongation, measures of growth potential, winter and spring freezing damage, and leaf characteristics. Multiple regression models accounted for as much as 85% of the variance among populations and described complex clines that were dominated by elevational and latitudinal effects. Although P. ponderosa, P. arizonica, and P. engelmannii were readily differentiated, the performance of progenies from one population suggested introgression primarily involving P. ponderosa and P. arizonica but also implicating P. engelmannii.