Systematics and genetic structure of Ponderosae taxa (Pinaceae) inhabiting the mountain islands of the Southwest

The systematics and genetic structure of taxa representing the Ponderosae subsection of genus Pinus were assessed for disjunct, isolated, and peripheral populations occupying the mountain islands of the Southwest. Wind-pollinated progenies of 290 trees were compared in common gardens according to ten variables reflecting allometric, needle, and phenologic characteristics of 2-yr-old trees. The tests also included populations of similar taxa from the Rocky Mountains to the north and the Sierra Madre to the south. Principal component and canonical discriminant analyses demonstrated that the taxa segregated into three distinct groups, one of which contained two subgroups. These groupings collectively accounted for all of the many and confusing taxonomic descriptions that exist for the Ponderosae of the southwest United States and northern Mexico. The results suggested that intertaxa hybrids or hybrid derivatives may have been segregating within the progenies of only three of the parental trees. Hybridization, therefore, appears to be infrequent and inconsequential to the interrelationships among taxa and to contemporary genetic structures of taxa. Univariate analyses showed that the three distinct groups displayed different genetic structures despite similarities in their geographic distributions. While genetic variation within populations of all groups was abundant, a group labeled “quinquefoliata” displayed little variation among populations; one labeled “engelmannii” had abundant interpopulation variation that was largely randomly distributed across the landscape; and in a group containing the subgroups called “scopulorum” and “taxon X,” abundant interpopulation variability was arranged systematically along moderately steep clines. These disparate genetic structures showed no apparent effects of the isolated, disjunct, and peripheral conditions under which populations of these taxa exist.