Genetic variation and hybridization of ponderosa pine

Abstract

Ponderosa pine's (Pinus ponderosa Laws.) geographic range is centered in the montane western United States, where it is the most widely adapted and ubiquitous conifer. A western variety, P. ponderosa var. ponderosa, extends from the mountains of southern California, northward, on the western and eastern sides of the Sierra Nevada-Cascade crest to Canada. It comes into contact with an eastern variety, P. ponderosa var. scopulorum Engelm., near the Continental Divide in west-central Montana. The eastern variety extends southward throughout mountains, plains, and basins to scattered stands in the Sierra Madre Occidental and Sierra Madre Oriental of northern Mexico. A 5-needled pine (P. arizonica Engelm.) of Mexico, with scattered populations in southern Arizona and New Mexico, was commonly represented as a variety of P. ponderosa, but recent field studies support species status for the taxon. Morphological observations, biochemical analyses, and growth responses of provenances in nursery and plantation trials indicate that ponderosa's varieties consist of well differentiated geographic races (Southern California, Pacific, and North Plateau races in the west, Rocky Mountain and Southwestern races in the east). Native ponderosa pines encounter and hybridize with other western yellow pines (P. jeffreyi Grev. & Balf., P. washoensis Mason & Stockwell, P. arizonica, and P. engelmannii Carr.) and one of the California big-cone pines (P. coulteri D. Don). Controlled crosses verify that Pacific race ponderosa pines will hybridize, but have strong barriers to crossing with P. jeffreyi; Pacific race pines are moderately compatible with the Rocky Mountain race, a rare and closely related species in California (P. washoensis), and five species in subsection Ponderosae in Mexico. The failure of ponderosa pine to cross with two Mexican species (P. teocote Schiede & Deppe and P. lawsonii Roezl) that were previously included in subsect. Ponderosae is evidence that those two species were misclassified. Since P. teocote crosses with P. patula (Schiede & Deppe), both P. teocote and P. lawsonii appear more closely aligned with the closed-cone pines in subsect. Oocarpae than with the pines of subsect. Ponderosae. Patterns of genetic variation among races and cross compatibility with Mexican pines suggest ponderosa's center of origin was Mexico. Ponderosa and its relatives appear to have been poorly adapted to the cool moist climates during full-glacial episodes of the Pleistocene; they are largely missing from the North American fossil record. Over the last 3,000 to 8,000 years, as western environments became warm and arid, ponderosa's drought and fire tolerances permitted it to become the most wide-spread and prevalent conifer of western United States.

Citation

Conkle, M. Thompson; Critchfield, William B. 1988. Genetic variation and hybridization of ponderosa pine. In: Ponderosa Pine: the species and its management, Washington State University Cooperative Extension, 1988: p. 27-43