Role of selection versus historical isolation in racial differentiation of ponderosa pine in southern Oregon: An investigation of alternative hypotheses

Journal Article

Development

Continuous populations identified as Pacific and North Plateau races of ponderosa pine (Pinus ponderosa P. Laws. ex C. Laws.) are parapatric along the crest of the Cascade Range in southern Oregon. A 3-year common-garden study of bud phenology and seedling vigor was performed to estimate the nature and magnitude of differentiation between races, to characterize the transition zone between them, and to relate responses between and within races to topography and climate. Principal component (PC) analyses identified two significant character complexes, PC-1 (phenological traits) and PC-2 (size traits), that explained 73% of the geographic race-related variation. The races were differentiated in two regards. First, PC-1 scores, which were highly correlated with frost-free season and summer–winter temperature differential, displayed a sharp discontinuity in the transition zone. Second, PC-2 scores were significantly correlated with physiographic and climatic variables in the North Plateau but not in the Pacific race, even though these variables had greater ranges in the latter. The data supported a narrow, adaptive transition between races for a complex of traits probably related to cold hardiness, and provided evidence that plant vigor traits were more closely adapted to environments in the North Plateau than in the Pacific region, possibly as a consequence of past climatic-stress selection in the former and competitive-stress selection in the latter.