Adaptive variation in Pinus ponderosa from intermountain regions. II. Middle Columbia River System

Seedlings representing 138 populations of ponderosa pine from drainages of the middle Columbia River system were grown and compared in common environments. Field studies of 3-year-old trees on mesic and xeric test sites detected population differentiation for traits reflecting growth and development. Populations that expressed the greatest growth potential under mesic culture suffered the greatest reduction in height under xeric culture. Consequently, a different set of populations was performing the best on the two sites when 3-year height was expressed as the growth from a common height at age 2. Greenhouse studies of the periodicity of shoot elongation revealed that populations of high growth potential achieved a large stature by means of a long duration, late cessation, rapid rate, and large amount of elongation. Multiple regression models accounted for as much as 60 percent of the variance between populations and described adaptive landscapes in which populations from low elevation have a high growth potential while those from high elevations have a low growth potential. Geographic variation at a common elevation indicated that populations of high growth potential come from areas where the frostfree period is long and precipitation is relatively high. Thus, growth potential is directly related to the length of the growing season, which can be truncated by either frost or moisture stress. Adaptive landscapes are used to construct guidelines for limiting seed transfer in artificial reforestation. In general, seed from a single source should not be transferred more than ±200 m in elevation.