Adaptive Variation in Pinus 

 ponderosa from Intermountain 

 Regions. II. Middle Columbia 

 River System 



G. E. Rehfeldt 



INTRODUCTION 



Populations that occupy contrasting environments tend 

 to differ genetically for numerous traits that convey en- 

 vironmental adaptation. In accordance, adaptive differen- 

 tiation in the widespread ponderosa pine {Pinus ponderosa) 

 is well documented (Wang 1977) and is related to the 

 elevation and geographic origin of the seed. For just the 

 Inland Northwest, populations from mild environments 

 tend to have a high growth potential while populations 

 from areas where growing seasons are short tend to have 

 a low growth potential (Wang 1977; Madsen and Blake 

 1977; Rehfeldt 1979b, 1980, 1986). 



Forest managers rely on geographic patterns of adaptive 

 variation to estimate the distance that seeds can be trans- 

 ferred from their origin before maladaptation begins to 

 limit the productivity of artificial regeneration. However, 

 previous studies were of insufficient power and scope for 

 providing managers with this information. Consequently, a 

 series of studies was begun to determine the appropriate 

 limits to seed transfer for the Intermountain Region. 

 While the first study of this series concerned populations 

 from central Idaho (results published in Forest Science, 

 Rehfeldt 1986), the present involves populations from the 

 middle Columbia River system of northeastern Washing- 

 ton, northern Idaho, and western Montana. 



MATERIALS AND METHODS 



Population differentiation was studied in seedlings from 

 138 populations that sampled the ecologic, geographic, and 

 elevational distribution within which the species is of com- 

 mercial importance in the Inland Northwest (fig. 1). 

 Genetic diversity of each population was sampled in a 

 manner to assure that seedling populations represented a 

 large number of parental trees. Most populations were 

 represented by an equal volume of wind-pollinated cones 

 from 10 trees. Some, however, were represented by 

 samples of cones collected from several squirrel caches. 

 Genetic diversity was assured by selecting cones with a 

 variety of morphologies, sizes, and colors. Seedlings were 

 used in separate studies of (1) growth and development in 

 the field and (2) the periodicity of shoot elongation in a 

 greenhouse. The results from both were used to assess 

 patterns of genetic variation. 



Growth and Development 



Seedlings were grown for 6 months in plastic containers 

 (65 cm^) in a shadehouse at Moscow, ID (lat. 48.5° N., 

 long. 116.7° W.). In the fall, seedlings were planted at two 

 sites at about 670 m elevation at the Priest River Experi- 

 mental Forest, 190 km north of Moscow. Although both 

 sites averaged 90 frost-free days, one was on sandy loam 

 soil and was maintained under a mesic cultural regime. 

 This site was tilled before planting, competing vegetation 

 was controlled, and the planting was irrigated twice each 

 summer during the course of the study. The second site, 

 on glacial till, was maintained under xeric cultural condi- 

 tions. Irrigation was provided early in the second growing 

 season to ensure seedling establishment, but water sup- 

 plemental to natural precipitation, which averages 81 cm, 

 was withheld thereafter. This site was also tilled before 

 planting, and competing vegetation, gophers, and white 

 grubs were controlled. Nine seedlings, planted in row 

 plots, represented each population in each of three blocks 

 at each site. At the mesic site, 0.5 m separated rows and 

 0.3 m separated trees within rows. To allow conversion of 

 the test to a demonstration plot, the corresponding 

 spacings at the xeric site were wider than at the mesic 

 site: 1 m and 0.5 m. 



Population performance was described by five variables: 



1. Height, the 3-year height (centimeters) of individual 

 trees. 



2. Leaf length, the length (centimeters) of a leaf from 

 near the center of the 3-year shoot of an individual tree. 



3. Adjusted height, 3-year height (centimeters) of in- 

 dividual trees adjusted by regression on 2-year height, a 

 value that depicts the 3-year height expected if all trees 

 had been the same height at age 2. Consequently, the 

 value is relatively independent of early environmental 

 effects (such as seed weight or transplanting shock) and 

 genetic effects and is capable of reflecting the adaptive 

 response of populations to a particular environment in a 

 relatively short time. 



4. Xeric height reduction, the amount (centimeters) by 

 which the height of trees cultured under xeric conditions 

 failed to reach the potential expressed under mesic condi- 

 tions. The value was calculated as the difference between 

 the xeric height of an individual tree and the mean mesic 

 height of the population from which the individual 

 originated. 



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