where 



AG = genetic gain in units of measure 



i = selection intensity; a constant relation to the proportion of the population 

 selected (Namkoong 1979) . 



hp,h2,o^ and a^p are defined in table 6. 



IDAHO CITY 



HOLCOMB 



< 



o 

 a: 



33 20 10 

 BOULDER CREEK 



15 r 



10 - 



33 20 10 



Individual Selection 

 * \ 



33 20 10 



jack's creek 



33 20 10 



PERCENT FAMILY SELECTION 



Figure 1. --Expected gains (ordinate) 

 in 16-year height from alternative 

 levels of family selection (bars) and 

 individual selection (shading) with- 

 in families for each planting site. 



Gains presented in figure 1 are expressed as a percentage of the mean height of only 

 those trees at each planting site that represent the seed zone within which that site lies. A 

 3 to 5 percent gain over local seed sources can be expected after intermatings of the tallest 

 families. And, gains of 10 to 15 percent seem likely in progenies of the best performing 

 individual trees within selected families. Relatively high gains will be realized, however, 

 only if seed orchards are composed of relatively few genotypes (fig. 2). And therefore, an 

 additional investment will be required to develop clonal seed orchards. 



These results attest to potential genetic gains at several levels of tree improvement for 

 ponderosa pine. First, seed transfer guidelines account for natural adaptation of populations 

 and reduce losses in productivity from maladaptation . These guidelines limit seed transfer to 



750 ft (230 m) elevation, ^^ 50 miles (0.7 degrees) latitude, and 60 miles (1.2 degrees) 

 longitude. Secondly, a 4 percent gain in 16-year height can be attained by collecting seeds 

 for reforestation from populations of proven genetic potential. And, thirdly, depending on 

 the intensity of selection, gains from 8 to 14 percent can be expected in the next generation 

 from seed orchards developed from select individuals within selected families. 



8 



