222 P. J. STEINHOFF 



Two trees in the arboretum at Placerville, California, average 

 38 feet tall and 7 inches in diameter at 29 years of age (Institute of 

 Forest Genetics, Placerville, Calif., unpublished data) . Seedlings from 

 several United States sources have been planted in the same provenance 

 trials with limber pine, in the north-central states. Provenance 

 plantings of 20 additional sources were made in the north-central states 

 and in Arizona, Utah, California, and Idaho in 1967, but initial survival 

 has been poor. 



Pinus strdbiformis has been crossed successfully with P. flexilis 

 and P. monticola (Little and Righter, 1965). Young hybrid P. monticola x 

 P. stvobiformris trees at Placerville were 12 feet tall at 12 years of age 

 (Little and Righter, 1965). They are intermediate in height between seed- 

 lings of the parental species. 



Southwestern white pine appears to have considerable potential for 

 planting outside its natural range. Once established it appears to be 

 winter hardy in the north-central states, California, and Idaho. It also 

 appears to have promise in hybrid combinations. Its relative blister rust 

 resistance is in question. Bingham 2 places it among "species deserving 

 more attention," saying that "resistance appears to be well above expecta- 

 tion,... and needs to be confirmed." 



Mexican White Pine (P. ayacahuite) 



Mexican white pine is distributed from the Mexican states of Jalisco 

 and Hidalgo southeastward to Guatemala, El Salvador, and western Honduras 

 (Critchfield and Little, 1966; Map 9). The species is considered by 

 many (Shaw, 1909; Martinez, 1948; Loock, 1950) to be composed of two 

 varieties. The type variety is found mostly in southern Mexico and 

 Central America. The variety veitchii Shaw is restricted to central 

 Mexico (Loock, 1950). Differences between the varieties are restricted 

 primarily to cone and seed characters and are not concerned with the 

 growth or form of the trees. 



One of the best timber trees of Mexico, Mexican white pine grows to 

 heights of 140 feet or more and diameters up to 5 feet (Loock, 1950). It 

 does best on deep moist soils. This species usually occurs as scattered 

 individuals in a mixture with other species so the volume per unit area 

 is not very high. 



Pinus ayacahuite is planted in Mexico and has been tried in a forest 



plantation in the Union of South Africa (Loock, 1950). At 25 years in 



South Africa the trees averaged 11.5 inches in diameter and 65 feet in 

 height (Streets, 1962). 



Trees up to 80 feet tall have been reported in British arboreta 

 (Dallimore and Jackson, 1967) and a specimen 26 inches in diameter in 

 Pennsylvania was reported by Wright (1958). Three trees at Placerville, 

 California, averaged 34 feet in height and 8 inches in diameter at 

 36 years (Institute of Forest Genetics, Placerville, Calif., unpublished 

 data) . 



The first known hybrids involving Mexican white pine resulted from 

 spontaneous crossing with P. griffithii in the arboretum at Westonbirt, 

 Great Britain (Jackson, 1933). These hybrids, P. x holfordiana A.B. 

 Jacks., were 70 to 80 feet tall at 50 years of age (Dallimore and 



