1909] PHILLIPS A STUDY OF PINON PINE 217 



sional specimens may be found even higher than this. The best 

 stands are found on coarse gravel, gravelly loam, or a coarse sand, 

 of i . 5 m (5 feet) or more in depth, on which humus and ground cover 

 are almost entirely lacking. The species often occurs on rocky 

 areas, where the soil is only 15 to 30 (6-i2 in ) in depth, and fre- 

 quently it is found growing in rock crevices. It is one of the first 

 trees to gain a foothold on the lava overflows which are known 

 throughout the southwest as mal pais. This rock in its disintegrated 

 form supports fair tree growth, but even before disintegration has 

 progressed very far, the junipers and pinon may be found encroaching 

 upon it. 



Another encroachment form of the pinon is to be found on small 

 mounds which rise 0.6 to 3 (2 to 10 feet) above the general level 

 of the desert-like tableland at approximately i5oo m (5000 feet) 

 elevation. On such islands as these, the pinon and one-seeded 

 juniper take possession and maintain a limited growth. The same 

 feature is noted at the bases of the hill and mountain slopes which 

 bound these tablelands. This remarkably distinct tension line seems 

 to be due to a greater soil porosity, less grass growth, and a smaller 

 alkali content, which are manifest in slightly higher elevations. The 

 distribution of these trees on such small mounds and limited in such 

 a distinctive manner presents an ecological problem for future investi- 

 gation. 



On slopes where site conditions are favorable for western yellow 

 pine, the pinon usually occupies the south and west aspects. Where 

 conditions become less favorable, it occupies the north and east slopes, 

 while the south and west slopes are bare or nearly so. This ability 

 to stand poor conditions is also shown on a large number of mountain 

 slopes ranging from 2830 to 3i35 m (6000 to 7000 feet) in elevation, 

 where scattering Douglas fir, of scrubby growth and badly affected 

 with witch's broom, is found in the canons; western yellow pine on 

 the middle slopes; and pinon on the ridges and upper slopes, where 

 the soil is scant and the soil moisture low. 



A distinctive peculiarity was observed between Servilleta and Taos, 

 New Mexico, in an open stand of the species in which approximately 

 two-thirds of the trees have constricted bases at the surface of the 

 ground. This constriction amounted to an average of i9 mm (o.75 in ) 



