84 BULLETIN 1105, U. S. DEPARTMENT OF AGRICULTURE. 



ameter bfeast high, mostly blackjack. 

 Where the usual groups of blackjack 

 are absent, resulting in a scattered ar- 

 rangement of seed trees over a lai-ge 

 area, the number required for wind 

 protection might exceed the minimum 

 required for seed production. Under 

 these circumstances, in the most ex- 

 posed situations, such as edges of 

 " parks " or wind-swept slopes, the 

 total number of trees above 11 kiches 

 in diameter should not be less than 

 . eight per acre. Where there is a con- 

 siderable amount of tall underbrush, 

 the number can be reduced if con- 

 sistent with seed requirements. 

 Siui and wind " Slopes exposed to drying-out in- 

 fluences of sun and wind " fall within 

 the scope of the above discussion, in 

 so far as the effects of wind are con- 

 cerned. The influences of insolation 

 have been discussed elsewhere in this 

 report. It is shown that seedlings so 

 situated as to be shielded from the 

 sun during the greater portion of the 

 day will be shaded out later in life. 

 Beyond the zone of maximum shade, 

 where the sunlight is sufficient to per- 

 mit normal development, the benefits 

 of partial shading will be at least in 

 part offset by root competition from 

 the trees. The only appreciable bene- 

 fit which the shade of standing trees 

 may be expected to yield is in assisting 

 germination and early establishment. 

 To take advantage of this benefit it is 

 necessary to fell the trees before the 

 seedlings begin to suffer from lack of 

 sunlight. Although such a silvicul- 

 tural system is generally impractical 

 under present economic conditions, it 

 might be applied to limited areas 

 whose value warrants special treat- 

 ment. 



In general, it may be said that for 

 western yellow pine in the Southwest, 

 the leaving of mature trees beyond 

 seed requirements, purely as a protec- 

 tion against excessive insolation, is 

 unwarranted. Since in this region the 

 sites exposed to excessive insolation ; 

 that is, steep slopes of southerly ex- 

 posure, ax'e also subject to unusually 



"^ 



