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



sions are strikingly confirmed under a different set of conditions in 

 a treeless clay flat surrounded by virgin forest (PL VII, Fig. 1). 

 This flat, which is virtually a small park about 150 yards in 

 diameter, is typical of a common condition in the pine forests of this 

 region. The surface is smooth, free from stones, and packed by 

 tramping. Perennial grasses have given way almost entirely to 

 "six-weeks grass," a short annual of small forage value. Clumps 

 of Senecio spartioides and pingue (Ilymenoxys floribunda), both 

 unpalatable weeds, occupy about 15 per cent of ground surface. 

 Counts in the middle of this opening in November, 1919, showed 

 13,000 seedlings of 1919 germination per acre ; whereas near the south 

 side, 75 feet from the edge of the timber, there were 25,000 per acre. 

 The tendency of seedlings to group themselves in and around the 

 weed clumps was so conspicuous that a 5 by 20 foot plot was charted 



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~ 0H 



OH 



HO 



K * 



O 

 G 







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Fig. 5. — Location of western yellow pine seedlings with reference to ground cover. 

 U=Hymenoxys floribunda; S= Senecio spartioides; X=Western yellow pine seedling, 

 1919 germination. 



to scale, showing the location of all seedlings and weed clumps 

 (Fig. 5). It was found that 17, or 55 per cent, of the 1919 seedlings 

 were in the weed clumps, which occupy approximately 13 per cent 

 of the total ground surface. On this basis it appears, therefore, that 

 the weed clumps are 8.2 times as favorable for germination of yel- 

 low pine seed as the intervening bare areas. One older seedling was 

 found in the weeds and none in the open spaces. Another plot nearer 

 the seed trees gave a ratio of 15 to 1 in favor of the weed clumps. A 

 survey of about one-half acre in the middle of the park showed 37 

 seedlings of ages estimated at from 4 to 10 years, nearly all seriously 

 injured by sheep. Of the total number, 19, or 51 per cent, were in 

 weed clumps, which occupy not to exceed 15 per cent of the ground 

 area, thus indicating that the chances for establishment and sur- 

 vival are roughly six times as great in the clumps as outside. The 

 greater abundance of seedlings near weed clumps is explained, as in 

 the case of the natural and cultivated plots, by the more favorable 

 conditions for germination and early survival afforded by the weed 



