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



of stock and increased their abundance during the second five-year 

 period. Plate X, Fig. 1 shows a patch of ground almost completely 

 covered by vetch. It appears highly probable that excessive damage 

 to pine reproduction on the bunch-grass range in this locality is in 

 a large measure due to the scarcity of these and other palatable 

 sheep forage plants. Observations indicate that the vetch and red- 

 and-yellow pea, though most abundant in the lower stages, may be 

 permanent members of the subclimax associations. 



Another characteristic difference due to grazing appears in the 

 second weed stage. Most of the representatives of this stage are 

 palatable to sheep; hence they are cropped very closely, especially 

 on stump patches, where they make luxuriant growth. Overgrazing 

 in this stage commonly causes a reversion to the first weed stage. The 

 two " six-weeks " grasses (Sporobolus ramulosus and Bouteloua pro- 

 cumbens), pingue (Hymenoxys floribunda), and Senecio spartioides 

 are common indicator species of such retrogression (PL X, Fig. 2). 

 The two last-named species are members of the second weed stage 

 which not only persists but flourish under overgrazing because 

 they are rarely eaten by stock when other forage is available. The 

 second weed stage may, because of its great luxuriance, become dis- 

 tinctly unfavorable to yellow pine reproduction. (PL IX, Fig. 2.) 



Although the study of succession has not been carried out in 

 sufficient detail to disclose the whole story of the development of 

 herbaceous vegetation, enough is known of the relation to western 

 yellow pine reproduction to warrant the following conclusions: 



Reproduction of western yellow pine in this region may occur 

 in any stage of the series. The chances for success are best in the 

 first weed stage or at the beginning of the second weed stage, when 

 there is sufficient cover to assist germination, but not enough to sub- 

 ject the seedlings to severe competition for moisture and light. If 

 moderate amounts of dead material, such as leaf litter or branches, 

 can be provided as a soil cover, the entire absence of growing plants 

 is preferable. During the second weed stage there is danger, especi- 

 ally on the richer soils, that the herbaceous plants will suppress the 

 pine seedlings. On the whole, the subclimax stage is less favorable 

 than the first weed stage, but is at least as favorable as the second 

 weed stage. According to the usual conception, succession is asso- 

 ciated with soil building; that is, the lowest stage occurs on the 

 poorest soil and paves the way for the next higher stage by building- 

 up the soil. Sampson (23) has shown that on areas subject to rapid 

 erosion plant retrogression and soil deterioration go hand in hand 

 and that before the subclimax stage can come back the soil must be 

 built up through the weed stages. This relation is the exception 

 rather than the rule in the yellow pine forests of the Colorado Plateau. 

 Here retrogression from the higher stages of plant cover is seldom 



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