68 Journal of Agricultural Research voi. xix. no. 2 



individual plants or between mats and bunches of species which grov/ in 

 that manner. The term cover (8), or ground cover (5), is frequently and 

 conveniently used in connection with discussions of vegetation. However, 

 when the term cover is applied in connection with grazing investigations 

 it should be defined, for it may mean one of two things: (i) basal cover, 

 or the ground surface limits of living vegetation, or (2) the foliage cover, 

 which is the plant layers above the basal cover. When the foliage cover 

 is removed, as by close grazing or clipping, the basal cover remains. 

 Plant layers as described by Clements (i) are vertical zones based on the 

 height of plants. On the prairie around Mandan two layers are impor- 

 tant — the ground layer, as Bouteloua and Carex, and the secondary layer, 

 as Stipa and Psoralea. 



Species that grov/ in mats or in bunches are most accurately expressed 

 in terms of basal cover. F'or example, Bouteloua basal cover would 

 refer to the amount of ground surface actually covered by Bouteloua if 

 the foliage were removed by grazing or clipping. In such species it is 

 possible to make the determinations with almost mathematical preci- 

 sion. Species that occur as individuals are best expressed in terms of 

 their abundance per unit area. Shantz (7) says in regard to this point: 



Those species which form mats can not be well represented in numbers per square 

 meter, and on this accoimt the percentage of surface covered is given instead. 



The foregoing statements in regard to basal and foliage cover are 

 very clearly illustrated in Plate 14. In 191 5 the foliage cover was very 

 heavy because growth conditions were favorable and the 

 ^IH^ area had not been grazed. An estimate of the total cover 

 iu^jl based upon the amount of foliage cover could easily have 



pT^i^ been made at that time. But in 191 6 on the same area, 



i i^L ! with the foliage cover removed, there would have been 



'( '(fj\ no basis for comparison with the 191 5 condition. This 

 \.^ illustrates the undesirability of utilizing the foliage cover. 



Fig. 1— Diagram Under all Conditions, as a basis for estimating the pos- 

 of grass mat: sibiHtics of foragc production and the consequent carry- 

 si from above.' iug Capacity. A clear distinction between basal cover 

 o. Basal cover; g^^^j foliage covcr is, therefore, necessary and important. 



0, foliage cover. . . r i • « 



The two illustrations of Plate 14 picture the same area, 

 but one illustrates a heavy foliage cover and the other only the basal 

 cover. However, the potential ability of the area to produce under 

 similar conditions as heavy a foliage cover as in 191 5 is unchanged. 



Figure I illustrates the difference between the basal cover and the 

 foliage cover. The limit of basal growth is a, while the limit of foliage 

 growth is b. In a given case the surface area of the foliage cover is greater 

 than that of the basal cover, yet the amount of forage is the same. The 

 basal cover is more permanent than the foliage cover, since the latter 

 may be readily removed by grazing. The quadrat map (fig. 2) in the 

 30-acre pasture, which was mapped in 191 5 and remapped in 191 6, shows, 



