Cover (Area Occupied) 



Cover, or specifically herbage cover, signifies primarily the 

 area of ground occupied by the leaves, stems, and inflorescences, 

 i.e., the above-ground parts of plants, as viewed from above. Each 

 layer of vegetation is considered separately, since overlapping 

 usually occurs, so that a tall plant is rated apart from one grow- 

 ing under it. Basal area, however, refers to the ground actually 

 covered by the crown only, or actually penetrated by the stems, 

 and readily seen when the leaves and stems are clipped at the 

 ground surface. "Basal area" has also been used to denote the area 

 occupied by the vegetation at 1 in., or at some other level, above 

 the ground (Figures 1-20 and 1-21). 



Discretion must be used in comparing cover data of various 

 species which may have been taken at different heights, and often 

 comparisons are not valid. ^^ Measurements of basal area may 

 vary considerably for the same plant, depending upon the height 

 at which the measurements are taken (Figure 1-2). For example, 

 in the same quadrat the area at the surface occupied by Aristida 

 longiseta was 382 cm^, and by Buchloe dactyloides, 408 cm^; at 

 a height of 1 in. the respective areas were 914 and 505 cm^; and 

 the total herbage covers of each were 1666 and 505 cm^, respec- 

 tively. The former grows in small bunches that have their maxi- 

 mum spread above 1 in., while the latter, a mat-former, often has 

 its maximum spread below 1 in., so the last measurements pre- 

 sent the most reliable comparison. ^°'^ Under the influence of pre- 

 vailing conditions herbage cover data are comparable, but it is 

 often difficult to make valid comparisons of basal-area data of dif- 

 ferent life-forms even in the same sample area. It may be desirable 

 in some studies, however, such as those on the effects of runoff, 

 erosion, and grazing, to use basal area; but the measurements 

 should then be taken of the maximum crown spread, which in 

 bunchgrasses such as Festuca arizonica and Muhlenbergia montana 

 may be at 3 to 5 in. above the ground, while in sodgrasses such 

 as Bouteloua gracilis it is often less than 1 in. Decreases in runoff" 

 and erosion may often be correlated better with crown spread of 

 bunchgrasses than with basal area. 



In research on grasslands in which permanent effects of graz- 



102 * TKe Coxnmimnity 



