274 



PLANT SOCIOLOGY 



French garigues, the Greek phrygana are convergent pasture types 

 of the Mediterranean region. They are marked by then- wealth 

 of aromatic Labiatae, Cistaceae, Rutaceae, Artemisiae, Euphor- 

 biaceae, thistle plants, and thorny bushes such as Bupleurum spinosuni, 

 Poterium spinosum, and Astragalus — all plants which are hardly 

 touched by sheep or goats. The zoobiotically conditioned communi- 

 ties are, unfortunately, even today still prevailing against the struggling 

 remnants of forest (Fig. 140). 



While in the temperate and tropical zones the consequences of 

 excessive pasturing are not so grossly apparent, owing to powerful 

 regenerative forces of vegetation, they are not hidden from the trained 

 observer. 



There is an extensive literature on the influence of pasturing upon 

 natural vegetation. 



Clements and Goldsmith (1924) have carried out experimental 

 investigations on the subject in the prairies of North America. For 

 three years they weighed the production of grass on similar areas under 

 examination. 



This quantitative study of vegetation included plots under three 

 conditions: (1) entirely ungrazed, (2) grazed by the rodent, prairie 

 dog {Cynomys gunnisoni zuniensis), and (3) grazed by cattle and prairie 

 dogs. The results are given in the following table : 



T.VBLE 31. — Influence of Grazing upon Grass Production 

 (In grams of forage per square meter) 



One of the dominant grasses, Agropyrum, bears pasturing fairly 

 well, thanks to its underground rhizomes, while Sporobolus disappears 

 almost entirely. The production of hay was reduced one-third to one- 

 fourth by the prairie dog, but by the prairie dog and cattle to one- 

 twentieth of the unpastured meadow. 



The duration and intensity of grazing are of very great importance. 

 According to the intensity of the pasturing process (for which unfor- 

 tunately exact data are not available), in most pastured districts of the 



