364 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



winter. The dominant Grasses are perennial, with axes on the 

 average about half-a-metre high, and typically involve such familiar 

 genera as Poa, Festuca, Lolium, Agrostis, Alopecurus, Phleum, and 

 many others. The associated forbs are also mostly perennial, or, 

 less frequently, biennial, and are often rosette-chamaephytes. Like 

 the Grasses, they may be coarse and 1-2 metres high in favourable 

 circumstances especially in damp situations, but rise scarcely at all 

 above the surface of the soil when intensely grazed ; such close 

 cropping cannot, however, be long withstood by most species. 

 Annuals occur chiefly in open patches that are due to overgrazing 

 or drought, while Mosses may form a weak ground-layer especially 

 m the damper situations. Woody plants, like geophytes, if present 

 are usually little in evidence — except where the biotic disturbance 

 is light and there is a tendency towards heath development or 

 reforestation. 



Semi-Deserts AND Deserts 



Although what amount practically to desert conditions are com- 

 monly developed very locally on dry rock and sand or other porou 

 surfaces, these usually show evidence of at least incipient succes 

 sional change. On the other hand, extensive desert areas tend to 

 be among the most stable vegetationally, owing to the general lack 

 of water for more growth than that of the plants already present. 

 Intermediate in water-relations between true deserts and the drier 

 among the areas whose vegetation has been described earlier in this 

 chapter, and usually intermediate also in geographic position, are 

 various semi- or near-desert areas such as the Sage-brush ones of 

 western North America. These are characterized by the Sage-brush 

 {Artemisia trident at a) and other low and often greyish shrubs having 

 herbaceous branches, that dominate a climax in regions having an 

 annual precipitation of 12-25 cm. (approximately 5-10 inches). 

 Such shrubs often belong to predominantly herbaceous families and 

 also thrive in areas of greater rainfall when the competition from 

 Grasses is eliminated. The scrub is low and often broken or of 

 more or less widely-spaced single bushes, the depauperated types 

 merging into the still more xerophytic ' desert-scrub ' developed 

 where the rainfall is limited to 7-12 cm. annually. In this latter 

 instance the dominants are bushy shrubs, particularly of Creosote- 

 bush [Larrea tridentata), usually |-2 metres high and spaced on 

 the average 5-15 metres apart through a widely-spreading root 

 system. 



