Plant Associations at the Desert Laboratory. 91 



Rkvikw oi' THE Plant Associations. 



It has been seen that the plant associations and habitats 

 of Tuniamoc Hill and the adjacent valley fall naturally into four 

 well-defined groups, namely, those of the river, of the flood-plain, 

 of the slopes, and of the hill. 



Associations of the river. — These are two in number: first, 

 the aquatic plants of the river itself and the irrigating ditches 

 fed from it, and second, the species of the river-banks. These 

 latter include not only willows and cottonwoods, but arrow- 

 weed and some other species characteristic of the arid Southwest. 

 The small number of species belonging to both of these associa- 

 tions, in comparison with those of corresponding habitats in 

 regions of greater rainfall, is significant. It is apparently due to 

 two causes; first, the greater difficulty which seedlings encounter 

 in starting where the water-supply is intermittent, and second, 

 the high rate of evaporation in connection with the inconstant 

 water-supply. 



Associations of the flood- plain. — No associations are more 

 characteristic than the two which belong to the flood-plain of 

 the vSanta Cruz and other rivers of the same region, nor could there 

 well be a case in which the delimitation of the smaller area oc- 

 cupied by one of these associations from the larger one in which 

 it lies is more clearly referable to special conditions. In this case 

 accumulations of alkali salts, commonly in areas of defective 

 drainage, constitute the essential peculiarity of the habitat of 

 the salt-bushes. 



Outside of these areas the flood-plain is the habitat of an 

 association composed of two groups of plants of very different 

 biological requirements. These are, on the one hand, the mes- 

 quite and Acacias, whose long roots extend to depths where a 

 sufficient water-supply is assured, and on the other, the Bigelowia 

 and various other plants of low growth, the roots of which occupy 

 more superficial soil-layers and are, as far as the soil relations 

 are concerned, subjected to more distinctively xerophytic con- 

 ditions. These differences of ecological requirements in plants 

 closely associated on the same ground are of fundamental im- 

 portance as regards competition. 



Associations of the slopes. — To this group belong the creosote- 

 bush association, the palo verde-catclaw association of the wash 



