SEASONAL SUCCESSION IN A DESERT COMMUNITY 235 



140. Seasonal Succession in a Desert Community . —The desert 

 vegetation in the vicinity of Tucson, xVrizona, has been more care- 

 fully studied than has that of any other ])lace in the world. Here 

 four seasons are recognized and these will be discussed in the fol- 

 lowing paragraphs. 



A. Winter Wet Season.— The secondary maximum of precipitation 

 for the year occurs in December and January and amounts to 2 

 or 3 inches of rainfall. The vegetation begins to awaken in January 

 and a large number of plants begin blooming early in February and 

 mature their fruits in ]\larch and April when there is a decrease in the 

 rainfall. The plants that bloom during the winter season may be 

 classed into two groups, namely, winter perennials and winter 

 annuals. The factor which usually checks the awakening of these 

 plants in the early part of the winter is the low night temperature 

 which often is several degrees below freezing. Sometimes, however, 

 a few warm days in December will bring forth a crop of low annual 

 herbs which may almost reach the blooming stage only to be blighted 

 by a frost early in January. The most outstanding characteristic 

 of these winter annuals which adapts them for desert conditions is 

 the remarkable power of resistance of the seeds. These seeds are 

 ripened and thrown onto the ground during ]\Iarch and April. The 

 surface layers of the soil reach temperatures above 100° F. during the 

 summer months. The summer rains soak both the soil and the seeds 

 but still no activity is shown. The summer cools into a rainless 

 autumn and finally the cooler nights are followed by the winter 

 rains of December and not until then do the seeds show signs of life. 



jNIany of the winter perennials lose their leaves with the approach 

 of the high temperatures of April and ^lay and the stems, bulbs, 

 or root-stalks go into a resting condition from which they do not 

 awaken until the following December or January, eight or nine 

 months later, the entire period of activity being limited to about one 

 hundred days. Some, however, such as the creosote bush {Larrea 

 tridentata), which has varnished leaves, and a few others with pro- 

 tective coatings of cutin or hairs, retain their leaves during the 

 greater part of the year. No plants without protected surfaces can 

 do this because during the summer the relative humidity is between 

 30 and 40 per cent and evaporation is, therefore, very rapid. 



B. The Dry Fore-summer.— The winter season may be said to end 

 about April 1st and is followed by the dry fore-summer which com- 

 prises April, INIay, and June with a total average precipitation of less 

 than 1 inch and maximum temperatures of 95° to 112° F. The 



