Environmental factors have great influence on phenological 

 behavior. For example, in the southern Arizona desert (Figures 

 1-12 and 3-1) plants begin to grow as soon as the temperature is 

 high enough in late winter or spring, and if there is enough mois- 

 ture the desert blooms with the yellow flowers of Mexican poppy 

 (Eschscholtzia neomexicana), pink penstemon (Penstemon parryi), blue 

 lupines {Lupinus spp.), yellow bladder-pod {Lesquerella gordoni), 

 asters (Aster spp.), mariposa lily {Calochortus aureus), mustards, and 

 many others. Outstanding forage plants at this season are Indian 

 wheat {Plantago argyrea) and alfilaria {Erodium cicutarium), the lat- 

 ter a native of the Mediterranean Region. Growth ceases for 

 these plants, many of which have shallow roots, when the April- 

 June drought period sets in. The July rains bring on the growth 

 of summer annuals and many perennials, including the annuals, 

 six-weeks grama {Bouteloua barbata) and needle grama {B. aristi- 

 doides), which take the place of Indian wheat and alfilaria as 

 forage plants.''^ 



Phenologic events may vary considerably in time of occurrence 

 from year to year. Over a period of 10 years in southern Idaho the 

 date of inception of growth varied from March 20 to April 24, 

 growth of herbage to a height of 6 in. fluctuated between April 

 26 and June 1, and formation of seeds, from May 12 to July 10.^^ 

 In the Wasatch Plateau in Utah inception of growth varied as 

 much as 45 days in various years, and a height growth of 6 to 8 

 in. of the chief grasses — the height considered suitable for the in- 

 itiation of the grazing season — varied as much as 47 days.^^ 

 Altitude also influences phenologic behavior, as seen again in the 

 Wasatch Mountains, where the rate of development of plant 

 growth was delayed 10 to 14 days for each increase of 1000 ft.^^ 



Phenologic behavior influences competition and association of 

 species. The occurrence of vegetative growth, flowering, and fruit- 

 ing at different seasons, or only slight overlapping of these activ- 

 ities, often reduces the intensity of competition and favors 

 association. For example, Agropyron spp., Poa spp., and Bromus 

 tectorum start growth and mature earlier than Bouteloua gracilis, 

 with resulting reduction in competition, because the last has a 

 higher threshold for inception of growth and a greater heat re- 



I 



88 • THe Commianity 



