ro18] ROBBINS—BOULDER PARK 499 
during dry years. The temperature extremes are great. Further- 
more, the occurrence of flood-waters postpones the invasion of 
pioneers. The freshly exposed stones and gravel possess no vegeta- . 
tion. Algae which may have been clinging to rocks while sub- 
merged are killed on exposure to the sun. There is no lichen 
stage on the rock surfaces. The first plants gain a foothold in 
Fic. 4.—Along South Boulder Creek: flood plain of cobblestones invaded by 
highly ieee plant community; note zone of Carex variabilis bordered outwardly by 
willow thicket 
the meager accumulation of fine sediment between the stones. 
Agrostis hiemalis (Walt.) B.S.P. and moss species usually initiate 
the succession. The individual grass plants form an interlacing,’ 
dense mass of fibrous roots which collect and hold sand and silt. 
These initial plants are followed by a highly mixed community 
composed of migrants from the sedge moor, willow thicket, meadow, 
and even dry grassland; in fact, there is no new habitat in the 
Park, except it is the roadside, where there is such a great number 
