1918] ROBBINS—BOULDER PARK 511 
situations. More important, however, are soil, temperature, and 
moisture conditions. The maximum seasonal variations of soil 
temperature in typical associations during the summer of 1909 (13), 
as seen in the following table, will give some explanation of the 
rapidity of change in the seasonal aspects. 
Sedee MiGOr. foci ee 7° F. 
Willow thickets oe oe i 8° 
Meadow 0th, 3. a 16° 
Herhaceots meadow oo i ee 20° 
Dry grawmlend 700 oe ie 24° 
It will be noted from this that the sedge moor has the least 
variable soil temperature throughout the growing season. This 
condition is due for the most part to the amount and texture of 
the vegetative cover, and also is intimately related to the soil water 
content. Sedge moor vegetation screens the soil efficiently. More- 
Over, as.a result of its high water content the specific heat of sedge 
moor soil is high and its conductivity of heat low. In the dry 
grassland, on the other hand, there is an absence of a dense vegeta- 
tive covering. Here the soil has a low specific heat, due to its 
dryness, and its heat conductivity is great. Dry grassland heats 
up more rapidly and cools off more readily and to a greater depth 
_ Sooner than either meadow or sedge moor. As regards soil tempera- 
ture and soil moisture, the sedge moor shows less seasonal variation 
than either meadow or dry grassland. This condition appears to 
be correlated with the lack of marked seasonal aspects. Edaphic 
conditions within a community control the seasonal changes of the 
vegetative covering. 
We have described the stages in the development of the flood 
plains of a mountain park. The oxbows and oxbow ponds are 
Prominent features of these flood plains. Boulder and gravel 
shores or sand and fine silt shores are the initial habitats. They 
culminate in a temporary meadow climax. Two exceptionally 
distinct ages of flood plains are represented: a recently formed one 
now in the willow thicket stage, and an older one on the middle 
terrace in the meadow or temporary climax stage. A consideration 
of the fate of the meadow will be discussed later, after treatment of 
the glacial lake and dry grassland series of succession. 
