1910] ROBBINS—VEGETATION IN COLORADO 277 
SAN LUIS VALLEY 
Precipitation: mean annual below 10 inches; about 75 per cent. during’ 
growing season; July and August wettest months; small amount of snow. Tem- 
perature: mean annual 40°-45°; mean summer 60°-65°; mean annual and daily 
Tange great. Average date of last spring frost May 24 to June 12; average length 
of growing season (period without frost) 3 months, 15 days. Relative humidity: 
low. 
This dry, level country is covered with Artemisia tridentata, 
which spreads over extensive areas, the plants often reaching a height 
of 8 feet. Chrysothamnus spp., Sarcobatus vermiculatus, and Atriplex 
Spp- are abundant, alternating in some places with bunch grasses. 
On the slopes occur pinyon pines, cedars, and Rocky Mountain 
yellow pines. The higher plant zones are very similar to correspond- 
ing zones in other parts of southern Colorado. 
The level parts of San Luis Valley have the same altitude as the 
eastern foothill region (7o00-go00 feet). The climatic conditions 
are practically the same, with the exception of the precipitation and 
the daily and annual ranges of temperature. In the eastern foothills 
the precipitation is 1 5-20 inches; in San Luis Valley below ro inches. 
In the eastern foothills the ranges of temperature are considerably less 
than in San Luis Valley. In comparing the monthly temperatures 
of Garnett, San Luis, and Saguache, all in San Luis Valley, with 
Idaho Springs, Georgetown, and Silver Cliff, all within the eastern 
foothills, it was found that, although the mean summer and mean 
annual temperatures were about the same, the winters of San Luis 
Valley are much severer than those of the foothills, the months of 
December, January, and February being 10°15° colder. As a 
tesult of these differences in range of temperature and rainfall, the 
foothills support a forest growth, while grass and sage predominate in 
San Luis Valley. 
MIDDLE PARK 
Climatological data for this region are not sufficient to be of much 
use. It is very probable, however, that the lower and flatter portions 
of the park have nearly ro inches of rainfall. There are great extremes 
of temperature. Artemisia tridentata produces the principal forma- 
tion on the dry, level stretches. It is associated with Chrysothamnus 
