1910] ROBBINS—VEGETATION IN COLORADO 273 
sion, the mesas are not subject to as late spring frosts as are the adjacent plains. 
Average length of growing season (period without frost) about 4 months, 21 days. 
Relative humidity: mean annual 48-50 per cent. 
The lower foothills and mesas of northeastern Colorado represent 
the meeting ground of grass and forest formations. The trees here 
are Rocky Mountain yellow pine and Douglas fir (Pseudotsuga 
mucronata [Raf.] Sudw.), the latter occurring in the moister situations, 
The Rocky Mountain yellow pine meets the grassland formation on 
the lower parts of the mesas; this formation consists chiefly of 
Bouteloua oligostachya and Koeleria cristata (L.) Pers. Cercocarpus 
parvijolius Nutt., the mountain mahogany, and Yucca glauca are 
common on ridges. Other common shrubs of this region are Prunus 
melanocar pa (A. Nels.) Rydb., P. pennsylvanica L.f., Symphoricar pos 
occidentalis Hook., Rhus trilobata Nutt., R. glabra L., Ribes longi- 
florum Nutt., R. pumilum Nutt., R. vallicola Greene, Opulaster spp., 
Ceanothus Fendleri A. Gray, C. pubescens (T. and G.) Rydb., Cra- 
laegus spp., Rosa Sayi Schwein., Berberis repens Lindl., and Edwinia 
americana (T. and G.) Heller. 
The lower foothills and mesas of southeastern Colorado differ 
from the corresponding regions in northern Colorado. The mesas 
are grassland composed chiefly of Bouteloua oligostachya. Scrub 
oak and mountain mahogany form a chaparral between the grassland 
and Rocky Mountain yellow pine formations of the lower foothills. 
Pinyon pine (Pinus edulis Engelm.), the one-seeded juniper (Sabina 
monosperma [Engelm.] Rydb.), and the Rocky Mountain juniper 
(Sabina scopulorum [Sarg.] Rydb.) are common in the lower portion 
of the foothills. The Rocky Mountain yellow pine and other foothill 
forms extend far out on the Arkansas-Platte Divide.'s 
EASTERN UPPER FOOTHILLS 
Precipitation: mean annual 15-20 inches; about 75 per cent. during the 
rowing season; the early summer is the season of maximum rainfall. Tem- 
perature: mean annual 40°-45°; mean summer 60°-65°; mean annual range 
"5 For an extended account of the mesas and foothills of southern Colorado see 
the following papers: SHANTz, H. L., A study of the vegetation of the mesa region 
fast of Pike’s Peak; the Bouteloua formation. Bor. GAZETTE 42:16-47, 179-207. 
CHNEIDER, E. C., The distribution of woody plants in the Pike’s Peak region. 
Pierce, Coll. Publ. Scien Series 12:137-170. 1909. 
