The Ecological Distribution of Cholla Cactus (Opuntia imbricata (Haw.) DC.) in El Paso County, Colorado
Abstract
The range of Opuntia imbricata (Haw.) DC., together with data from 250 weather stations, was mapped revealing that the cactus is essentially confined to a region receiving 28 to 48 cm annual precipitation and experiencing mean January temperatures above --1'C. The distribution of the cactus was quan- titatively mapped for El Paso County, Colorado, the northernmost county of its range. Here the cactus achieves greater densities upon soils of high clay content and upon areas of sloping and concave surface and southern aspect. An analysis of published data from New Mexico reveals a greater abundance of cholla upon fine textured soils in that state as well. Individual plants are dispersed according to a Poisson distribution-at least within quadrats 4 to 100 m2 in area. Within 25 m2 quadrats the cactus asociated positively, and significantly, with Sitanion hystrix, Hilaria famesii, Muhlenbergia torreyi, and Yucca glauca. Negative associations were observed with nine species, most of which are commonly regarded as weeds or indicators of range deteriora- tion. The quadrats were objectively segregated into several groups, and the species composition of each group has been tabulated. The successional status of Opuntia imbricata has not been established, nor has its status as a pasture pest been determined.
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