干旱区生态环境知识资源中心

Vegetation plots in experimentally grazed pastures at the Desert Experimental Range in west-central Utah have been mapped periodically since 1935. The pastures have been grazed at the same intensity and in the same season by sheep from 1935 to the present. Records show that Bromus tectorum and Salsola pestifer were observed on mapped plots in 1958, although the latter species was reported from the range in 1937. Halogeton glomeratus appeared first in 1969. All of these species had become common an both grazed and ungrazed plots by 1989, although plant density and vigor of these aliens were reduced on ungrazed sites. Frequency of occurrence of these species has increased steadily since their first appearance. Experimental studies suggest that soils from areas dominated by Halogeton and Salsola are associated with lower survival of Ceratoides lanata seedlings than soils from healthy Ceratoides stands.