Scientists are dismayed by the massive decrease in the size and extent of shrub growth over the past five years in certain areas of the Great Basin.

Termed "dieback,"' the condition has severely decreased the numbers of livestock that can be grazed on those areas.Utah State University researchers Jim Dobrowolski and Kern Ewing of the Range Science Department have been working with the U.S. Bureau of Land Management to monitor plant populations and determine underlying reasons for the dieback.

Dobrowolski said the dieback has been especially severe in Utah in Skull, Rush, Puddle and Pine valleys south of the Great Salt Lake. The dominant plant in the valleys is shadscale, which provides winter grazing for sheep and cattle, and is of limited use for free-ranging pronghorns.

The scientists have concentrated their studies in Puddle Valley, between the Lakeside and Grassy mountains.

"Among the possible trigger mechanisms we've been looking at are saltwater intrusion from the Great Salt Lake and the waterlogging of soils caused by the above-average precipitation from 1982 to 1985,"' he said.

"Because many soils in Great Basin Desert valleys don't drain well, standing water may saturate surface soil horizons and deplete oxygen,"' Dobrowolski said.

The USU researchers suggest that oxygen stress may weaken the shadscale in saturated areas, allowing fungi, bacteria or viruses to invade, which then spread the dieback to plants growing in drier sites.

The severely cold winter of 1983 and the summer droughts of 1984 and 1985 may also be contributing factors, and the stress on the plants may be increased by grazing livestock and plant-eating insects, he said.

Ewing said the U.S. Department of Agriculture shrub laboratory in Provo has isolated several fungal pathogens, including water mold, from the dying plants, but the role of the fungi in the dieback is not clear.

Recent vegetation surveys show that affected stands of shadscale may have five dead shrubs to one live shrub, while in healthy stands, the ratio is reversed. The death of shadscale has favored an increase in growth of cheatgrass and annual plants that are of little or no forage value.