干旱区生态环境知识资源中心

The salt desert shrub ecosystem has suffered increasing invasion by exotic annual weeds, especially cheatgrass, for the last 25 years or longer. After the extensive shrub dieoffs of 1982 to 1984 in the Great Basin and Colorado Plateau, many shadscale (Atriplex confertifolia) and winterfat (Ceratoides lanata) populations failed to regenerate. Several facultatively pathogenic fungi were isolated from the rhizosphere of shadscale. Levels of soil Fusarium and pythiaceous fungi in intact and dieoff shrubland areas were measured with positive correlation between fungal density and dieoff severity of shadscale in one study and winterfat in a separate study. Shadscale seedings in three shadscale sites invaded by annual weeds in the Utah western desert resulted in significant seedling emergence followed by 100 percent establishment failure. Seedlings with wilt symptoms were observed in each of these studies. In each case soil moisture was adequate for seedling establishment. A greenhouse test was conducted to study seedling establishment of these two species in 10 valley bottom soils, four from recruiting shrub populations and six from defunct shrub sites invaded by cheatgrass or halogeton. Autoclaved and fungicide-treated soil treatments significantly increased winterfat seedling survival (P > 0.001) in soils from weed-invaded sites. No effect of soil treatment was seen in shadscale seedling numbers in this study. Winterfat seedling weight was not affected by soil treatment and did not differ between soil sites. Shadscale seedling weight was significantly increased by soil treatment in intact soils but not in weedy soils. In a separate emergence study shadscale fruits were planted in soil from two intact and two weed-invaded shadscale sites. After moist chilling at 2 degreesC, more than twice as many live seedlings were observed in the two soils from intact sites as in the two soils from weed-invaded sites. Work in progress will attempt to isolate a causal organism from dying shadscale seedlings. Shadscale sensitivity to fungicides will also be investigated.