PS 53-144 - Exploring the causes and effects of cheatgrass die-offs in the Great Basin, USA

Wednesday, August 8, 2012
Exhibit Hall, Oregon Convention Center
Owen W. Baughman, Natural Resources and Environmental Sciences, University of Nevada Reno, Reno, NV, Susan E. Meyer, Shrub Sciences Laboratory, USDA Forest Service, Rocky Mountain Research Station, Provo, UT and Julie Beckstead, Biology Department, Gonzaga University, Spokane, WA
Background/Question/Methods

The ecologically destructive, highly invasive annual grass Bromus tectorum (cheatgrass) occurs in monocultures across large areas of the semiarid West. An observed but little-studied phenomenon in B. tectorum monocultures is a discrete ‘die-off’ event, where the seed bank of this grass suddenly and inexplicably fails to produce even a sparse emergent crop for one or more growing seasons. Such die-offs represent an opportunity for researching B. tectorum ecology, epidemiology, and biological control.  We present two studies which 1) aimed to determine if the primary causal factors of die-offs were biotic or abiotic using a baited-seed greenhouse experiment involving sterilized and nonsterilized field soil from two 2009 die-off sites in Nevada and Utah, and 2) used seed bank sampling to explore the role of a generalist seed pathogen Pyrenophora semeniperda – currently under investigation for use as a biological control for B. tectorum – in causing die-offs at ten sites in Nevada, Utah, and Washington in 2008. Both studies used immediately adjacent healthy monocultures as non-die-off references.

Results/Conclusions

In the greenhouse, no significant inhibition of B. tectorum was observed in sterilized soil from die-offs compared to sterilized reference soil, indicating that abiotic soil characteristics were not the direct cause. Significant mortality of baited seeds in nonsterilized die-off soil was also not observed, meaning no direct evidence of a biotic cause was found. The die-off phenomenon was found to cause a significant fertilizer effect, with soil nitrate being over four fold higher in both die-offs than in their respective references. We discovered that analyzing seed bank fractions proved to be the best way to determine die-off age, which became a crucial detail for making inferences. Pyrenophora semeniperda did not appear to be causal, as no significant differences in fungus-killed seed densities were found between die-offs and references. While failing to pinpoint a causal factor in B. tectorum die-offs, the findings of these exploratory studies are guiding current and developing investigations of this intriguing phenomenon, and have identified exciting restoration potential at these heavily degraded sites.