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

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.