Understanding the role of resource limitation in restoration of sagebrush ecosystems dominated by cheatgrass (Bromus tectorum)

Background/Question/Methods

Invasion and expansion of cheatgrass (Bromus tectorum L.) into cold desert shrublands is often linked to increases in resources, and restoration ecologists have suggested that decreasing nitrogen availability and restoring a more conservative nitrogen cycle should decrease the competitive advantage of cheatgrass.  Repeated burning decreases soil carbon and mineral nitrogen in grassland and forest systems, but use of repeated burning to decrease soil resources in cheatgrass dominated systems has not been tested.  In a five year field study, we examined effects of repeated burning, litter removal, and seeding with an annual competitor (common wheat) after burning on soil mineral N, litter and cheatgrass biomass, and plant and seed density.  We used generalized linear mixed-effects models to examine differences among treatments over time. We developed a multivariate conceptual model of effects of environmental factors (precipitation, temperature) and restoration treatments on soil mineral N and cheatgrass success and we used structural equation modeling (SEM) to evaluate component hypotheses. We asked: (i) What are the direct and indirect effects of restoration treatments aimed at reducing resources on soil mineral N and cheatgrass success over time?  (ii) How do abiotic and biotic factors influence trends in soil mineral N and cheatgrass success over time? 

Results/Conclusions

We hypothesized that burning would result in a pulse in soil mineral N due to heat-induced soil organic matter denaturation, but that subsequent fires would volatilize biomass N resulting in progressive N deficiency over time. However, soil mineral N in burned plots was never lower than pre-burn levels likely because surface soil temperatures during burns were too cool to cause volatilization of N from soils or aboveground biomass. Repeated burning did decrease litter biomass but, contrary to our hypothesis, there was generally no difference in soil mineral N between litter intact and litter removed plots. Litter did have a negative effect on cheatgrass success likely due to effects on seed entrapment and plant establishment rather than N mineralization. As hypothesized, plots seeded with wheat generally had the lowest cheatgrass success, but mineral N was similar to other treatments and decreased cheatgrass success was likely due to reduced litter. In this semi-arid shrubland, long-term trends in soil mineral N and cheatgrass success were most strongly associated with precipitation and winter minimum temperatures.  Management strategies aimed at decreasing litter and seed banks and increasing competitive interactions may be more effective at reducing cheatgrass success than current approaches for reducing soil nitrogen.