Alien Invasion: Effects of Atmospheric Nitrogen Deposition on Sagebrush Steppe Vegetation Dynamics at Upper Columbia Basin Network Parks

Allen, Edith; Bell, Michael

Agricultural operations within the Snake River Plains cause emissions of nitrogen (N) -to the atmosphere. This nitrogen is deposited through a combination of wet and dry deposition both locally and downwind from emission sources. Nitrogen additions to the ecosystem can cause changes to nutrient cycling and vegetation community dynamics. In this project, we measured atmospheric concentrations of nitrogen species NOx, NO2, NH3, and HNO3 at 10 sites within the Upper Columbia Basin Network Monuments and compared them to 2006 CMAQ modeled deposition values. We also measured perennial and annual vegetation to determine if the spread of the invasive grass Bromus tectorum (cheatgrass) was correlated with the nitrogen deposition gradient. The passive atmospheric samplers were consistent with modeled values in that NH3 was the dominant N species in the area and that the highest concentrations were near the concentrated animal feeding operations near the Snake River. Bulk deposition samplers were contaminated by birds where NH3 concentrations were high, but a high correlation between the noncontaminated sites allowed us to calculate expected deposition values. The N deposition at all sites was higher than was previously modeled. This was especially true at the low deposition sites where measured values were 200% higher than modeled values. There was an increase in cheatgrass cover as nitrogen deposition increased across the region. As cheatgrass increased, there was concomitant decrease in native forb and native grass species. There was also a decrease in shrub cover as nitrogen deposition decreased. The combinations of these vegetation changes have the potential to increase fire risk in high deposition areas due to an increase in standing biomass of cheatgrass in the summer. The degradation of native vegetation may also lead to changes in higher trophic levels in the ecosystem. Our results raise these park specific concerns and management recommendations: Craters of the Moon National Monument (3.0-8.1 kg/ha/yr) has the largest gradient of N deposition within its boundaries due to its size and north/south orientation. The southern and western regions of the park are at highest risk as they are closest to emission sources. These areas also seasonally grazed; which disturbs the soil and can increase the potential for cheatgrass invasion. The northern region of the park near the Visitor’s Center currently has low levels of deposition, but the levels are considerably higher than previously modeled. As this area is protected by the National Park Service, there is no grazing. If N levels increase there may soon be shifts in vegetation communities toward more cheatgrass. Hagermann Fossil Beds National Monument (10.4 kg/ha/yr) and Minidoka National Historic Site (11.2 kg/ha/yr) are most at risk due to vegetation changes due to N deposition. They are both located in high deposition areas and are adjacent to large agricultural operations. Restoration would require control of invasive species, a difficult task when their productivity is increased under high N deposition. If the number of CAFOs increases the impacts of nitrogen on the ecosystem will become more severe. City of Rocks National Reserve (4.0 kg/ha/yr) consistently had the lowest concentrations measured during the project. The northwest region of the park may have higher nitrogen concentration than have been modeled due to emission sources to the west. This area of the park should be watched closely as it is the most likely path of a future invasion front.

Alien Invasion: Effects of Atmospheric Nitrogen Deposition on Sagebrush Steppe Vegetation Dynamics at Upper Columbia Basin Network Parks

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