|Sheley, Roger - MONTANA STATE UNIVERSITY|
Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 14, 2000
Publication Date: N/A
Interpretive Summary: Cheatgrass was introduced into the interior Pacific Northwest during the late 1800's and has come to dominate large tracts of rangeland. In many areas there has been limited recovery of the native vegetation after cheatgrass initially gains dominance. We compared soil and plant nitrogen levels in native bunchgrass communities, and adjacent areas that had been dominated by cheatgrass for 40+ years. If long-term dominance by the introduced annual grass were to change fertility of a site, then restoration to native grasses might be especially difficult. We found that the conversion from perennial native grasses to the introduced annual had not changed site fertility. This suggests that the competitive nature of cheatgrass, rather than fertility changes, has kept the native vegetation from reestablishing on these sites.
Technical Abstract: It is often assumed that displacement of native perennial vegetation by exotic annuals will alter nutrient cycling. We compared nitrogen dynamics of native bunchgrass vegetation and adjacent stands of the exotic annual grass Bromus tectorum on three sites in southeastern Washington, USA. The stands of B. tectorum had dominated the sites for at least 40 years. We want to emphasize that these sites were not prone to frequent wildfires as can be the case in some B. tectorum dominated ecosystems. Over a two-year period we found very few consistent differences between the two vegetation types in aboveground standing crop, root mass, in situ N mineralization, extractable soil N, or total soil C or N. B. tectorum aboveground plant mass and litter tended to have lower C/N ratios than did the native vegetation, but the results were not consistent over time or site. It appears that the exotic annual adapted to the resources on the site and at least at the fairly gross level we measured, had little impact on soil nitrogen. Our results suggest that caution must be used in assuming that a change in vegetation type and growth form will necessarily alter soil N levels.