Our study was designed to better understand how the removal of Russian olive (Elaeagnus angustifolia) affects soil N and plant communities in mixed grassland, riparian ecosystems. Previous work has shown that Russian olive invasion is associated with elevated soil N levels, due to its N-fixing ability, and higher proportion exotic plant and annual forb cover than areas outside the tree’s canopy. However, it was unclear how these changes to the ecosystem respond after Russian olive removal. To address this, we measured ionic soil N concentration and plant cover in plots underneath and outside Russian olive canopy, from 2010 through 2014, along the South Fork of the Republican River in eastern Colorado. We used a mixed model ANOVA to analyze soil N concentrations and non-metric multidimensional scaling (NMDS), multi-response permutation procedure (MRPP), and species indicator scores to compare plant communities.
Prior to removal (2010 and 2011), plots underneath Russian olive were associated with higher ionic soil N concentration, as well as higher proportion exotic plant and annual forb cover than plots outside the tree’s canopy. In the winter and spring 2012, Russian olive was mechanically removed from half of our plots. After removal, available soil N showed no significant difference between removed and unremoved plots, and the proportion exotic and forb cover increased for all plots. Kochia (Bassia scoparia) was particularly driving this pattern, as its cover increased substantially for all plots. The increase in kochia was highest in plots in the removal treatment where soil was disturbed, and the elevated soil N associated with Russian olive may have increased the effects of soil disturbance. These results indicate that (1) the effects of Russian olive on soil N and plant cover persist in the third year following its removal, (2) annual forb species, and in particular kochia, have increased dramatically in areas where the soil was disturbed by the removal process, and (3) this disturbance effect seems to be exacerbated by the high soil N levels associated with Russian olive.