During recent decades in semi-arid riparian areas of Nebraska and the Great Plains, invasive woody species like eastern redcedar (Juniperus virginiana) and Russian olive (Elaeagnus angustifolia) have encroached and, in some cases, replaced existing vegetation, and altered ecosystem composition, structure and function. This study assessed changes in the understory species composition after removing eastern redcedar and Russian olive from several stretches along the Republican River cottonwood (Populus deltoides) riparian forest in Nebraska.  In April 2008, six plots were established in Red Willow County. Invasive woody species were removed from three treatment plots and the other three were left as controls.  In August 2008, the overstory and nearest understory species were recorded every 2 m along five line transects in each plot.  To test whether understory vegetation changed with removal treatments and/or overstory conditions group, rare species were grouped together and then the square root of the species/group frequencies were arsine transformed and analyzed using Detrended Correspondence Analysis (DCA). 

Results/Conclusions In control plots, cottonwood and canopy gaps accounted for most of the overstory.  Small eastern redcedar individuals occurred more frequently under cottonwoods (73%) than under canopy gaps (65%).  In contrast, Russian olive and green ash (Fraxinus pennsylvanica) occurred more frequently under canopy gaps (20% and 11% respectively) than under cottonwoods (13% and 6% respectively).
A total of 34 forb, grass and sedge species were encountered along transects.   More species were tallied in removal (22 ± 2) than in control plots (13 ± 2).   Control plots had lower Axis 1 values, associated with the occurrence of native woody species, nutsedge, invasive tree species and bare spots.  Treatment plots had higher Axis 1 values associated with invasive forb species such as bull thistle (Cirsium vulgare), ground cherry (Physalis virginiana), and common mullein (Verbascum thapsus) and invasive warm season grasses such as barnyard grass (Echinocloa crus-galli).  Smooth brome (Bromus inermis) frequency decreased under eastern redcedar in both treatments.  In treatment plots, invasive forbs increased at locations where eastern redcedar had been removed while warm season grasses increased where eastern redcedar had been absent. We conclude that type of overstory significantly influences understory vegetation and its recovery after removal of invasive woody species.