Breaking the Cycle: can glyphosate and willow cuttings disrupt positive feedbacks that sustain reed canarygrass monocultures?

Background/Question/Methods

Restoring ecosystems invaded by persistent invasive plant species often requires the elimination of internal positive feedbacks created by the invader.  Reed canarygrass (Phalaris arundinaceae, hereafter RCG) creates monocultures in disturbed wetlands, currently dominating more than 40,000 ha in Wisconsin.  Previous studies have documented that a thick litter layer created by RCG monocultures prevents native plant establishment.  RCG also preferentially invades sites with high nitrogen (N) availability, suggesting that plant-soil feedbacks resulting in elevated N could be important for this species.  While many researchers have sought effective management methods to reduce RCG dominance, few have used live willow (Salix sp.) stakes as a natural competitor.  We are conducting an experiment in a site dominated by RCG in the Upper Mississippi River (UMR) floodplain south of La Crosse, WI, U.S.A., 43°44’N  91°13’W.  We have three primary objectives in this experiment: 1) to determine the effects of fall glyphosate herbicide application on RCG productivity during the following growing season, 2) to evaluate how resource availability responds to glyphosate herbicide application, and 3) to examine the effects of glyphosate treatment, willow stake height, and planting date on willow (Salix nigra) stake success.

Results/Conclusions

In the first year of the experiment, glyphosate significantly reduced RCG % cover in April, May and June, and decreased below-ground biomass in August.  Thus, fall glyphosate application has the potential to limit both above-ground and below-ground competition from RCG the following growing season.  Ground-level light availability was significantly higher in plots sprayed with glyphosate than in unsprayed plots.  Available soil N and soil moisture levels were not significantly affected by glyphosate at any point during the growing season.  Thus, it is unlikely that glyphosate application is altering plant-soil feedbacks that lead to increased N levels.  Willow stake survival was significantly affected by planting date, with few fall-planted stakes surviving.  Survival of stakes planted in spring was significantly greater for short (90 cm) than for tall (150 cm) stakes, but was not affected by glyphosate application.   Willow average height and canopy % cover were significantly higher in plots where glyphosate was applied.  Thus, glyphosate prohibits the performance of RCG while aiding the establishment of a shrubby willow canopy, especially if short stakes are planted in spring.  We hope reduced RCG biomass will reduce its internal feedbacks; thus, breaking the cycle.