Print PageThe Arkansas River watershed, in southeastern Colorado, accounts for over 70% of the area infested by tamarisk in Colorado. Previous studies on passive re-vegetation and site restoration following tamarisk removal have either been retrospective meta-analyses, or were carried out in regions of the southwestern US with very different climates and infestation characteristics. Therefore, this study is novel both in its direct spatial comparison of commonly used removal methods, and the actual project area. Four sites were chosen in the watershed to investigate the effect of removal methods on passive understory re-vegetation, with four adjacent 0.5-1 hectare primary treatment plots (aerial herbicide application, Hydro-Axe mulching, excavation, untreated control) installed at each site. Within each treatment plot six multi-scale, nested sampling plots were randomly located to monitor tamarisk re-generation and understory vegetation recruitment and establishment. Additionally, at three of the sites we are evaluating the influence of tamarisk canopies on aerially applied herbicide (imazapyr) retention and soil dissipation. For this companion study artificial trees outfitted with blotter paper were installed at sites prior to treatment to collect applied herbicides, and soil samples taken before and after treatment to measure imazapyr soil dissipation under natural field conditions.
Results/Conclusions
Preliminary results have indicated that the average tamarisk canopy retained 74% (P < .0001) of aerially applied imazapyr, and that retention significantly (75%, P < .0001) reduced soil residue levels beneath the canopy. Also, soil dissipation underneath the canopy occurred at slower rates than dissipation in open areas. Soil residue levels one YAT varied, but in some samples imazapyr persisted at concentrations (200 ppb) known to be phytotoxic to desirable plants. Herbicide applications resulted in significantly reduced plant species abundance (richness: 65%, P < 0.05; basal cover: 73%, P < 0.05) and diversity (62%, P < 0.05). Mechanical treatments increased plant species richness (28%, NS), a result that was possibly due to a positive effect from the layer of debris created by this removal method. Mechanically treated plots that contained debris had significantly higher species richness (81%, P < .0001) than did adjacent cleared plots. In conclusion, our preliminary findings suggest that various removal methods do influence subsequent understory re-vegetation patterns, and that in general there is a higher capacity for desirable passive re-vegetation than was previously thought. These sites will be monitored for several years to provide a better understanding of the interaction between tamarisk removal methods and resulting plant species recruitment and re-establishment.
