Management efforts in natural systems are increasingly focused on suppressing invasive plants which have overrun vast areas worldwide. Underlying these efforts is the common assumption that suppression of the target species will release native system components from negative impacts of invasion, thereby shifting conditions closer to the pre-invasion state. Weed control tools like broadleaf herbicides are frequently used to aid in the goal of restoring native plant communities, yet these tools can have unintended effects by harming native taxa or promoting invasion of secondary exotic species. The positive versus negative effects of weed control measures must be quantified in a comprehensive manner to determine efficacy of restoration efforts.
We conducted a long-term study to evaluate the efficacy of a fall, broadcast application of picloram for restoring prevalence of native species within grasslands invaded by spotted knapweed (Centaurea maculosa). To consider a range of local-scale conditions, sampling was conducted in plots at four pairs of sites initially containing variable levels of knapweed. The herbicide picloram was aerially applied to one site in each pair after one sampling year, and treatment effects on native functional groups were evaluated at differing baseline levels of knapweed invasion for three subsequent years and again at six years post treatment. This approach allowed us to examine treatment effects as a function of initial conditions and relate changes in prevalence of native functional groups to baseline declines associated with knapweed invasion.
The herbicide treatment effectively suppressed spotted knapweed, setting the stage for release of native species from invader impacts. However, treatment effects were complex and depended on initial levels of knapweed invasion. Treatment appeared to alleviate impacts of knapweed invasion on perennial grass cover, but these positive effects were only significant in plots with the highest initial level of knapweed invasion. At the same time, picloram application had predominately negative effects on native perennial forbs, the group most sensitive to knapweed invasion (Ortega and Pearson 2005). In plots where knapweed was initially absent, treatment reduced cover and species richness of perennial forbs to levels generally found in highly invaded plots. However, at moderate and high invasion levels, negative treatment effects on perennial forb cover were countered by positive effects of knapweed suppression. Non-target effects on the dominant forb, arrowleaf balsamroot (Balsamorhiza sagittata) extended to seed production and abundance of seedlings, potentially causing population declines if prolonged (Crone et al. 2009). Annual forbs, which are relatively insensitive to knapweed invasion, were also negatively affected by treatment. Furthermore, treatment released cover of the invasive grass, downy brome (Bromus tectorum), which initially occurred at low levels and replaced spotted knapweed as the dominant species. Most treatment effects persisted through the sixth and final post-treatment year and would likely be reinforced by repeated applications used to maintain knapweed control. Our results suggest that broadcast picloram applications, alone, may not restore conditions found in non-invaded communities, and that costs versus benefits of treatment may depend on initial invasion levels. More selective application methods such as spot spraying would better focus suppression on target weeds, maximize release of native species, and localize side effects.
- University of Montana
- State of Montana
- Montana Department of Natural Resources and Conservation
- Lolo National Forest
- Plum Creek Timber Company.