The Effects of Targeted Triclopyr Application on Habitat Quality in Boreal Saskatchewan Transmission Rights-of-Way

Abstract

Vegetation management along transmission rights-of-way in remote northern forests across Canada is challenging. Mechanical removal of vegetation is often ineffective as many boreal species regenerate rapidly upon physical disturbance. Limited information on herbicide impacts in northern regions and on boreal vegetation makes communicating risks and benefits to local stakeholders and Indigenous communities difficult. Herbicides directly enter the ecosystem through deposition on vegetation and soils following application. Treated vegetation can be a vector of contamination to browsing herbivores, and herbicides can indirectly enter the soil ecosystem upon leaf abscission. Litter decomposition is critical to soil nutrient cycling and ultimately ecosystem health. The indirect effects of herbicides on habitat quality in boreal ecoregions remains poorly understood. Working in collaboration with SaskPower and the Lac La Ronge Indian Band, the influence of targeted applications of the herbicides, Garlon RTU and Garlon XRT (active ingredient triclopyr) were studied in northern Saskatchewan. Triclopyr drift and dissipation in foliage were assessed following a targeted low-volume foliar (Garlon XRT) or basal bark (Garlon RTU) application. Greater drift concentrations localized at the stem base were observed with basal bark treatments. These effects may be exacerbated with high stem density, especially in conjunction with sandy soil prevalent in northern Saskatchewan which increases the potential of herbicide mobility and off-target effects. Concentrations in foliage were higher following low-volume foliar applications but dissipated to 50% of initial concentrations within a week (DT50 = 5.7 days and DT90 = 34.6 days). A hazard quotient risk assessment for moose (Alces alces) and snowshoe hare (Lepus americanus) indicates browsing on triclopyr treated foliage with the residues detected in this study are unlikely to result in acute toxicity (extrapolated from concentrations that caused 50% mortality in rats and rabbits, respectively); however, long-term browsing may cause adverse chronic effects (extrapolated from the concentration with no observable effects in a two-generation reproduction rat study with a safety factor of 100). Basal bark application is ideal when stem density is lower and toxic effects for herbivores is of concern, and low-volume foliar applications are best suited in areas with higher stem density when off-target herbicide deposition is less acceptable. The indirect impacts of triclopyr on habitat quality were also examined through litter mass loss and quality (carbon:nitrogen ratios) as was the response of boreal invertebrates (Folsomia candida and Oppia nitens) in microcosms and avoidance tests. Higher concentrations of nitrogen (lower carbon:nitrogen) were observed in field treated foliage resulting from triclopyr repression of natural leaf senescence processes. Litter breakdown rates were not significantly different within a year of treatment despite nitrogen profile differences between field treated and untreated samples. Triclopyr concentrations entering the ecosystem upon leaf abscission were below conservative avoidance endpoints for boreal invertebrates. The triclopyr concentrations that caused 50% of tested F. candida and O. nitens to avoid treated litter were above field application rates. At field application concentrations there were no differences in survival and reproduction rates of F. candida. Therefore, field application rates of triclopyr are not expected to impair ecosystem services and habitat quality based on the parameters evaluated in these studies. The results from these studies suggest the overall risk of targeted triclopyr use in northern Saskatchewan rights-of-way is low. These findings have improved our knowledge concerning triclopyr use in boreal ecosystems to support risk communication and informed integrative vegetation management decisions.