COS 255-2 - Long-term glyphosate persistence in forest plant tissues and related impacts on plant nutrients

Background/Question/Methods

Glyphosate-based herbicides (GBHs) are highly effective, nonselective herbicides used in the Canadian forest industry to remove vegetation competing with commercially important conifer trees during reforestation of harvested cutblocks. Persistence of glyphosate in forest ecosystems had not been monitored beyond one year prior to our study, although literature suggests glyphosate may be stored in tissues of plants that received a sublethal dose of GBH. The primary objective of our research was to determine the duration (up to twelve years) of glyphosate persistence in the roots, shoots, and fruits of selected plant species in GBH-treated forest cutblocks. Due to the ability of glyphosate to form insoluble complexes with metal ions, it was suspected that metal nutrient concentrations in surviving plants might be affected by persistent glyphosate. The second main objective of this research was to determine if concentrations of calcium, iron, magnesium, manganese, nickel, and zinc were significantly altered by GBH treatment. Samples from five perennial species growing in two distinct biogeoclimatic (BEC) zones of northern British Columbia were analyzed for glyphosate content, and comparisons were made between species, plant tissue type, and BEC zone. Shoot samples from three species in one of the zones were further analyzed for nutrient concentrations.

Results/Conclusions

Glyphosate was detected in all plant tissues, including new shoot and fruit growth, for a minimum of one year after treatment, and at least six years in the shoots and 12 years in the roots of some species. Samples from the drier, colder, more northern BEC zone retained significantly higher levels of glyphosate for longer than samples collected from the warmer BEC zone. Nutrient concentrations were more variable over time in treated samples than in control samples, and comparison of treated and control samples resulted in significant differences in 47 % of all cases. Generally, Ca, Mg and Ni decreased with treatment in all three species; Zn increased with treatment for each species; and Fe and Mn had inconsistent changes with treatment. Both the persistence of glyphosate and related changes in plant nutrient concentrations may have cumulative health effects that should be considered when assessing exposure of humans and wildlife to chronic, low concentrations of glyphosate and other chemicals in the environment. Northern boreal forests should be of special concern, where glyphosate may persist in both soils and plant tissues for longer than previously expected.