Persistence of glyphosate, and associated influence on available nutrients, in native plant tissues following sublethal herbicide application in northern British Columbia forests

Glyphosate-based herbicides (GBH) are highly effective, nonselective herbicides that have been used in British Columbia’s forest industry since the early 1980s to remove deciduous vegetation competing with commercially important coniferous tree seedlings. Long term persistence of glyphosate and its metabolic byproduct aminomethylphosphonic acid (AMPA) in the forest ecosystem has not previously been monitored beyond one year, although it is known that glyphosate may be stored in tissues of plants that received a sublethal dose of GBH. The primary objective of this research thesis was to determine the duration (up to twelve years) of glyphosate and AMPA persistence in the roots, shoots, and fruits of selected perennial 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 thesis was to determine if concentrations of calcium (Ca), iron (Fe), magnesium (Mg), manganese (Mn), nickel (Ni) and zinc (Zn) were significantly altered by GBH treatment. Plant tissue samples from five forest understory perennial species growing in two distinct biogeoclimatic (BEC) zones of northern BC were analyzed for glyphosate and AMPA content, and comparisons were made between species, plant tissue type, and BEC zone. Residues persisted for up to 12 years in some tissue types, and root tissues generally retained glyphosate residues longer than shoot tissue types. Samples from the drier, colder, more northern Boreal White and Black Spruce (BWBS) BEC zone retained significantly higher levels of glyphosate for longer than samples collected from the warmer Sub-Boreal Spruce (SBS) BEC zone. Shoot samples from three species in the SBS zone were further analyzed for nutrient contents. 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, and in 61 % of year one samples. 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 for wildlife. More research should be conducted in this field, as many questions remain unanswered.