Reduced function in Chamaenerion angustifolium after sub-lethal glyphosate exposure

Chamaenerion angustifolium (fireweed) is an ecologically important plant in the northern hemisphere. It provides food across forest openings for many wildlife species including bumblebees, which are important pollinators to North America. Fireweed also acts as a significant food source for honeybees and is used by many North American Indigenous people as food and medicine. In forested areas managed for timber, fireweed is often incidentally exposed to glyphosate-based herbicides (GBH) in post-harvest vegetation management. We studied the response of fireweed to sub-lethal GBH exposure in a controlled experiment and in standard operational field conditions to determine impacts on specific aspects of growth and reproduction of the species. We aimed to determine if GBH-related stress symptoms would significantly impact the fluorescence of fireweed flowers, and/or the nutritional quality of pollen, which would have consequences for pollinators. Results showed that fireweed is negatively impacted by sublethal exposures of GBH including reduced photosynthetic efficiency, reduced height, and reproductive shoot dieback. In operational environments studied, pollen viability was reduced one-year after applications and anther fluorescence was altered. The amino acid concentration of flowers was reduced, and glyphosate residues remained present at low concentrations in floral tissues at two years post-treatment. It was concluded that these changes to fireweed growth and reproduction reduce its function as a primary source of good quality food for pollinators.