Traffic patterns, more than adjacent land use, influence element content of roadside forbs for insect pollinators

Alexander M. Shephard, Lauren Agnew, Annika Herdtle, Timothy S. Mitchell, Elizabeth T. Borer, Emilie C. Snell-Rood

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


Roadsides are targeted for restoration of pollinator-friendly plants. Yet, roads are sources of macronutrient, micronutrient and heavy metal pollution that may contaminate roadside plants. Adjacent landscape features such as railroads and agriculture provide additional macronutrient and heavy metal pollution that may exacerbate traffic effects. However, we lack perspective on how roads combine with rural landscape features to influence nutrition of roadside plants, which could have implications for pollinator health. We surveyed roadsides across Minnesota, USA and measured foliar levels of dietary macronutrients (nitrogen, phosphorous and potassium), a micronutrient (sodium) and metals (iron, zinc, copper, chromium, nickel, lead, aluminium and cadmium) in six abundant roadside forb species used by insect pollinators: Asclepias syriaca, Dalea purpurea, Monarda fistulosa, Ratibida pinnata, Solidago spp. and Trifolium pratense. We aimed to determine (1) how road variables (traffic volume and distance from road) combine with adjacent land use (railroad and agriculture) to influence element content of roadside forbs and (2) whether some forb species show consistent differences in their accumulation of potentially toxic heavy metals, which could inform selection of species to plant along roadsides. We found that foliar concentrations of nine elements increased with greater traffic volume (nitrogen, phosphorous, iron, zinc, copper, chromium, nickel, lead and aluminium), and concentrations of six elements declined with distance from the road (nitrogen, phosphorous, potassium, iron, zinc and copper). Leaves collected adjacent to railroad had less phosphorous, potassium, iron, nickel and aluminium than leaves collected from sites not adjacent to railroad. Additionally, leaves collected from sites adjacent to agriculture had lower copper levels than leaves from sites without adjacent agriculture. We found no evidence that particular ford species along roadsides consistently rank higher than other species in their accumulation of heavy metals. Our results show that traffic alters more elements in roadside plants than does adjacent landscape context, alleviating concerns that landscape features exacerbate pollutant levels in roadside pollinator habitat. However, nutrient contamination of most roadside plants is unlikely to reach toxic levels for insect pollinators. This work is consistent with the positive conservation potential of low to moderate traffic roadsides for pollinators.

Original languageEnglish (US)
Article numbere12195
JournalEcological Solutions and Evidence
Issue number4
StatePublished - Oct 1 2022

Bibliographical note

Funding Information:
Alex Shephard led writing, data analysis and visualization. Lauren Agnew collected data and assisted with project conceptualization and manuscript editing. Annika Herdtle assisted with data collection and manuscript editing. Tim Mitchell assisted with conceptualization, analysis and manuscript editing. Elizabeth Borer led conceptualization and contributed to analysis and manuscript editing. Emilie C. Snell‐Rood led grant funding and project conceptualization and contributed to writing, analysis and manuscript editing.

Publisher Copyright:
© 2022 The Authors. Ecological Solutions and Evidence published by John Wiley & Sons Ltd on behalf of British Ecological Society.


  • forb
  • heavy metals
  • herbivory
  • macronutrients
  • micronutrients
  • pollinator
  • right-of-way
  • roadside


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