Changes in biomass and root:shoot ratio of field-grown Canada thistle (Cirsium arvense), a noxious, invasive weed, with elevated CO2: implications for control with glyphosate

Authors: Ziska, Lewis; Faulkner, Shaun; Lydon, John

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2004
Publication Date: 9/1/2004
Citation: Ziska, L.H., Faulkner, S.S., Lydon, J. 2004. Changes in biomass and root:shoot ratio of field-grown Canada thistle (Cirsium arvense), a noxious, invasive weed, with elevated CO2: implications for control with glyphosate. Weed Science. 52(4):584-588.

Interpretive Summary: Plants use atmospheric carbon dioxide (CO2) as fuel. They "breathe" CO2 in much the same way that humans breath oxygen. Since the start of the industrial revolution, humans have poured a lot of CO2 in the atmosphere. Since plants like CO2 this will result in increased plant growth. However not all plants are good. Some plants result in negative effects on human society. We call these plants weeds. So if weeds also grow more, as CO2 increases, won't this be bad? It has been assumed by most scientists that even as CO2 goes up, weeds will still be able to be controlled by chemicals. We tested this assumption by growing a noxious weed (Canada thistle) at present and future CO2 levels and spraying with different herbicides ("Round-up" and "Liberty"). At the higher CO2 values the weeds did not die at the recommended concentration. This suggests that as atmospheric CO2 increases, weeds may be harder to control using chemicals. This may have a bad effect on crop production in the United States and other countries.

Technical Abstract: To determine how rising CO2, a principle greenhouse gas, alters chemical weed control, Canada thistle (Cirsium arvense L. Scop.) was grown under field conditions at ambient and elevated CO2 concentrations (ambient and 350 ppm above ambient, respectively) for 2000 and 2001. Under field conditions, elevated CO2 per se resulted in significant increases in below ground biomass (+70-150%) but no consistent effect on above-ground (shoot) growth relative to the ambient CO2 control for both years. Post-emergent applications of glyphosate (Roundup) and glufosinate (Liberty) at recommended rates (2.24 and 0.376 kg ai ha-1, respectively), resulted in significant reductions in above ground (shoot) biomass and below-ground (root) biomass relative to unsprayed plots under ambient CO2 for both herbicide treatments following a 6 week regrowth period. However, in contrast to the ambient CO2 treatment, glyphosate, and glufosinate applications at commercially recommended rates had no effect on shoot and root growth, respectively, when Canada thistle was grown at the elevated CO2 concentration. Because of the differential response, the ratio of root and shoot biomass at elevated CO2 relative to that at ambient CO2 significantly increased (2-4x for shoot, 7-8x for roots) as a result of herbicide application. Overall, these data indicate that rising atmospheric CO2 could increase herbicide tolerance of Canada thistle, a widely recognized noxious weed species. Subsequent changes in herbicide efficacy could increase costs associated with control of this species.