Predicting population-level changes in ragweed (Ambrosia artemisiifolia L.) response to elevated carbon dioxide

Background/Question/Methods

           Assessing ecotypic variation in plant response to predicted levels of carbon dioxide (CO₂) is a priority for understanding climate change effects on plant species, especially those that can affect human health. Common ragweed, a native annual North American plant, is a well known allergenic weed that increases growth and reproduction in experimentally elevated CO₂ conditions.  However, it is not well known whether plants from environmentally distinct populations respond differently to elevated carbon dioxide.  Ecotypic variation in CO₂ response was tested here by exposing plants from 24 populations across a latitudinal gradient in the Northeastern US to three levels of CO₂: 400 ppm, 600 ppm, and 800 ppm. The objective of the study was to test for differences among ecotypes in 1) changes in timing and duration of flowering, and 2) stimulatory effects on biomass and reproductive allocation.  Wild-collected seeds from each population were planted in pots in outdoor growth chambers and evaluated weekly for phenological and growth data.  Experimental design of 4 replications of each of the 3 carbon dioxide levels and 4 replications of the 24 populations within each chamber resulted in assessment of 1,152 plants.  Plants were harvested and dried for biomass at the end of the growing season.

Results/Conclusions

            Overall, northern populations showed the most stimulation while southern populations showed the least in response to elevated CO_2.  Preliminary results (n=300) indicate that plants flowered earlier and longer in elevated CO₂ concentrations; this was especially evident in the northern most populations where the timing was earlier and longer than the mid- and low- latitudes. There was a significant increase in shoot and root biomass for all populations with elevated CO₂; the greatest stimulation occurred in the northern populations.  Northern populations invested the most mass in seed production and southern populations invested more mass in vegetative growth at 800ppm CO₂ while mid-latitudes showed a switch from seed investment to vegetative investment with increasing CO₂.   The implications for male flower and pollen production are currently under study. It can be concluded from the current analysis that future CO₂ concentrations may have the greatest effect on allergy season in northern parts of the study range due to the longer flowering season and enhanced reproductive effort.