Could recent increases in atmospheric CO2 have acted as a selection factor in Avena fatua populations? A case study of cultivated and wild oat competition

Projected increases in atmospheric carbon dioxide concentration [CO₂] may lead to differential selection and competition between weeds and crops. Yet, the current level of atmospheric [CO₂] already reflects a rapid rise (~25%) from mid‐20th century levels. To assess whether this increase could have already resulted in differential selection between weeds and crops, two temporally distinct populations of Avena fatua (wild oat) from the same geographic location, one from the 1960s (WO_old) and one from 2014 (WO_new), were grown with and without a cultivated oat (CO) line, Clintland 64 (Avena sativa) at current and mid‐20th century [CO₂] levels (ca 320 and 402 μmol mol^?1 respectively). Monocultures of each WO population differed in their response to recent increases in atmospheric [CO₂], with WO_new showing a significantly higher response for all vegetative parameters relative to WO_old. Assessment of competitive outcomes indicated that at 315 μmol mol^?1 cultivated oat was at a competitive advantage relative to either A. fatua population for leaf area and total above‐ground biomass. However, at current levels of CO₂, an overall increase in WO_new competitive ability was observed relative to the cultivated oat line. Overall, these differences are consistent with, but not conclusive for, improved evolutionary fitness and increased early competitive ability of A. fatua relative to cultivated oat as a function of the recent increase in atmospheric [CO₂]. While additional empirical studies are needed, these preliminary results indicate that weeds may be adapting rapidly to rising levels of atmospheric carbon dioxide.     

What criteria are relevant for predicting the invasive capacity of a new agricultural weed? The case of invasive American species in France

In Europe, exotic species are assessed for their potential threat only after they have already become weeds. To test hypotheses underlying risk assessment procedures, 274 North and South American exotic species naturalized in France were compared to differentiate environmental from agricultural weeds and to characterize weeds of several crops. Most agricultural exotic weeds belonged to the Asteraceae and the Poaceae, but proportionally to the total number of invasive species from a family the most successful families were the Amaranthaceae, Euphorbiaceae and Poaceae. Weed status in North and South America was the best predictor of agricultural weed status in France, and a pre-1900 date of introduction was associated with a greater likelihood of having become a major weed. The characteristics of 78 species present in field crops were examined in relation to management practices. In annual crops, the life-cycle compatibility of weeds appeared to be the main factor of selection. Exotic species differed in their C4 photosynthesis pathway in maize and rice. In vineyards, many ecological strategies are possible because the habitat is heterogeneous. Intrageneric comparison of exotic and native species showed a superior relative growth rate and higher biomass production under nutrient-rich conditions for exotics. However, it is still difficult to find a predictive function that fits well with all observed cases of weed invasion.