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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 ( WOold) and one from 2014 ( WOnew), 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 WOnew showing a significantly higher response for all vegetative parameters relative to WOold. 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 WOnew 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. [ABSTRACT FROM AUTHOR]

The establishment and spread of invasive plants are often associated with a 'general-purpose genotype', with a corresponding high degree of phenotypic plasticity when introduced to a new environment. Evolutionary potential of invasive species following introduction should also be considered, however, as such changes can facilitate rapid range expansion. In this study, we utilised seed from three geographically distinct mid-Atlantic populations of an invasive species, Microstegium vimineum ( Japanese stiltgrass), to assess whether populations varied in their biomass and reproductive responses to light and nitrogen under neutral glasshouse conditions. To assess evolutionary adaptation, we quantified and correlated the abiotic environment from each collection location with the final biomass and seed production of that population under common glasshouse conditions. For M. vimineum, growth and reproductive responses to a common set of abiotic parameters (e.g. light) under these conditions indicated considerable phenotypic variation between and within populations. Interestingly, the degree of variation of seed production and final biomass among populations in the glasshouse was strongly correlated with growing season length and cumulative degree days recorded near each population location. These latter data indicated that M. vimineum, introduced into this region <100 years ago, may be undergoing adaptive evolution. Although additional populations of M. vimineum need to be examined, this study suggests that local evolutionary adaptation may be occurring rapidly and could, potentially, be an important aspect in the establishment and spread of this invasive species. [ABSTRACT FROM AUTHOR]

V alerio M, T omecek MB, L ovelli S & Z iska LH (2011). Quantifying the effect of drought on carbon dioxide-induced changes in competition between a C3 crop (tomato) and a C4 weed ( Amaranthus retroflexus). Weed Research 51, 591-600. Summary Recent and projected increases in atmospheric carbon dioxide concentration ([CO2]) and subsequent effects on climate are likely to alter competitive outcomes of weeds and crops. Rising [CO2] per se could increase the competitive ability of C3 crops relative to C4 weeds. However, such an outcome may depend on other climatic variables. In this study, tomato, a C3 crop species, was grown from emergence to anthesis using replacement series mixtures with Amaranthus retroflexus, a C4 weed species at three different [CO2], 400, 600 and 800 μmol mol−1, with and without water stress. Under well-watered conditions, leaf photosynthetic rates and plant height, leaf area and biomass all increased with elevated [CO2] for tomato relative to A. retroflexus, consistent with the kinetics of C3 photosynthesis. However, if water was limiting, a significant positive effect of [CO2] was noted for plant height and biomass of A. retroflexus with increased competition. This result may be related to a greater increase in leaf water potential with rising [CO2] for A. retroflexus relative to tomato under water stress. Overall, these are the first data to suggest that increases in atmospheric CO2 could still exacerbate crop losses from a C4 weed, even with a C3 crop, if drought occurs. [ABSTRACT FROM AUTHOR]