Background
It is a well-known fact, as the author indicates, that C4 plants benefit from elevated temperature; and he thus writes that many people believe that "since the majority of the 'world's worst weeds' are C4 plants and most crops are C3 plants, global warming can be assumed to strengthen the competition from these weeds in the future." But is this seemingly logical conclusion really correct?

What was done
In a test of the hypothesis that he conducted in Finland, Tyvonen studied the degree of success of Amaranthus retroflexus (a C4 weed) growing both with and without spring barley (a C3 crop) at current and elevated (+3°C) temperatures in a greenhouse experiment, where A. retroflexus was sown at a number of different starting times relative to the time of sowing of the barley in the various competition runs.

What was learned
The Finnish researcher discovered that although the growth of barley decreased somewhat with the rise in temperature, "the growth and seed production of A. retroflexus in competition with barley was minimal." Furthermore, as he continues, "climate warming will advance the sowing times of C3 crops (Kaukoranta and Hakala, 2008), thus reinforcing the competitive benefit of C3 crops in spring time."

What it means
From the results of his study, Hyvonen concludes that "A. retroflexus is unlikely to take hold in spring cereals," which he says is confirmed by field observations that reveal that "none of the 188 weed species found in a Finnish weed survey (Salonen et al., 2001) was a C4 species." In addition, he notes that "none of the C4 species belong to the most important weeds of C3 crops in Europe (Schroeder et al., 1993)," concluding that "C3 crop species could act as a barrier to invasion by the 'world's worst weeds' into the boreal region." We would only add to this conclusion that the additional preferential benefit that the ongoing rise in the air's CO2 content provides to C3 as opposed to C4 plants makes his view of the matter all the more likely to be correct.

References
Kaukoranta, T. and Hakala, K. 2008. Impact of spring warming on sowing times of cereal, potato and sugar beet in Finland. Agricultural and Food Science 17: 165-176.

Salonen, J., Hyvonen, T. and Jalli, H. 2001. Weeds in spring cereal fields in Finland - a third survey. Agricultural and Food Science in Finland 10: 347-364.

Schroeder, D., Mueller-Schaerer, H. and Stinson, C.S.A. 1993. A European weed survey in 10 major crop systems to identify targets for biological control. Weed Research 33: 449-458.