Background
Elevated CO2 typically stimulates the growth of nearly all plant species in monoculture, including those deemed undesirable by humans, such as weeds and various invasive species. Consequently, as the air's CO2 content continues to rise, climate alarmists have a penchant for predicting the worst when it comes to competitive relations among weeds, invasive species and C3 vs. C4 plants growing together in mixed-species stands.

What was done
In a study designed to explore this complex issue, the authors grew thale cress (Arabidopsis thaliana) either by itself or together with either the C3 grass Bromus inermis or the C4 grass Andropogon gerardii in small pots placed within open-field FACE arrays at the Cedar Creek Ecosystem Science Reserve, Minnesota (USA) -- which were maintained at atmospheric CO2 concentrations of either 368 or 560 ppm -- from the time of emergence to the time of senescence of A. thaliana.

What was learned
At the time of harvest, the CO2-induced biomass stimulation of A. thaliana was approximately 42% when grown alone, but 46% when grown together with A. gerardii and 50% when grown together with B. inermis, while corresponding stimulation values were 1%, 3% and 4% for leaf number, 15%, 17% and 21% for plant height, 11%, 21% and 20% for stem number, and 25%, 43% and 39% for fruit number, indicative of greater CO2-induced benefits for A. thaliana when it was grown in competitive mixtures with other species.

What it means
Based on the above-quoted results, as well as many other findings of their complex and comprehensive study, Lau et al. conclude that "elevated CO2 reduces the effects of competition on mean fitness ... and minimizes the strength of competition as a selective agent [italics added]." Therefore, it may logically be expected, all else being equal, that ecosystem species richness or biodiversity should at least remain stable, or possibly even increase, in response to continued increases in the air's CO2 content.