Contemporary evolution of an invasive grass in response to elevated atmospheric CO 2 at a Mojave Desert FACE site

Elevated atmospheric CO 2 has been shown to rapidly alter plant physiology and ecosystem productivity, but contemporary evolutionary responses to increased CO 2 have yet to be demonstrated in the field. At a Mojave Desert FACE (free‐air CO 2 enrichment) facility, we tested whether an annual grass weed ( Bromus madritensis ssp. rubens ) has evolved in response to elevated atmospheric CO 2 . Within 7 years, field populations exposed to elevated CO 2 evolved lower rates of leaf stomatal conductance; a physiological adaptation known to conserve water in other desert or water‐limited ecosystems. Evolution of lower conductance was accompanied by reduced plasticity in upregulating conductance when CO 2 was more limiting; this reduction in conductance plasticity suggests that genetic assimilation may be ongoing. Reproductive fitness costs associated with this reduction in phenotypic plasticity were demonstrated under ambient levels of CO 2 . Our findings suggest that contemporary evolution may facilitate this invasive species' spread in this desert ecosystem.