Gas exchange, growth, and defense responses of invasive Alliaria petiolata (Brassicaceae) and native Geum vernum (Rosaceae) to elevated atmospheric CO 2 and warm spring temperatures

• Premise of Study: Global increases in atmospheric CO 2 and temperature may interact in complex ways to influence plant physiology and growth, particularly for species that grow in cool, early spring conditions in temperate forests. Plant species may also vary in their responses to environmental changes; fast‐growing invasives may be more responsive to rising CO 2 than natives and may increase production of allelopathic compounds under these conditions, altering species’ competitive interactions. • Methods: We examined growth and physiological responses of Alliaria petiolata , an allelopathic, invasive herb, and Geum vernum , a co‐occurring native herb, to ambient and elevated spring temperatures and atmospheric CO 2 conditions in a factorial growth chamber experiment. • Key Results: At 5 wk, leaves were larger at high temperature, and shoot biomass increased under elevated CO 2 only at high temperature in both species. As temperatures gradually warmed to simulate seasonal progression, G. vernum became responsive to CO 2 at both temperatures, whereas A. petiolata continued to respond to elevated CO 2 only at high temperature. Elevated CO 2 increased thickness and decreased nitrogen concentrations in leaves of both species. Alliaria petiolata showed photosynthetic downregulation at elevated CO 2 , whereas G. vernum photosynthesis increased at elevated temperature. Flavonoid and cyanide concentrations decreased significantly in A. petiolata leaves in the elevated CO 2 and temperature treatment. Total glucosinolate concentrations and trypsin inhibitor activities did not vary among treatments. • Conclusions: Future elevated spring temperatures and CO 2 will interact to stimulate growth for A. petiolata and G. vernum , but there may be reduced allelochemical effects in A. petiolata .