High growth temperatures and high soil nitrogen do not alter differences in CO 2 assimilation between invasive Phalaris arundinacea (reed canarygrass) and Carex stricta (tussock sedge)

PREMISE OF THE STUDY: Global change in temperature and soil nitrogen availability could affect plant community composition, potentially giving an advantage to invasive species compared to native species. We addressed how high temperatures affected CO 2 assimilation parameters for invasive Phalaris arundinacea and a sedge, Carex stricta , it displaces, in natural and controlled environments. METHODS: Photosynthetic parameters were measured in a wetland in Indiana, USA during the abnormally warm year of 2012. In a growth chamber, photosynthetic parameters were measured on the plants grown under three levels of nitrogen and exposed to optimum temperatures followed by 2012‐like summer conditions and then hot temperatures with an autumn‐like photoperiod. KEY RESULTS: In the wetland, C . stricta exhibited signs of midsummer leaf senescence, whereas P . arundinacea maintained CO 2 assimilation at ambient pCO 2 ( A amb ) through mid‐October. In the chamber, 2012‐like conditions reduced A amb for both species through reductions in maximum carboxylation ( V cmax ) and electron transport ( J max ) without further change during subsequent hot, autumn‐like conditions, whereas the quantum efficiency of carbon assimilation ( qe ) declined throughout the experiment. However, P . arundinacea had higher values of A amb , J max , and qe than C . stricta . A general, the positive effect of increasing nitrogen availability occurred for photosynthetic processes for both species in hot conditions. CONCLUSIONS: Our data suggest that C . stricta is more susceptible to excessive light stress than P . arundinacea during hot, sunny periods, leading to leaf senescence. Field confirmation of this idea is needed, but frequent heat waves should favor P. arundinacea over C. stricta with or without eutrophication.