anthropogenic activities have been increasing the atmospheric co2 concentration globally and the increase is predicted to continue to rise in the future. increased co2 has been shown to increase photosynthetic rates as well as water use efficiency in trees as well as induce changes in the global climatic conditions. climate change is also expected to alter the world’s hydrologic cycles, leading to increased flooding events in many regions. prolonged flooding events can cause negative effects on even flood tolerant species such as picea mariana and larix laricina, because of the lack of soil aeration. the lack of air in the soil causes the roots to turn from aerobic root respiration to fermentation. fermentation can lead to root tissue die back as a result of carbohydrate use inefficiency. the goal of this study was to determine if the benefits gained by increased co2 could overcome the problems caused by flooding. to test the effects of increased co2 and flooding on p. mariana and l. laricina changes in morphology and physiology were examined by exposing seedlings to double atmospheric co2 and a 28 day flood with a 35 day recovery. overall no interactive effects on the morphology of either seedling species were found. there was only one significant interaction between co2 and flooding seen on the physiology of the seedlings; though no long lasting effects were observed. the flooding treatment, however, significantly reduced the quantum yield of photosystem ii and root respiration in both species and under both co2 concentrations. the photosystem ii function recovered 35 days after the termination of the flood treatment, but root respiration did not. the results suggest that the co2 elevation did not alleviate the flood injuries or enhance the subsequent recovery following the flood treatment. it is also found that the two species has similar tolerance to flooding despite that the flood treatment induced the development of adventitious roots in l. laricina but not in p. mariana.