Summary When the exotic Acer platanoides L. (Norway

maple) and the native A. saccharum Marsh. (sugar maple) grow together in the understories of urban Quercus forests in the eastern USA, average annual height growth increments are nearly twice as large in A. platanoides as in A. saccharum, 19.26 ± 3.22 versus 10.01 ± 1.69 cm. We examined several ecophysiological mechanisms that might be associated with the superior invasive ability and growth of A. platanoides in two urban oak forests in Pennsylvania. Leaf longevity was 12 days greater in A. platanoides than in A. saccharum. In addi-tion, leaf mass/leaf area ratio was greater in A. platanoides than in A. saccharum (2.67 ± 0.03 versus 2.32 ± 0.02 mg cm–2); however, leaf thickness was significantly lower in A. pla-tanoides than in A. saccharum suggesting that A. platanoides contains more dense palisade and mesophyll cell layers than A. saccharum. Field net photosynthesis (mass basis) and pho-tosynthetic light response curves (area basis) indicated signifi-cantly greater carbon assimilation, and nitrogen and phosphorus use efficiencies in A. platanoides than in A. sac-charum. Acer platanoides also exhibited higher water use efficiency than A. saccharum (0.88 ± 0.12 versus 0.32 ± 0.09 mmol CO2 mol–1 H2O). Acer platanoides exhibited signifi-cantly lower osmotic potentials than A. saccharum, but a similar relative water content at zero turgor. We conclude that A. platanoides utilizes light, water and nutrients more effi-ciently than A. saccharum