Complementary responses of morphology and physiology enhance the stand-scale production of a model invasive species under elevated CO2 and nitrogen

1. Elevated atmospheric carbon dioxide (eCO(2)) concentrations and nitrogen (N) enrichment are known to enhance plant productivity and invasion. However, the implications of their interactive effects for plant productivity are not well understood, especially at the stand scale, presumably because morphological and physiological responses to these global change factors are rarely studied together in the field or assessed at the stand level. 2.We first determined how leaf-level morphological and physiological traits responded to factorial combinations of ambient and elevated CO2 and N. We collected trait data from the model invasive species Phragmites australis (common reed) that were measured over 3years in a long-term global change field experiment. We then combined the trait data and additional descriptions of P.australis canopies in a simulation model of carbon assimilation to determine how morphology and physiology contribute to P.australis' stand-scale productivity. At the leaf level, we found that light-saturated rates of photosynthesis were strongly stimulated by eCO(2) (37%) and that this effect was enhanced by increasing salinity. N had a smaller effect (17% stimulation) on physiological responses than eCO(2), but leaf morphological traits responded primarily to N; plant height increased by 27% and leaf area increased by 47%. 3. Stand-scale simulations demonstrated that that morphological and physiological adjustments induced approximately additive responses when P.australis experienced both eCO(2) and N enrichment. The simulations also indicated that morphological changes (which were primarily associated with canopy size) influenced stand-scale carbon assimilation more than physiological changes. Moreover, 97% of the N response was due to changes in morphology, whereas 62% of the eCO(2) response was caused by physiological shifts. 4. Our analysis indicates that morphological and physiological trait responses to elevated CO2 and nitrogen are likely to enhance the productivity of P.australis in complementary ways, potentially accelerating its invasion in North America. Furthermore, our data suggest that changes in morphological traits may have a greater influence on carbon gain than leaf-level physiology under near-future environmental conditions. Our study also highlights the importance of accounting for both morphological and physiological responses when attempting to infer global change responses from leaf-level data.